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The variability of boundary layer ozone over Beltsville, Maryland in the summer of 2010
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Jose D. Fuentes, Xiao-Ming Hu, David Doughty
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Department of Meteorology, The Pennsylvania State University, University Park, Pennsylvania
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Everette Joseph
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Department of Physics and Astronomy, Howard University, Washington, DC
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Abstract
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To investigate the variability of the boundary layer ozone in summer in the Mid-Atlantic
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region, chemical species and meteorological variables were measured at a research site in
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Beltsville, MD (39.06N, 76.88W) in August 2010. Measurements were obtained from a tower
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and ozonesondes. The average maximum O3 mixing ratio (~60 ppbv) occurred at 15 EDT while
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the average minimum O3 mixing ratio (~16 ppbv) occurred at 6-7 EDT. During August 2010,
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the O3 mixing ratio at 6-7 EDT remained below 10 ppbv on 37% of days. Only on 18% of days,
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the O3 mixing ratio reached above 25 ppbv at this time. Higher nighttime ozone was observed
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under the conditions of passage of fronts and “leaky inversion”. During the passage of the
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fronts, daytime surface O3 was suppressed on a time scale of 1-day to 2-3 days due to suppressed
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solar radiation and/or suppressed NOx concentration. Under the nighttime “leaky inversion”
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condition, surface O3 mixing ratios were increased by 10-30 ppbv during half an hour to 3 hours.
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Excluding these conditions, the mean O3 mixing ratios decreased from 22.4 to 11.8 ppbv from
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0AM to 6AM with a depletion rate of 1.8 ppbv/hr. The three dimensional chemistry model,
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WRF/Chem, was run to investigate the importance of regional transport to local high ozone
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episode. Two different regional transport mechanisms contributed to higher daytime O3 mixing
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ratios in the Mid-Atlantic region: the O3 is transported along the Urban corridor region or O3 is
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transported from the mid-west to the eastern US. A single column model is being run to
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investigate the contribution of vertical transport, dry deposition and chemical production to the
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variability of boundary layer O3 in Beltsville, Maryland.
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understanding of relative importance of various processes to the formation of summer time high
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O3 concentrations in the Mid-Atlantic region.
This study will improve our
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Introduction
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Objectives
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To investigate the impact of atmospheric stability on the boundary layer ozone mixing ratios, i.e.,
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to investigate the contribution of vertical transport, advection, and chemical production to the
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boundary layer O3 over Beltsville, Maryland.
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Methods
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Two episodes with higher O3 mixing ratios could be identified in 2010: August 9-11 and August
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16-19, 2010 (Figures 1 and 3). Enhanced O3 production in the boundary layer in the afternoon
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on August 9-10 is apparent (Figure 2). WRF/Chem simulations will be conducted for the period
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of August 9-11 (see domain configuration in Figure 3). A few sensitivity simulations will be
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conducted also (e.g., turning off gas phase mechanism or/and dry deposition).
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Thus the
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contribution of vertical transport, chemical production, and dry deposition to the boundary layer
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O3 mixing ratio could be identified from those sensitivity simulations.
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WRF/Chem simulation will be also conducted for another case with suppressed boundary layer
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ozone but relatively high NOx (e.g., August 24-27). The reason for the suppressed boundary
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layer ozone will be investigated, less chemical production or less downward transport from the
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free troposphere?
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Results
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The mean diurnal variation of O3 mixing ratio in Beltsville in August 2010 is shown in
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Figure 1. The average maximum O3 mixing ratio (~60 ppbv) occurred at 15 EDT while the
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average minimum O3 mixing ratio (~16 ppbv) occurred at 6-7 EST. O3 near surface was
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depleted during nighttime due to dry deposition and NO titration. Thus it reached minimum at
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dawn before the surface O3 replenish processes occur: photochemical O3 generation and
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downward transport of O3 due to the increased turbulence during daytime. During August 2010,
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the O3 mixing ratio at 6-7 EDT remained below 10 ppbv on 37% days (Figure 2). Only on 18%
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days, the O3 mixing ratio reached above 25 ppbv at this time (Figure 2). Figure 3 shows O3
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diurnal variation on two days, August 18 and 22, on which O3 mixing ratio at 6-7 EDT remained
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above 40 ppbv. Turbulence data was measured between August 18 and August 26. σw normally
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reminded low (below 0.2 m s-1) during nighttime and reached high (above 0.6 m s-1) during
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daytime on clear days. However on the night of August 17 and 21, σw showed relatively high
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values (Figure 3). Such high value of σw means stronger turbulence thus stronger vertical mixing
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on those nights. Stronger vertical mixing brought down O3-richer residual layer air to surface,
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thus kept the surface O3 mixing ratio higher than the other days. However such higher O3 mixing
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ratio at dawn didn’t contribute to a higher daytime O3 mixing ratio. On the contrary the O3
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mixing ratio during daytime on August 18 and 22 remained relatively low. This is due to the
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precipitation on August 18 and 22 and low NOx emission on August 22. The photochemistry
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was suppressed on both days. The precipitation on August 18 and 22 was associated with the
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passage of fronts. The sharp increase of O3 of 37 ppbv and decrease of temperature around 3
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AM on August 18 is due to the storm associated with the passed cold front. Recent field
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investigations (Betts et al., 2002; Sahu and Lal, 2006; Grant et al., 2008a) reported that
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convection process could downward transport O3 from the free troposphere to the ABL and
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increase ground-level O3 levels by 20-30 ppbv. When the time O3 mixing ratio increased sharply
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at 3 AM on August 18, temperature decreased sharply while relative humidity and wind speed
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increased sharply.
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decreased temperature.
Evaporation associated with rainfall increased relative humidity and
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On August 11, surface O3 mixing ratio increased by around 30 ppbv from 1-4 AM. At the
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same time surface NOx mixing ratio decreased by ~25 ppbv. Afterward, surface O3 mixing ratio
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decreased to 24 ppbv before the daytime O3 replenish processes occurred. During the night of
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August 10 and most time of August 11, wind direction is from north. A front passed the research
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site from north to south on August 12. If the increase O3 is from the advection of upstream
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plume, we would have observed higher mixing ratio of other pollutants, such as CO and NOx.
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However both CO and NOx decreased as O3 increased. Thus the increased O3 is less likely from
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the advection of plume. The short-time (a few hours) O3 increase during nighttime is very
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similar to the “leaky inversion” as observed in Thompson Farm in New Hampshire (Talbot et al.,
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2005), which is due to the vertical exchange of air between the surface layer under the nocturnal
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inversion and the residual boundary layer above it. Since the residual layer has higher O3 and
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lower mixing ratios of other pollutants, so the vertical exchange decrease surface NOx and CO
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and increased surface O3. During the nighttime, the vertical gradient of O3 near surface in
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Beltsville is 0.06-0.08 ppbv/m (Figure 2). Since surface O3 was increased by 30 ppbv, we could
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imply that downward mixed air mass is transported from at least 400 m above ground. Due to
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the stronger coupling between surface layer and the layers above, the temperature near surface
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did not decrease substantially.
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temperature of this month, 297K, is recorded. It is just lower than 297.6K recorded on night of
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August 21, which is due to the warm front on that night. At the level of 31m above ground on
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the night of August 10, highest minimum temperature of this month is recorded, i.e., 300K.
On the night of August 10, the second highest minimum
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On the night of August 13, the “leaky inversion” occurred in a quicker way. In around 10
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minutes around 3AM on August 14, surface O3 mixing ratio increased from 9 to 19 ppbv. At the
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same time, surface temperature increased by 1 oC and specific humidity drop by 1 g kg-1 in half
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an hour. The downward transport occurred in such a quick way that the air mass from above was
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not mixed substantially with ambient air when it reached surface, thus still had higher O3 mixing
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ratio, higher potential temperature and lower specific humidity.
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Exclude these cases, the mean O3 mixing ratios decreased from 22.4 to 11.8 ppbv from
0-6AM with depletion rate of 1.8 ppbv/hr.
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Figure 4 shows the mean diurnal variation of nitrogen oxides (NOx). NOx peaked at 8
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AM. Emission from morning traffic and lower boundary layer height at this time contributed to
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the NOx peak. In summer, rapid photochemical reactions rapidly convert NO to other nitrogen
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species and lead to short lifetime of NO (Munger et al., 1998). Short time window when NO
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was high also suggests a NOx limited environment in summer that restricts local O3 production
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(Talbot et al., 2005). The correlation between O3 and T2 and NOx will be calculated tomorrow!
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An O3 episode occurred on August 9-10. Maximum O3 mixing exceeded 80 ppbv on both
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days (Figure 1). Apparent O3 production in the PBL occurred in the afternoon on both days
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(Figure 2). In another word, gas phase chemical reactions generated O 3 in the PBL. Ozone
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mixing ratio showed positive vertical gradient in the lowest 1 km in the afternoon, which implies
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that dry deposition removed O3 at the surface. On the night of August 8 and 9, surface O3 was
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depleted to the level below 10 ppbv (Figure 1). The measured profile of O3 during the nighttime
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shows strong vertical gradient below 1 km (Figure 2), which indicates that dry deposition or/and
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NO titration removed O3 at/near the surface.
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WRF/Chem simulations have been conducted to investigate the air chemistry over
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eastern U.S. Three one-way nested domains are set up as shown in Figure 3. Ozone at ~540 m
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above ground in the morning and at surface in the afternoon on three days is shown in Figure 4.
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Figure 4 illustrates that the surface O3 mixing ratio in the afternoon is highly correlated to the O3
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mixing ratio in the upper layer (here is 540 m above ground) in the morning. In the morning of
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August 9, the plume of O3 in the upper layers was along the eastern urban corridor. Thus in the
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afternoon, the surface O3 mixing ratio in the urban corridor area reached high. In the morning of
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August 10, the eastern US was impacted by the plume from mid-west in the upper layer. The
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surface O3 mixing ratio from DC to New York City also reached high in the afternoon of August
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10. On the contrary, the upper layers in the morning on August 8 were relatively clean in the
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eastern US. The surface O3 mixing ratio is relatively lower in the eastern US on this day. So the
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O3 in the residue layer contributed to the surface O3 during daytime when the PBL grow.
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On August 9 and August 10, we noticed two different regional transport mechanisms. On
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August 9, the O3 is transported along the corridor while on August 10, O3 is transported from the
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mid-west to the eastern US.
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Philadelphia to New York City, there are heavy emissions (Figure 5). So if transport occurred
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along corridor, the O3 may accumulate to a high level in the afternoon. Transport from mid-west
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(northern Ohio River Valley) is also shown to contribute to the O3 episodes in the eastern US
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(Stehr et al., 2000; Taubman et al., 2006; Hains et al., 2008).
Along the corridor from Richmond to DC to Baltimore to
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So the wind direction in the residue layers in the previous night is very important to the
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O3 mixing ratio on the second day. The simulated wind fields in the residue layer at 07:00 UTC
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on August 9 and 10 are shown in Figure 6. On both days the southwesterly low level jet is
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simulated by the WRF/Chem model (Figure 6), which is in agreement with the observations
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(Figure 7, 8, 9).
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Wind profiler data confirms these measurements for both August 9 and 10. During the
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night, lower troposphere (< 2km) wind velocities reach a maximum of 20-30 knots near 300
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meters, before decreasing to 10 or less knots at 2km, for most cases. The strength of this jet is
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stronger on August 10th than on august 9th, and the august 10th jet also covers a wider breadth of
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heights. Surface winds on August 9th start out from the south-southwest, and August 10 surface
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winds are from the southwest. On both days, winds veer to become westerly at 2km.
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A.T. Gaye (2008a), Ozone transport by mesoscale convective storms in western Senegal,
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Atmos. Environ., 42(30), 7104 -7114.
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Hains, J. C., B. F. Taubman, A. M. Thompson, J. W. Stehr, L. T. Marufu, B. G. Doddridge and
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R. R. Dickerson (2008), Origins of chemical pollution derived from mid-Atlantic aircraft
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profiles using a clustering technique, Atmos. Environ. 42, 1727–1741.
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Munger, J., S.‐M. Fan, P. Bakwin, M. Goulden, A. Goldstein, A. Colman, and S. Wofsy (1998),
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oxidation and deposition with season and distance from source regions, J. Geophys. Res.,
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Figure 1. Observed time series of O3
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Figure 2
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Case of August 18
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Case of August 22
Figure 3.
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Case of August 11
Case of August 14
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Aug 7?
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Figure 4.
Extra cases with higher nighttime O3
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Figure 4.
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Figure 2. Observed vertical profile of O3 on August 9-10.
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Figure 3 Domain configuration of WRF/Chem
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Figure 4. Simulated O3 at (top) layer 7 at 14:00 UTC and (bottom) layer 1 at 17:00 UTC on (from Left to Right)
August 9, and 10.
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Figure 5. NO emission from 2005 national emission inventory (NEI).
Figure 6. Wind field in the residue layer (~370m above ground) at 07:00 UTC on August 9 and
10.
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Figure 7. Observed profiles of wind direction and wind speed.
Figure 8: MDE wind profiler data from Beltsville, MD, during the night of August 9, 2010
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Figure 9: MDE wind profiler data from Beltsville, MD, during the night of August 10, 2010
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Control
NoChem
Figure 8. Simulated vertical profiles of O3 from control run and NoChem sensitivity run. In the
NoChem sensitivity run, boundary layer O3 is reduced due to the dry deposition process.
Boundary layer O3 at 14:00 EDT on the four days are 55, 49, 46, 37 ppbv. On average dry
deposition removes 6 ppbv O3 per day from afternoon boundary layer.
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Figure 9. Simulated vertical profiles of O3 from control run and NoDep sensitivity run. The
NoDep run predicted higher O3 mixing ratio in the boundary layer at 20 UTC on 4 days by 4.1,
7.5, 9.4, and 12.8 ppbv. The different O3 mixing ratio is due to deposition of O3 and subsequent
different chemical reactions. So in one-day interval, dry deposition and subsequent effect caused
difference of 3.4, 1.9, and 3.4 ppbv O3
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