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Tracking Potential Sources of Peak Ozone Concentrations in the Upper Troposphere over
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the Arabian Gulf Region
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Teresa K. Spohn1,2,+ and Bernhard Rappenglück1,2,
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1) Department of Earth and Atmospheric Sciences, University of Houston, 4800 Calhoun Rd,
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Houston, TX, USA, 77204-5007
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2) Qatar Environment and Energy Research Institute, P.O. Box 5825, Doha, Qatar
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Abstract – In August 2013, the Qatar Environment and Energy Research Institute (QEERI), was
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the first to launch temporally highly resolved ozonesondes in the Middle East region. The data
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from 20 launches consistently show changes in meteorological parameters at about 5.5 km above
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the surface, which are more pronounced following a change in synoptic conditions on 15 August
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2013, including temperature inversions, corresponding change in potential temperatures, relative
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humidity, and significant wind shear. These changes are typically associated with a large scale
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subtropical subsidence layer in accordance with previous aircraft studies in this region. Below the
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inversion layer, the ozone follows typical patterns for lower tropospheric measurements, starting
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in the surface layer up to 0.5 km above the ground level around noon at about 66±15 ppbv.
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However, above the subsidence inversion, ozone mixing ratios begin to increase to 79±13 ppbv
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
now with the National University of Ireland, Galway.
Corresponding author. Tel.: +1 713 893 1298; Fax: +1 713 748 7906; E-mail address: brappenglueck@uh.edu
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between 6-12 km with maximum values ~ 100 ppbv around 8 km, then decreasing again before
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reaching the stratosphere.
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Three-day HYSPLIT back trajectories indicate that ozone levels are typically about 17% lower in
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the 6-12 km range under wind flow conditions from the East than in cases when trajectories came
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from the Mediterranean. High pressure may lead to subsidence of ozone from the upper
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troposphere/lower stratosphere and eventually cause an increase of ozone mixing ratios by ~18%
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above average between 6-7 km, i.e. slightly above subtropical subsidence layer. Under the impact
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of regional convective activity and associated lightning, ozone mixing ratios can increase by
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more than 35% averaged over the 9-12 km altitude range. In both cases maximum ozone in the
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mid to upper troposphere reached more than 100 ppbv.
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Keywords: Ozone, Middle East, Qatar, Tropospheric Transport, Lightning
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1. INTRODUCTION
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Ozone (O3) in the troposphere is of interest because of its negative effects on human health and
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global warming (US Environmental Protection Agency, 2012). Tropospheric ozone is primarily
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formed photo-chemically through the break-down of nitrogen oxides (NOx) by sunlight in the
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presence of volatile organic compounds (VOCs) (e.g. Lelieveld, et al. 2009). Contrary to the
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tropospheric ozone, ozone in the stratosphere is formed through photolysis of oxygen and forms a
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barrier protecting the Earth from the sun’s ultra-violet radiation; however, it is sometimes mixed
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into the troposphere through Stratospheric-Tropospheric Exchange (STE) (e.g. Lelieveld, et al.
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2009).
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There are very few studies of ozone in the Arabian Gulf region (Li et al., 2001; Lelieveld et al.,
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2009; Zanis et al., 2014). While they all concur that there is unusually high ozone in the mid to
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upper troposphere in the Middle East region, they do not agree on the reasons for it.
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Li et al. (2001) focus on a study in July 1997. They conducted sensitivity simulations to
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determine contributions to ozone at various levels within the troposphere, as well as distinguish
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between the influence of anthropogenic and lightning sources. Li et al. claim that there are
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unusually high summertime levels of ozone, over 80 ppbv in the mid to upper troposphere in the
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Middle East region. The reason for this is the anti-cyclonic circulation over the Arabian Peninsula
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and the Indian sub-continent “funneling” in pollution from Europe and nitrogen oxides (NO x)
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generated by lightning outflow from the Indian Monsoon. Pollution from eastern Asia transported
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in the Easterly Tropical Jet Stream is said to contribute as well. Results indicate that the largest
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source of ozone (35-50%) is due to production in the upper troposphere and large-scale
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subsidence in the region. Losses in these areas due to mid-level outflow are only little according
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to the Li et al. study. They estimate that 20-30% of the tropospheric ozone column has been
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caused by anthropogenic sources and 10-15% is from lightning. According to the paper,
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stratospheric ozone is not thought to be a major contributor, although it seems to have a greater
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impact than in other regions of the world.
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Li et al.’s study used the GEOS-CHEM (Goddard Earth Observing System) global 3-D model of
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atmospheric chemistry in conjunction with vertical ozone profiles collected by the MOZAIC
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(Measurement
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http://www.iagos.fr/web/rubrique2.html) program over the regions of Tel-Aviv, Dubai, and
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Tehran. Data from the NASA (National Aeronautics and Space Administration) Data
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Assimilation Office for the years 1993-1997 were used as input for the model along with an
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anthropogenic base emissions inventory from 1985, adjusted for the study period, which included
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Ozone
and
Water
Vapour
on
Airbus
in-service
Aircraft;
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NOx, Non-Methane Hydrocarbons (NMHC), and Carbon Monoxide (CO). Lightning-NO
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production associated with deep convection was parameterized following Price and Rind (1992).
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Lelieveld et al. (2009) postulate that the Middle East is an ozone “hotspot” due to long-range
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transport of pollutants as indicated by the tracer CO, unusually strong STE, substantial natural
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upwind sources of NOx such as lightning, the lack of precipitation, and contribution from local
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emissions such as those from oil and gas refineries combining to create ideal conditions for ozone
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production and entrainment. They find that there is a distinct ozone maximum between the
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surface and 750 hPa, which is even more pronounced in the summertime when conditions favor
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photochemistry. The average ozone mixing ratio in the mid to upper troposphere in the summer is
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around 80 ppbv. A comparison with other subtropical areas showed that the diel variation of mid-
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tropospheric ozone in Bahrain was related more to the long-range transport of pollutants than to
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local production, and that after removing anthropogenic sources from the model, the region still
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had higher ozone than the other areas. Unlike Li et al. (2001), this study suggests that
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stratospheric ozone does have a major impact on the regional tropospheric ozone column, making
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up about two thirds of it in winter and one quarter in the summer.
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The Lelieveld study uses the EMAC (European Center-Hamburg 5th generation model MESSy
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[Modular Earth Submodel System] Applied to Atmospheric Chemistry) model in conjunction
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with satellite-retrieved ozone imagery from the Tropospheric Emission Spectrometer (TES;
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http://tes.jpl.nasa.gov/) and Scanning Imaging Absorption Spectrometer for Atmospheric
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Chartography (SCIAMACHY; http://www.sciamachy.org/) to identify where the highest ozone
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concentrations are. The model is nudged toward the meteorological conditions in 2006 based on
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re-analysis data from the European Centre for Medium-Range Weather Forecasts (ECMWF) and
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uses EDGAR 3.2 (Emissions Database for Global Atmospheric Research 2000) for inputs of
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anthropogenic emissions. The model includes a stratospheric ozone tracer as well and follows its
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transport and destruction in the troposphere. However, it does not include recycling processes of
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that stratospheric ozone tracer in the troposphere.
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The most recent study by Zanis et al. (2014) indicates that the enhanced tropospheric summer
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ozone levels in the Eastern Mediterranean and Middle East are the result of stratospheric ozone
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being transported to the troposphere through subsidence induced by high pressure systems in the
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region. While local photochemical formation is still the dominant contributor to ozone
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concentration, stratosphere to troposphere transport (STT), a type of STE, plays a critical role in
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places with favorable conditions.
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The Zanis et al. (2014) research was conducted in a similar way to the previous Lelieveld et al.
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(2009) study, using the EMAC model with ECMWF inputs and a stratospheric ozone tracer, and
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comparing the results to TES satellite data, but this time for a 12 year climatological study from
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1998-2009. Both the model and satellite show pools of higher ozone concentration in the upper
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and middle troposphere over the Eastern Mediterranean, with the stratospheric ozone tracer
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indicating a 40-45% contribution from the stratosphere to the middle troposphere. This is
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attributed to the large-scale subsidence and limited outflow resulting from the anti-cyclonic
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motion of the high pressure systems prevalent in the area during the summer months.
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Reid et al. (2008) report about dust profiles obtained through aircraft measurements, mostly over
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the area of the United Arab Emirates and vicinity, and also give a detailed overview of
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meteorological patterns in the region, including the mention of a subtropical subsidence
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inversion. While the focus of the paper was on aerosols and dust in the region, their flights
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encountered polluted air masses from Europe, India, and possibly Africa as well.
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All of these publications agree that the Middle East area is of specific interest due to some unique
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meteorological conditions and presumably elevated ozone levels which may have a large-scale
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impact and contribute to global warming as the ozone is exported from the region. Due to the
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scarcity of data sets in that area, they all call for more in situ measurements.
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In this paper we present data analysis from ozonesondes launched from Doha, Qatar, in August
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2013 and explore potential source areas for ozone in the upper troposphere.
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4. METHODS
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4.1 Data Collection
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Electro Chemical Concentration (ECC) ozonesondes (Droplet Measurement Technologies, 2013)
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in conjunction with iMet-1 radiosondes (InterMet Systems, 2008) attached to a 1.2 kg Kaymont
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brand weather balloon were used. Sondes were equipped with GPS (Global Positioning System).
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The iMet-1 radiosonde system (InterMet Systems, 2013) provides data for pressure (accuracy: 0.5
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hPa), temperature (accuracy: 0.2°C), relative humidity RH (accuracy: 5% RH), wind speed
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(accuracy: 0.1 m s-1), wind direction (accuracy: ≤ 5 degrees for wind speeds < 14 m/s, and ≤ 2
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degrees for wind speeds > 14 m/s), altitude (accuracy: 15 m), and position (accuracy: 5 m). The
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principle of ECC sondes are described thoroughly in Russell III et al. (1998). ECC sondes were
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used because they routinely outperform other types of ozonesondes such as the Brewer-Mast and
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KC96 Carbon Iodine Cell, as demonstrated in the 1996-2000 Jülich Ozone Sonde
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Intercomparison Experiment (JOSIE) (Smit and Kley, 1996; Smit et al., 2007) and are currently
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the most commonly used world-wide (Global Atmospheric Watch, 2013). Depending on the
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launch conditions and amount of helium, the balloon rose at a rate of 2-5 m s-1 and reached
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around 30 km above sea level (asl) before burst. For the altitude ranges considered in this study
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the accuracy of the ECC sondes varies from ±5% (1000 hPa) to ±12% (200 hPa) and the
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precision from ±4% (1000 hPa) to ±12% (200 hPa) (Droplet Measurement Technologies, 2013).
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Smit et al. (2007) report even slightly lower values. Comparison studies with routine lidar
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measurements made at the Observatoire de Haute Provence during 1990-1995 show that for
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fifteen simultaneous ECC versus lidar profiles, the mean of the differences observed between 4
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and 7 km was 2.5 ± 1.8 ppb (4 ± 3%) (Ancellet and Beekmann, 1997).
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In the period from 4 to 21 August 2013, 20 ozonesondes were launched from the College of the
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North Atlantic-Qatar campus in Doha (25.36ºN, 51.48ºE) at various times of day, including early
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morning (7:00 LT (local time)), mid-day (between 12:00-13:00 LT), and night (23:00-24:00 LT).
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Local time is UTC (Universal Time Code) + 2 hours. Data validation and verification were done
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in accordance with standardized methods listed in the U.S. Environmental Protection Agency’s
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Manual EPA-QA/G-8. (US Environmental Protection Agency, n.d.)
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5. OBSERVATIONS
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The radiosonde data consistently show a marked change in temperature, relative humidity, wind
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direction, and potential temperature at an altitude of about 5.5 km asl - factors indicative of a
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strong inversion. Previous observational and modeling studies (Li et al., 2001; Reid et al., 2008;
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Lelieveld et al., 2009) have shown that this phenomenon at about 5 km is in fact a subtropical
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subsidence inversion, which seems to occur persistently during the summer months. This is also
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seen in regular radiosonde launches from other locations in the Middle East area, e.g. Al-
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Qaisumah and Riyadh (Saudi-Arabia), Abu Dhabi (United Arab Emirates) and Kuwait City
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(Kuwait) (not shown, but retrieved from the University of Wyoming global radiosonde
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depository at http://www.weather.uwyo.edu/upperair/mideast.html). Figure 1 displays the average
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ozone profile, including the 25% and 75% percentiles, for all 20 Doha ozonesondes launched
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from 4 to 21 August 2013, segregated into 100 m altitude bins. This figure shows that above the
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subsidence inversion, ozone begins to increase significantly within the free middle troposphere,
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peaking around 8 km altitude range and then decreasing again before entering the stratosphere
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between 14 -16 km (see Table 1 for more information). This is fairly in agreement with modeled
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height of tropopause crossings for the Middle East region by Tyrlis et al. (2014).
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Ozone in the troposphere is mainly formed locally near the surface in complex reactions
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involving NOx and VOC precursors, however it has been found that pollution in the surface layer
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can reach upper tropospheric levels, due to strong convective processes (Baehr et al., 2003;
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Borbon et al., 2012). The persistent subsidence inversion over the Middle East area during the
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summer acts largely to suppress convective processes and the upper troposphere is therefore
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decoupled from the lower troposphere (Li et al., 2001; Reid et al., 2008; Lelieveld et al., 2009).
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As in-situ formation of ozone is limited in the upper troposphere, ozone aloft is likely due to
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transport processes. Apart from horizontal transport, vertical transport (e.g. lifting or subsidence)
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can play an important role. Through STE processes, stratospheric ozone can also contribute.
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Previous research has postulated that ozone in the mid to upper troposphere is being transported
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from Europe and the Mediterranean (Li et al., 2001).
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Henceforth we focus the discussions on the ozone distribution in the upper troposphere to trace
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the potential source areas of the elevated ozone found in the mid to upper troposphere between 6
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and 12 km over the Arabian Gulf area. For this purpose we consider mid-day launches only and
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do not include early morning launches done in addition after 15 August. This way there is
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consistency of only one launch per day at approximately the same time. From the remaining 14
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mid-day launches we needed to remove one launch (09 August) due to problems in the GPS data
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acquisition which led to unreliable wind data. In the following data analysis we therefore focus
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on an overall dataset of 13 mid-day launches.
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For data analysis we applied a series of 72 hour HYSPLIT (Hybrid Single Particle Lagrangian
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Integrated Trajectory Model) (Draxler and Rolph, 2013) back trajectories at altitudes of 6, 9, and
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12 km, based on the GPS coordinates of each ozonesonde when it was at these altitudes. In order
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to identify meteorological patterns for the month of August, the World Wide Lightning Location
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Network (WWLLN) lightning data (WWLLN Management Team, 2013), as well as National
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Oceanic and Atmospheric Administration (NOAA) surface, 200 and 500 hPa weather analysis
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charts of the region (NOAA National Climatic Data Center, 2013) were used to determine what
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factors might have influenced the long-range transport of ozone.
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5.1 HYSPLIT Back Trajectories
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Figure 2 shows the results of the HYSPLIT back trajectory analyses. Of the 13 launches used in
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this study, 6 days between 5 and 13 August showed trajectories coming from the East, from
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northern India and the Himalayas. On two days, 14 and 15 August, the trajectories curled around
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locally, indicating a shift in large-scale wind flows. After 15 August, the remaining 5 set of
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trajectories came from the northwest, the Mediterranean and North Africa. Uncertainties of the
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HYSPLIT trajectories are assumed to be in the range of 15-30% of the travel distance (Draxler
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and Rolph, 2013). Although, there might be some limitations associated with the accuracy of
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backward trajectories, they at least may provide information about major regimes. The HYSPLIT
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back trajectories showed generally that each air packet ended up within approx. 20-25% of its
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initial altitude.
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5.2 Weather Charts
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Figure 3 displays 500 hPa and 200 hPa weather charts for the Middle East area. The surface level
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analysis and lifted index charts are characterized by a constant “heat low” in the region due to
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high surface temperatures. This is capped by high pressure, indicated by elevated geopotential
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heights on the 500 hPa chart, preventing convective lifting, and resulting in thin altostratus clouds
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at about 6 km altitude most days. On 12 and 13 August, a stronger ridge moved in over Qatar,
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indicated by the yellow arrow on the 500 hPa chart. This ridge began to move out of the Qatar
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region on 14 August and had receded completely by 15 August. At the 200 hPa level, there is a
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persistent powerful jet streak north of Qatar between 40 and 45ºN from the beginning of August
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until 15 August, when it suddenly disappears, after which an upper level trough forms, pulling in
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air from the Mediterranean.
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5.3 Lightning Data
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Lightning data for the northern hemisphere for the month of August was obtained from the World
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Wide Lightning Location Network (WWLLN; http://wwlln.net/new/network/) and plotted in
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Google Earth. These data are gathered by a global network of sensing stations using VLF (Very
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Low Frequency between 3-30 KHz) to detect “sferics”—electromagnetic discharges caused by
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lightning. With a current network of 40 stations, the WWLLN is capable of detecting only about
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30% of all lightning strikes. This may be considered as an upper limit; it may be likely less in the
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in the area of this study. Due to this limitation, any lightning activity which is detected can be
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considered a lower limit and the actual lightning activity may be largely underestimated, and
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therefore the potential lightning-influenced ozone production in these areas as well.
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The Indian Summer Monsoon took place during August, and there was in fact heavy lightning
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activity over India, often along the HYSPLIT trajectories. There was however also considerable
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lightning in the Middle East, even in Qatar itself on some days. In particular, on 6 August the
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country experienced some thunderstorms (Figure 4).
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5.4 Balloon Data
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As illustrated in Table 1, the average measured ozone for all 13 launches between 6 and 12 km
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was 80±13 ppbv, with a median of ~79 ppbv. This is higher than the average total tropospheric
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ozone between the surface and 14 km of 72±14 ppbv. The average ozone below 500 m was
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~64±15 ppbv. Note that for consistency, and to ensure none of the data were above the boundary
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layer,500 m was used as a representation of surface ozone, rather than the entire planetary
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boundary layer which varies between 1-2 km.
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All launches were characterized by pronounced changes in relative humidity above 5.5 km as
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shown exemplary in Figure 5. Unlike the launches before 15 August which showed only weak
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temperature inversions, the launches after 15 August all showed a marked temperature inversion
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of 2-3ºC over a distance of less than 0.5 km at around 5.5 km altitude.
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On 7 of the 13 days, the ozonesondes measured maximum ozone over 100 ppbv between 6 and
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12 km. Three of these launches took place before 15 August, at which point the large scale
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synoptic condition changed and thus did the wind direction throughout most of the troposphere.
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On 6 August a peak ozone mixing ratio over 100 ppbv occurred at an altitude above 12 km. On
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12 August the peak ozone mixing ratio was 105 ppbv between 8 and 10 km, and on 13 August a
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nearly identical “plume” between 6 and 8 km (Figure 6) can be visually discerned.
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6. DISCUSSION
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In this section we will focus on three examples to explain the causes of high ozone concentrations
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in the Arabian Gulf region. Each case has specific characteristics which can indicate the likely
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source of the ozone peaks.
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6.1 Case 1: Ozonesonde launch on 6 August and the effect of lightning
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In this study we do not verify a distinct impact on ozone due to lightning related to the Indian
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Monsoon as this would require a comparison with transport processes from the Indian
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subcontinent under non-Monsoon conditions. However, this study shows that lightning from the
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Indian Monsoon is not associated with as high of ozone values at upper-tropospheric levels in the
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Arabian Gulf region as are observed when back trajectories point to the Eastern Mediterranean
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region. Air masses originating from the Eastern Mediterranean region contain about 30-40 ppb
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higher ozone values than air masses impacted by the Indian Monsoon and associated lightning
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activities as shown in Figure 8. Figure 8 also demonstrates that these different ozone regimes are
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clearly accompanied by different relative humidity regimes. While high relative humidity values
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can be expected for air masses impacted by the Indian Monsoon, it is remarkable that air masses
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originating from the Eastern Mediterranean contain extremely low relative humidity for the entire
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preceding 72 hours. This distinct feature is visible in all the other profiles of the corresponding
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specific synoptic conditions. As mentioned before, the days 14 and 15 August were transitional
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days between these large-scale flows, which also showed transition of the relative humidity
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profiles. The Eastern Mediterranean is a well-known hot-spot for enhanced tropospheric ozone
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levels (e.g. Kouvarakis et al., 2002; Kourtidis et al., 2002; Lelieveld et al., 2002; Lelieveld et al.,
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2009). This is largely due to large-scale subsidence in the summer (Zanis et al., 2014; Tyrlis et
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al., 2014), low tropopause height, and enhanced tropopause folding activity over the Levantine
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region during the summertime, in connection with the Asian monsoon (Tyrlis et al., 2014). The
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numerical simulations by Zanis et al. (2014) indicate that there is a large contribution of
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stratospheric ozone to the pool of high ozone values over the Eastern Mediterranean/Middle East
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in the middle and lower free troposphere and Sprenger et al. (2007) showed that stratospheric air
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mixed into the upper troposphere can remain there up to a few days. The extremely low relative
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humidity values in air masses coming from the Eastern Mediterranean area are in support of these
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previous modeling studies.
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Apart from these ozone transports in the upper troposphere, however, also local thunderstorm
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activity in the Arabian Gulf could be contributing to higher ozone. On 6 August there was
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considerable lightning activity in the entire region, about 50% more than on other days before 15
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August, and the HYSPLIT back trajectory for the ozonesonde, which was launched that day,
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either passed directly through (trajectory at 9 km altitude in Figure 9) or close to a nearby
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thunderstorm in the Arabian Gulf with correspondingly increased ozone at 12 km (Figure 9). It
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should be noted that the location, altitude, spatial extension of the lightning activity as well as the
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number of strikes is indicative for the overall lightning activity. As mentioned above the
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WWLLN data may only detect about 30% of all lightning strikes. In addition, uncertainties
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associated with the back trajectories are in the range of 15-30% of the travel distance (Draxler
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and Rolph, 2013). Thus it is likely that both trajectories, at 9 km and at 12 km altitude, might
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have been impacted by lightning. The peak ozone mixing ratio that day was 105 ppbv at 12 km,
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with a smaller 101 ppbv peak at 10 km (Figure 9). This is significantly higher than on the
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preceding day when under the same flow conditions about 50-60 ppbv ozone were observed from
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top of the boundary layer height around 2 km altitude up to 10-14 km altitude. Lightning is an
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important source of NOx in the upper troposphere worldwide (Schumann and Huntrieser, 2007;
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Huntrieser et al., 2012), including the Middle East area (Li et al. 2001) and the balloon’s actual
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trajectory took it directly downwind from this source. Huntrieser et al. (2007) report up to 60-70
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nmol mol-1 enhancement of ozone in the anvil outflow over a horizontal scale of about 400 km.
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This also due to the fact that the atmospheric lifetime of NOx in the upper troposphere is
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significantly longer than in the boundary layer and that photochemical processes are largely NOx
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limited (Huntrieser et al., 2007 and references therein). The amount of NOx produced also
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depends on the lightning stroke length, peak current, release height, and vertical wind shear
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(Huntrieser et al., 2008). Figure 9 shows that ozone exceeds 60 ppbv, the preceding day's average
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tropospheric value observed under similar Easterly flow conditions, around 7 km altitude and
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then continuously increases and reaches maximum values. This coincides with strongest vertical
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wind shears between 9-12 km, which encompasses the top of the cloud deck, as reflected in the
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relative humidity data (not shown). Ozone mixing ratios averaged over 9-12 km altitude were
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66.8±2.2 ppbv on 5 August versus 91.2±2.4 ppbv on 6 August, which corresponds to an increase
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of about 36.5% over this altitude range. These values rank among higher values found in other
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studies on lightning impact on ozone in the upper troposphere (Cooper et al., 2007; Morris et al.,
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2010 and references therein). In conclusion, this suggests that ozone produced as a result of this
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lightning activity may have contributed appreciably to the concentrations measured by the
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ozonesonde over Qatar.
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6.2 Case 2: Ozonesondes launched on 12 and 13 August
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The 12 and 13 of August were the only two days of the month during which a strong ridge was
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centered over the Arabian Peninsula, as shown on the 500 hPa chart in Figure 3. Zanis et al.
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(2014) discuss how the anti-cyclonic activity associated with high pressure systems causes ozone-
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rich air to be pulled downward isentropically out of the stratosphere into the upper troposphere
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through subsidence in the Eastern Mediterranean and Middle East region. Their study used a
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stratospheric ozone tracer in the model which showed the air mass descending over time.
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The observations on 12 and 13 August show an example for subsidence, as the ozone plume is
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between 8 and 10 km on 12 August, and has fallen to between 6 and 8 km only 24 hours later, yet
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maintains its distinct shape (see section highlighted by the rectangle in Figure 6). In Figure 6 we
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included the nighttime ozonesonde launch on 12 August so that this figure displays a sequence of
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ozonesondes almost every 12 hours apart. Note that the ozone values between 6 and 12 km
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altitude are significantly higher than the average ozone profiles at these altitude ranges (see
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Figure 1). Both days show subsidence inversions (not shown) at similar altitudes (5.5 km on 12
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August and 6 km on 13 August, i.e. close to the 500 hPa level). These inversions are associated
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with wind shear, mostly northerly below, whereas aloft winds come from the east throughout the
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upper troposphere coincident with the large scale synoptic situation as shown in Figure 3. Both
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inversions are also associated with generally decreasing relative humidity aloft, typical for
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subsidences. Also, starting with these inversions ozone mixing ratios increase with increasing
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altitude reaching maximum values between 8-11 km altitude on 12 August and between 6 and
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slightly above 8 km altitude on 13 August. The levels with enriched ozone are confined by
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another thermal inversion at about 12 km altitude on 12 August and slightly above 8 km altitude
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on 13 August. On 12 August the layer of maximum ozone is accompanied by low relative
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humidity around 10-15%, similar to the relative humidity values observed in the lower
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stratosphere on that day. Due to these overall low relative humidity values the thermal inversion
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about 12 km altitude does not show a pronounced change in relative humidity. This is in contrast
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to 13 August, when the lower thermal inversion can be clearly identified as a subsidence
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inversion with decreasing relative humidity values aloft. This is also reflected in the HYSPLIT
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analysis for that day (Figure 7), which shows that air masses arriving at the ozone sonde location
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at about 9 km altitude have originated above 11.5 km altitude and experienced a drastic decrease
345
in relative humidity along the trajectory. While on 13 August the relative humidity profile shows
346
some variation within the layer between 6 km and slightly above 8 km altitude ozone mixing
347
ratios are largely anticorrelated with relative humidity in that layer indicative for sustaining
348
similar atmospheric properties from the previous day. The anticorrelation of ozone with relative
349
humidity can be best seen in the nighttime ozone profile on 12 August between ~8 km to ~9 km
350
altitude. The main difference between the vertical meteorological profiles of those two days is the
351
change in the upper tropospheric subsidence inversions from around 12 km (12 August) to
352
slightly above 8 km altitude (13 August) within 24 hours, which would translate into an
353
entrainment of enriched ozone layers from upper tropospheric areas on the order of 4-5 cm/s
16
354
estimated as an upper boundary. Based on the HYSPLIT analysis an entrainment velocity on the
355
order of 1 cm/s can be estimated.
356
While on 12 August ozone mixing ratios were about 93.0±11.8 ppbv averaged over the altitude
357
range of 8 - 11 km, they decreased to 74.3±9.4 ppbv on the subsequent day for the same altitude
358
range, which is a decrease by ~20%. On the other hand, ozone mixing ratios averaged over 6 - 7
359
km were 66.1±4.2 ppbv on 12 August and increased to 77.9±12.5 ppbv on 13 August (increase by
360
~18%). The altitude range 7 - 8 km represents a transitional region with limited change as the
361
plume of enriched ozone passes this region during subsidence. This subsidence process will be
362
accompanied by horizontal dispersion, which is reflected by the fact that the ozone mixing ratios
363
in the ozone lamina slightly decrease and are confined to a shrinking altitude range.
364
365
6.3 Case 3: Ozonesonde launched on 19 August
366
The majority of cases of high ozone concentrations in the region during the measurement period
367
occur when the HYSPLIT back trajectories come from the Mediterranean, as exemplified by the
368
profile of the ozonesonde launched on 19 August (Figure 8), and corroborated by all launches on
369
and after 15 August. Here the ozone begins to increase at a lower altitude, around 7 km, and
370
remains high up to 12 km. In these instances the ozone peak is about 100 ppbv, and most likely
371
the result of long-range transport of pollutants (Li et al., 2001) in the 6-12 km range (72 hour
372
back trajectories in this altitude range point towards the Eastern Mediterranean). The strong
373
temperature inversion at 6 km acts as a boundary layer, preventing mixing from below or transfer
374
from aloft. Between 11.5 - 12 km altitude wind direction changes by about 180° (not shown)
375
which coincides with a sharp drop in ozone mixing ratios and which caps the high ozone levels
376
observed beneath, i.e. in the 6-12 km range.
17
377
Table 2 shows general results of the two large-scale flow regimes observed during the
378
measurement period as evident in Figures 2 and 8. The flow regime denoted as "East" refers to
379
the time period impacted by Monsoon activity over India, whereas the flow regime denotes as
380
"West" refers to the source region over the Eastern Mediterranean. From Table 2 it is evident that
381
Westerly flow regimes are associated with higher ozone mixing ratios than Easterly flow regimes
382
during the measurement period of August 2013. Compared to Easterly flow conditions, ozone
383
values in Westerly flow conditions are enhanced by about 17% through the entire troposphere
384
and in particular in the 6-12 km altitude range, which is double the enhancement in the surface
385
layer. It is noteworthy to keep in mind that ozone values under Easterly flow conditions are
386
impacted by thunderstorm activities related to the Monsoon season. As a consequence an even
387
higher ozone enhancement in Westerly versus Easterly flow regimes in the 6-12 km altitude
388
range may have been masked by Monsoon lightning activities. The observed ozone enhancement
389
associated with back trajectories point towards source regions over the Eastern Mediterranean
390
and support recent model results (Lelieveld et al., 2009; Zanis et al., 2014).
391
392
7. CONCLUSIONS
393
During August 2013, 20 ozone sondes were launched from Doha, Qatar. A subset of 13 mid-day
394
launches were further analysed. Although limited to one month in one summer season some
395
interesting observations were obtained. Upper tropospheric ozone in the Arabian Gulf region in
396
August 2013 was on average 80±13 ppbv, as predicted by models used in previous studies (Li et
397
al., 2001; Lelieveld et al 2009); however the source of high ozone is more in agreement with
398
Lelieveld et al. (2009) and Zanis et al. (2014) than postulated by Li et al. (2001). The extremely
399
low relative humidity of a few percent observed in air masses originating from the Eastern
400
Mediterranean in our study is in support of the modeling studies by Lelieveld et al. (2009) and
18
401
Zanis et al. (2014) who suggested high ozone levels in the middle and upper troposphere as a
402
result of stratosphere-troposphere transport over the Eastern Mediterranean and subsequent
403
transport from this area to the Arabian Gulf under continued subsidence processes. Elevated
404
ozone levels in the upper troposphere occur because of the meteorological conditions allowing
405
precursors to be transported into the area, and as a result of subsidence.
406
When the wind flow in the upper troposphere is from the East, the ozone levels are about 17%
407
lower in the 6-12 km range than when the wind is from the Mediterranean. High pressure may
408
lead to subsidence of ozone from the upper troposphere/lower stratosphere and eventually cause
409
an increase of ozone mixing ratios by ~18% between 6-7 km, i.e. slightly above the subtropical
410
subsidence layer. Under the impact of regional convective activity and associated lightning,
411
ozone mixing ratios can increase by more than 35% averaged over the 9-12 km altitude range. In
412
either case maximum ozone in the mid to upper troposphere reached more than 100 ppbv.
413
This study was limited to 13 launches in the month of August. There are no seasonal
414
comparisons. More work needs to be done in order to learn more about the temporal variability in
415
regional ozone patterns and long-term trends.
416
417
Acknowledgements – We are grateful for the support provided by Qatar Foundation and the
418
Qatar Civil Aviation Authority. Logistical and technical support provided by Messrs. M.A.
419
Ayoub, L. Ackermann, and Mrs. M. Shalaev from QEERI is greatly appreciated. The research
420
results have not been subject to QEERI scientific and policy review and therefore do not
421
necessarily reflect the views of QEERI and no official endorsement should be inferred. Many
422
thanks also to Dr. B. Holzworth for providing the WWLLN data for the time period of the study.
423
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FIGURE CAPTIONS
Figure 1. Average ozonesonde profiles in the time period 4 - 21 August 2013. Data are all 20
ozonesonde launches segregated into 100 m altitude bins.
Figure 2. HYSPLIT back trajectories in Google Earth (Google Inc., 2013). Altitude color coding:
red 6 km, blue 9 km, green 12 km altitude of the ozonesonde upon arrival time of the air
parcel.
Figure 3. NOAA 12:00UTC weather charts sequence for 11-14 August depicting a ridge over
Arabian Peninsula on 12 and 13 August 2013 on the 500 hPa charts as indicated by the
yellow arrow (left panel, top to bottom) and jet streak movement at 40-45ºN around 15
August 2013 on the 200 hPa charts from 13-16 August (right panel, top to bottom).
Figure 4. Lightning on 6 August 2013 (WWLLN Management Team, 2013).
Figure 5. Profiles of ozone, relative humidity, air temperature, potential temperature, wind speed,
and wind direction as obtained from the ozonesonde on 19 August 2013 at 10:10 UTC (12:10
LT).
Figure 6 Above: Comparison of vertical ozone profiles on 12 and 13 August 2013. Note the
plume height at 9-10 km on 12 August and 8 km on 13 August. Below: Vertical profiles of
ozone and relative humidity in the altitude range 6- 12 km.
Figure 7 Above: Altitude plot of the 72-hour backward trajectory ending at the location of the
ozone sonde at about 9 km altitude. Below: relative humidity along this trajectory.
Figure 8. Comparison of high lightning and low ozone (5 August 2013, 10:07 UTC (12:07 LT);
upper left plot and blue line in graph) with low lightning and high ozone days (19 August
2013, 10:10 UTC (12:10 LT); upper right plot and red line in graph). Lightning data for 5
Aug (blue), 4 Aug (pink), 3 Aug (green), and 2 Aug (yellow); Lightning data for 19 Aug
(blue), 18 Aug (pink), 17 Aug (green), and 16 Aug (yellow). Upper RH trajectory is for 5
August, lower RH trajectory for 19 August. In addition, vertical profiles of relative humidity
on the corresponding days as well as along 72-hour backward trajectories ending at the
ozonesonde location at about 9 km altitude are shown.
Figure 9. Above: lightning and HYSPLIT back trajectory on 6 August 2013, 10:00 UTC (12:00
LT), highlighting the 9 km trajectory passing through the lightning storm. Below: vertical
ozone profile on 6 August 2013.
23
576
577
578
TABLE 1. Median, mean, standard deviation, and range of measured ozone for all 13 launches at
various altitudes. Launch times close to local noon.
579
Ozone [ppbv]
Layer
580
Median
Mean
Standard
deviation
Maximum
Minimum
Number#)
Upper troposphere
(6 ≤ → 12 km)
79.4
80.0
12.8
109
46
2878±183
Entire troposphere
(0 ≤ → 14 km)
72.4
71.6
14.4
109
27
5767±355
Surface layer
(0 ≤ → 0.5 km)
66.4
64.3
14.8
95
29
165±25
#)
Number of data points per launch
581
582
583
584
585
586
TABLE 2. Mean and standard deviation of measured ozone at various altitudes separated into
different source regions according to the 72 hour back trajectories as shown in Figures 2 and 7. The
transition days August 14-15 (see Figure 2) are not included. Launch times close to local noon.
Ozone in ppbv.
587
LaunchesAugust 5-13#)
("East")
Layer
588
589
590
591
592
Mean
Standard
deviation
Launches August 16-21##)
("West")
Mean
Standard
deviation
Upper troposphere
(6 ≤ → 12 km)
74.0
6.9
86.2
8.9
Entire troposphere
(0 ≤ → 14 km)
66.1
5.0
77.6
3.0
Surface layer
(0 ≤ → 0.5 km)
61.3
2.1
66.7
4.5
#)
based on 6 ozonesondes
based on 4 ozonesondes
##)
24
593
594
595
596
597
598
599
600
601
Figure 1. Average ozonesonde profiles in the time period 4 - 21 August 2013. Data are all 20
ozonesonde launches segregated into 100 m altitude bins.
25
602
603
604
605
Figure 2. HYSPLIT back trajectories in Google Earth (Google Inc., 2013). Altitude color coding:
red 6 km, blue 9 km, green 12 km altitude of the ozonesonde upon arrival time of the air parcel.
26
606
607
608
609
610
Figure 3. NOAA 12:00UTC weather chart sequence for 11-14 August depicting a ridge over the Arabian Peninsula on
12 and 13 August 2013 on the 500 hPa charts as indicated by the yellow arrow (left panel, top to bottom) and jet streak
movement at 40-45ºN latitude around 15 August 2013 on the 200 hPa charts from 13-16 August (right panel, top to
bottom).
27
611
612
613
614
615
Figure 4. Lightning on 6 August 2013 (WWLLN Management Team, 2013).
28
616
617
618
619
620
621
622
623
Figure 5. Profiles of ozone, relative humidity, air temperature, potential temperature, wind speed, and
wind direction as obtained from the ozonesonde on 19 August 2013 at 10:10 UTC (12:10 LT).
29
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
Figure 6 Above: Comparison of vertical ozone profiles on 12 and 13 August 2013. Note the plume height at
9-10 km on 12 August and 8 km on 13 August. Below: Vertical profiles of ozone and relative humidity in
the altitude range 6- 12 km.
30
676
677
678
679
680
681
682
683
Figure 7 Above: Altitude plot of the 72-hour backward trajectory ending at the location of the ozone sonde
at about 9 km altitude. Below: relative humidity along this trajectory.
31
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685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
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710
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712
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714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
Figure 8. Comparison of high lightning and low ozone (5 August 2013, 10:07 UTC (12:07 LT); upper left
plot and blue line in graph) with low lightning and high ozone days (19 August 2013, 10:10 UTC (12:10
LT); upper right plot and red line in graph). Lightning data for 5 Aug (blue), 4 Aug (pink), 3 Aug (green),
and 2 Aug (yellow); Lightning data for 19 Aug (blue), 18 Aug (pink), 17 Aug (green), and 16 Aug (yellow).
Altitude color coding for back trajectories: red 6 km, blue 9 km, green 12 km altitude. In
addition, vertical profiles of relative humidity on the corresponding days as well as along 72hour backward trajectories ending at the ozonesonde location at about 9 km altitude are shown.
32
736
737
738
739
740
741
742
743
744
Figure 9. Above: lightning and HYSPLIT back trajectory on 6 August 2013, 10:00 UTC (12:00 LT),
highlighting the 9 km trajectory passing through the lightning storm. Below: vertical ozone profile on 6
August 2013. The start point of the back trajectory indicates the position of the ozonesonde at that given
time. Altitude color coding for back trajectories: red 6 km, blue 9 km, green 12 km altitude.