Analysis of Air Transport and Oxidation Chemistry in the North-Atlantic Region from
Interpretations of Non-Methane Hydrocarbon (NMHC) Measurements at Pico Mountain,
Azores
D. Helmig1, D. Tanner1, R.C. Owen2 and R. E. Honrath2 and D. Parrish3
1
Institute of Arctic and Alpine Research (INSTAAR), University of Colorado, Boulder, CO 80309, USA
Department of Civil and Environmental Engineering, Michigan Technological University, Houghton, Michigan, USA
3
Chemical Sciences Division, National Oceanic and Atmospheric Administration, Boulder, CO 80303, USA
2
Manuscript in preparation for Journal of Geographical Research
May 29, 2006
Abstract
One year of continuous measurements of non-methane hydrocarbons at the mountaintop PICO-NARE observatory on Pico Island,
Azores were used to investigate seasonal oxidation chemistry and transport patterns in the central North Atlantic Region. NMHC at
this site exhibited seasonal and short-term variations and cycles that reflect the distance of the island from continental sources of
NHMC emissions and oxidation of NMHC by the seasonally highly variable OH radical. Substantially enhanced NMHC levels
during the summer of 2004 were attributed to the impact of long-range transport of biomass burning plumes resulting from Northern
Canada and Alaskan wildfires. During summer, air samples were the most photochemically processed. NMHC concentrations and
their relative ratios were valuable in identifying transport situations where anthropogenically influenced air from the mid and western
U.S. was transported to Pico in 5-8 days. Interpretations of NMHC ratios for use as a relative scale for photochemical processing
(‘photochemical clock’) was shown to yield results in agreement with trajectory analysis interpretations. Ozone in excess of 35 ppbv
was observed at PICO-NARE throughout all seasons. Enhanced ozone levels were observed in air that had relatively ‘fresh’
photochemical signatures (e.g. ln [propane]/[ethane] > -2.5). Ozone at lower levels (< 40 ppbv) was always correlated with more
processed air patterns (‘older’ air with ln [propane]/[ethane] < -2.5.
1. Introduction
1
Non-methane hydrocarbons (NMHC) in the atmosphere show considerable variations on spatial and temporal scales, their
concentrations being determined by the strength of emission sources and atmospheric removal processes. Atmospheric oxidation is
mostly due to reaction with the OH radical, with reaction rate constants increasing significantly with the molecule size.
Consequently, lighter hydrocarbons (having the slowest reaction rates) exhibit much longer lifetimes and their atmospheric
concentrations decline at slow enough rates for NMHC concentrations to remain high enough after several days of transport to impact
air chemistry at remote downwind locations. Since many individual NMHC have common emission sources and their emission ratios
vary comparatively little, changes in absolute concentrations and NMHC ratios can be used as tools to decipher atmospheric transport
and oxidation chemistry. Several researchers have investigated this utility and have presented a framework for the interpretation of
light (C2-C6) hydrocarbon observations. Selected NMHC can be used as tracer for specific emission sources or events. For instance,
isoprene is a selective tracer for biogenic emissions (ref), acetylene has been found to be significantly enhanced in biomass burning
plumes (DeGouw ref, RICHARD?) and light, saturated and unsaturated NMHC (e.g. ethane, propane) have been used to identify
influences from urban energy use and petrochemical industries (Blake and Rowland, 1995; Jobson et al., 2004). Diurnal
concentration changes of light, unsaturated NMHC (ethene, propene) allowed to identify occurrences of upslope and downslope flow
conditions at Mauna Loa Observatory (Greenberg et al., 1996).
Ratios of certain NMHC (e.g. butanes) are emitted at rather similar emission ratios. Due to their very similar atmospheric removal
rates, atmospheric ratios show little variations. Their analytical data can therefore be used as a quality control tool in NMHC
measurements (Parrish et al., 1998). The variability of NMHC concentrations can provide information on the impact or distance of a
measurement site from pollution sources. A ‘remoteness’ scale has been proposed, that is derived from a plot of the ln of the standard
deviation of ambient NMHC concentrations at a given site versus their estimated lifetime (Jobson et al., 1999). Changes of the ratio
of branched versus straight n-alkanes have been used to infer the importance of halogen and nitrate radical versus OH radical
chemistry as the reaction rates of these two different oxidation routes are significantly different enough to cause changes in the
atmospheric concentration ratios of these isomeric compounds (Penkett et al., 1993; get Finlayson-Pitts et al. reference for halogen
chemistry, RICHARD?).
NMHC ratios and concurrent measurements of ozone were also applied in studies of changes in the oxidation chemistry of the
atmosphere. In particular, the relative increase of ozone at observed NMHC ratios was used as an argument for an increased ozone
production (respectively reduced ozone loss rates) in long-range transport across the Pacific Ocean (Parrish et al., 2004). NMHC
measurements from remote, marine environments have also been applied for estimating mean OH radical fields during transport of air
in the marine boundary layer and lower free troposphere (Ehhalt et al., 1998; Williams et al., 2000).
The possibilities for using NMHC data for interpretations of atmospheric oxidation processes are particularly promising in situations
where observations can be obtained in air that has traveled for extended periods of time without being influenced by recent emissions
2
or surface processes. Hence, remote islands that are high enough to probe free tropospheric air are ideal locations. These
aforementioned considerations motivated the monitoring of NMHC at the mountaintop PICO-NARE site on Pico Island, Azores.
These measurements commenced in the summer of 2004 and have been continuous since during most times when the station was on
power. In this paper we present data from the first year of these new observations and examples of interpretations that demonstrate
the utility of the NMHC data for interpretations of oxidation and transport processes in the North Atlantic region.
2. Methods
2.1. PICO-NARE Station
The PICO-NARE observatory is located in the summit caldera of the inactive Pico Mountain volcano (38.xoN, 28.xoW), the highest
mountain on Pico Island, and in the Azores, Portugal. At 2225 m asl, lower, free tropospheric air is sampled at the station during most
times. More information on the geo- and topographical features are provided by Kleissl et al. (2006). A detailed analysis of boundary
layer height, and of mechanical uplifted and buoyant flow conditions showed that during less than 25% of the time conditions were
such that air from lower elevations was potentially lifted up to the station height. Furthermore, chemical measurements of nitrogen
oxides and carbon monoxide at the observatory showed very little (resp. negligible) influence from island emission sources even
during upslope conditions, which infers that even during upslifting events, mostly lower tropospheric air is transported to the station
(Kleissl et al., 2006). Data and interpretations from other research at PICO-NARE have been presented previously (Honrath et al.,
xxxx; Fialho et al., xxxx) and in other contributions to this special issue (Val Martin et al., 2006).
2.2 NMHC Measurements
The remoteness of the PICO-NARE site and the limitations for power and for supply of cryogen and consumable gases determined the
design of an analytical system that was tailored towards this unique situation. All consumable gases and blank air were prepared at
the site in situ with low-power gas generators. The instrument is designed to follow automated startup and shuddown procedures and
can be remotely controlled from our Boulder, CO offices. Ozone was removed by flowing the sample air through an ozone scrubber
prepared from sodium-thiosulfate-impregnated glass wool. After sample drying and NMHC focusing on a mulit-stage solid adsorbent
trap, NMHC were subsequently analyzed by thermal desorption with gas chromatography (GC) separation and flame ionization
detection (FID). The instrument was calibrated by regular injections of a multi-component, rural air standard that was quantified prior
to shipment against a gravimetric hydrocarbon standard scale in the NOAA Aeronomy Laboratory. A second, clean, ambient air
standard was injected every 3-4 days for quality control.
3
Sample volumes of 600 ml (10 min collection time) and 3000 (50 min collection time) ml were alternated for quantification of ethane
and NMHC > C2, respectively. Typically, a total of 12 ambient air samples, one standard and one blank sample were analyzed daily.
Data were transferred daily to our Boulder, CO offices for instant quality control and analysis. The primary calibration standard was
returned to Boulder in spring 2006 and the control analysis on the independently calibrated NOAA GC system showed that C2-C6
NMHC mixing ratios were within +/- xx % (DAVID, WE PROBABLY WILL HAVE TO PUT OUR BEST GUESS NUMBERS
HERE AS WE STILL DON’T HAVE THIS ANALYSIS BACK FROM PAUL? ANY UPDATE ON THAT? WE DEFININTELY
NEED TO HAVE THIS DONE BY THE TIME THE REVIEWS COME BACK) of the values determined two years earlier, prior to
the shipment to Pico. NMHC were quantified using compound-specific FID response factors, as determined from the primary
standard injections. Quantified NMHC included ethane, propane, n-butane, i-butane, i-pentane, n-pentane, n-hexane and isoprene (the
ethane record doesn’t begin until in fall 2004 when some modifications in the focusing procedure allowed its quantitative analysis).
From the above described experiments analytical precision and accuracy were estimated to be better than 5% and 10% for mixing
ratios > 100 ppt and approximately a factor of 2 higher for levels between the detection limit (which typically were ~ 30, 11, and 1-2
pptv for C2, C3, and C4-C6, respectively) and 100 pptv. More instrumental details have been provided elsewhere (Tanner et al.,
2006).
RICHARD, CAN YOU DESCRIBE TRAJECTORY ANALYSIS?
Two simulations with the particle dispersion model FLEXPART (Stohl et al., 2005) were run for the case scenario studies presented
below. The first simulation included all anthropogenic emissions for the entire US (EDGAR emissions in the box of 25oN to 48oN and
-126oW to -65oW), the second with western US emissions only (same box with -100oW as the eastern boundary).
3. Results and Discussion
3.1 NMHC Mixing Ratios
Plots with the individual sample data (representing a total of xxxx air samples DAVID??) for ethane, propane, and n-butane from Aug.
2004 – Sept. 2005 were presented by Tanner et al. (2006). Here we combined these data to monthly whisker plots that show the
minimum, 5, 25, 50, 75, and 95 percentile, and maximum of measured mixing ratios during each month of available measurements
(Fig. 1).
These data show the typical Northern Hemisphere seasonal cycle of NMHC with lower mixing ratios in the summer and maximum
values in late winter. This behavior is driven by the annual concentration changes of the OH radical, which is closely linked to
latitudinal diurnal solar radiation cycle. High variability in NMHC mixing ratios was observed at any given time of year. It is
4
noteworthy that all of these features show relations and dependencies toward the NMHC reactivity with OH and the resulting NMHC
lifetime. The longest-levied NMHC, ethane, shows the relatively smallest seasonal amplitude of the calculated median concentrations
(e.g. difference between mean winter and summer mixing ratios), and the smallest relative variability on short (e.g. weeks) time
scales. All of these features increase with increasing molecule size (respectively shorter OH lifetime). It is also noteworthy that the
seasonal maximum and minimum of ethane occurs the latest of all compounds (calendar days xx and xx, respectively, DAVID,
PLEASE ADD), as due to its slower OH reaction, ambient levels respond with a longer delay to the seasonal OH cycle. These
features in the Pico NMHC data are in agreement with data from a number of other sites, which, along with their seasonal OH
dependencies, have been presented and discussed in detail in the literature (e.g. Goldstein et al., 1995; Gautrois et al., 2003).
Comparison of the NMHC >C3 for summer 2004 with data from the corresponding period during 2005 shows a higher variability as
well overall higher mixing ratios during 2004. As discussed in detail in other contributions to the ICARTT issue (Ref, RICHARD),
the summer of 2004 was characterized by an unusually high occurrence of boreal wild fires in Northern Canada and Alaska, outflow
of which was frequently observed at Pico (Val Martin et al., 2006). Substantial enhancements in NMHC, at times reaching increases
of x times (DAVID, WHAT’S YOUR BEST GUESS) over their seasonal background levels, were observed in these boreal fire
plumes.
A number of data have been presented in the literature that allow comparisons and further evaluation of the Pico measurements. The
probably most related study are the seasonal measurements made at Mauna Loa Observatory during MLOPEX-2 (Greenberg et al.,
1996). Similar to Pico, the Mauna Loa site is a remote island location where, during downslope conditions, free tropospheric air is
sampled that has traveled over the ocean for several days. MLOPEX-2 data included in Figure 1 are consistently lower for all NMHC
and during all seasons. The differences in Pico and MLO NMHC mixing ratios increases with decreasing lifetime, e.g. while ethane
mixing ratios compare to within ~20%, n-butane values at MLO are more than 5 times lower than at Pico.
MLO has a more prominent diurnal upslope-downslope cycle and data presented by Greenberg et al. was broken up into the
occurrences of these two flow regimes. Included data in Fig. 1 resulted from downslope (e.g. free tropospheric air) conditions.
Upslope data for MLO typically were higher, with relative enhancements increasing with decreasing molecule liftetime. Higher
NMHC levels at Pico (compared to MLO) are likely due to several reasons. 1. Aircraft profiles have shown that NMHC mixing ratios
generally decline with height within the free troposphere (e.g. Blake et al., 1997), with larger concentration changes being observed
for shorter-lived compounds. MLO is at about 1200 m higher altitude than the PICO-NARE station, consequently lower NMHC
mixing ratios would be expected. Secondly, as shown by Rudolph (1995) lower tropospheric NMHC mixing ratios decrease towards
lower latitude (MLO is xo RICHARD? further south than Pico). Of further importance is the distance to the adjacent continents,
which is about two times as much for MLO, and which will cause transport and photochemical processing times from continental
sources to be longer, resulting in more depleted NMHC ratios at MLO.
5
Besides MLO, comparison of the Pico data with six other data sets from higher northern latitudes in Canada, the Atlantic Region and
Europe (as summarized by Gautrois et al. (2003)) shows that Pico NMHC levels are unanimously lower, both during the winter and in
the summer season compared to these locations.
The cumulative distribution of NMHC during fall 2004 (date-date, DAVID, PLEASE ADD), winter 2004-2005 (date-date), spring
2005 (date-date) and summer 2005 (date-date) is shown in Figure 2. Data series that do not extend in the lower percentage range
resulted from respective fractions of these data being reported below the instrument detection limit. The regression line slopes
through these individual data series give an indication of the variability of the atmospheric concentration of a given compound.
Steeper slopes are observed for long-lived compounds (e.g. ethane) as these compounds have a higher atmospheric background
concentration which reduces the relative variability caused by emission influences. It is noteworthy that regression line slopes through
these data are lower for the summer, which likely can be attributed to the shorter seasonal atmospheric lifetime, respectively lower
background concentration.
Results for isoprene measured at the station were presented by Kleissl et al. (2006). Isoprene, was typically not detected (< 2 pptv) in
winter and nighttime samples. During spring, isoprene was occasionally observed in samples collected during morning to evening
hours. Occurrences and mixing ratios of isoprene increased towards late summer. During August 2005, isoprene was detected on xx
% of all days in the afternoon with maximum mixing ratios reaching up to 27 pptv. The isoprene data clearly show seasonal and
diurnal dependencies that are determined by both the expected seasonal changes in isoprene emission rates from vegetation growing at
lower elevation on Pico island and by occurrences of buoyant and mechanical uplift flow that transports air from lower parts of Pico to
the observatory (Kleissl et al., 2006).
3.2 Ratios of NMHC
Iso-butane and n-butane reaction constants with OH are very similar and the ratio of these two compounds in the atmosphere changes
very little during transport and photochemical oxidation. The ratio of these two compounds in the data from Pico, differentiated by
the four seasons and plotted against the n-butane mixing ratio is shown in Figure 3. The correlation between these two compounds is
very obvious. Deviations and larger scatter at lower (e.g < 50 pptv) mixing ratios can mostly be attributed to the loss of precision,
respectively to the i-butane mixing ratio dropping below the detection limit. No statistically significant difference was found in the
regression line slope between the four seasonal data series. The regression line slope for all data was calculated to be 0.4xx +/- 0.xx
(DAVID T., PLEASE ADD). Similar values (range 0.37 – 0.55) have been reported in data from a multitude of other sites in both
continental and marine environments (e.g. Bottenheim and Shepherd, 1995; Bottenheim et al., 1997; Greenberg et al., 1996; Parrish et
al., 1998).
6
Correlation plots of all C2-C6 NMHC are shown in Figure 4 and results for the linear regression analysis are given in Table 1. The
common feature in these data is that regression line slopes of (NMHCA/NMHCB with carbon number NMHCA < NMHCB increase
monotonically with increasing carbon number of NMHCB (which is expected as atmospheric mixing ratios of these NMHC generally
decrease with increasing carbon number). These same regression line slopes become larger toward the summer, as the longer-chain
NMHC drops faster/more in concentration as the more stable, shorter-chain NMHC. Regression coefficients generally decrease
towards the summer, as shorter liftetimes, lower concentrations and higher relative variability cause the correlation between individual
compounds to become weaker.
The distribution of NMHC data in a double natural logarithm plot of [n-buante]/[ethane] versus [propane]/[ethane] (Figure 5) can be
used to investigate the degree of photochemical processing that occurred in air reaching Pico. Data in these plots is distributed
between two theoretical lines that are determined by the assumptions that air with a common ratio of these compounds at a source
would have only been altered by OH photochemistry (kinetic line) or by dilution with air that has zero concentrations of both
compounds of consideration (dilution line). The seasonal differences in NMHC oxidation are clearly visible in these data. During
winter, most data have larger ratios and are less variable, indicative of less photochemical processing, and/or more homogenously
distributed source regions and air transport. In contrast, spring and summer data are more scattered (weaker R2 values); the lower
[NMHC]/[ethane] ratios are indicative of the higher degree of air processing that occurred during transport. Regression line slopes
through the four seasonal data series are statistically not different (at P>0.95) DAVID, IS THIS CORRECT?. The regression through
all data yields a slope of xxxx, which is within the range of slopes reported for this analysis in several other experiments (Parrish et al.,
2004).
3.3 NMHC Variability
The variability of NMHC during each of the four differentiated seasons is directly related to the slopes of regression lines through the
data in Figure 2. The standard deviation of the natural logarithm of determined mixing ratios is calculated from
DAVID, GIVE EQUATION OF HOW lnx WAS CALCULATED FROM DATA IN FIG. 2 AND DEFINE PARAMETERS GIVEN
IN EQUATION (ref.). The seasonal lifetime of the respective NMHC was determined from the seasonal OH radical concentration.
[OH] was estimated using the algorithm presented by ref et al. (xxxx). DAVID, CAN YOU EXPLAIN WHICH LATITUDE,
LONGITUDE YOU USED AND HOW YOU DERIVED THE VALUES THAT YOU USED FOR FALL, WINTER, SPRING AND
SUMMER? Reaction rate constants were adjusted to the temperatures measured at the PICO-NARE station during the respective
season. Please note that this local lifetime represents an estimate for the conditions at the receptor site, the actually encountered
lifetime during transport of a NMHC to Pico may have deviated from this estimate dependant on the geographical and atmospheric
7
conditions during the transport path. Also note that this analysis in not very sensitive towards the applied [OH], but more so towards
the relative differences between individual compounds. Consequently, errors in the estimated, total [OH] will only have little effect
on the results for the regression coefficients (Jobson et al., 1999). Four pairs of  lnx – lifetime estimates were obtained for each
compound. The results shown in Figure 6 show well correlated linear relationships for all four seasons; seasonal differences in the bvalues were not statistically significant at the p>95% level (DAVID, PLEASE CHECK), the best fit through all data yields lnx = xx xx
DAVID, PLEASE FILL IN VALUES.
The exponent b in this equation has been noted to describe the importance of sink terms in the regional variability budget whereas the
coefficient A can be related to the degree of photochemical aging; A-values have been used to derive estimates of transit times for
different sample sets (Jobson et al., 1990).
Interpretation of observed values for b from different sites has shown that b approaches 0 near urban areas, where the variability is
strongly influenced by differences in the strength of local emission sources, whereas b-values close to 1are found in stratospheric data
sets, where the variability is low and dominated by chemical loss alone. The mean Pico value of 0.xx (DAVID) compares well with
data from three aircraft data sets collected over other diverse remote areas, including the Arctic Boundary Layer (ABLE3A), the
equatorial Atlantic (TRACE-A) and the western Pacific (PEM-West B) experiment, which resulted in b-values of 0.46 – 0.53. This
comparison illustrates a rather high similarity between the continuous, seasonal Pico data and the results from the comparatively short
aircraft campaigns.
3.4 NMHC Processing and Ozone
Ozone has been noted to exhibit a large variability at PICO-NARE, in particular during the summer (RICHARD, ref?). Enhanced
ozone, with mixing ratios in excess of 35 ppbv has been observed at PICO-NARE throughout all seasons. In Figure 7, we analyze the
dependency of observed ozone levels on the degree of hydrocarbon photochemical aging, using the natural logarithm of
[propane]/[ethane] as the indicator of photochemical processing. The much higher variability in ozone data under spring and summer
conditions (as already noted by Honrath et al., (xxxx) is clearly evident in this analysis. These comparisons also show that enhanced
ozone levels were consistently observed in air that had relatively ‘fresh’ photochemical signatures (e.g. ln [propane]/[ethane] > -2.5).
Transport of air with lower ozone (< 35 ppbv) was always correlated with more processed air patterns (‘older’ air with ln
[propane]/[ethane] < -2.5); such situations were not encountered during the fall and winter. These comparisons suggest that a certain
photochemical aging threshold (where ln [propane]/[ethane] < -2.5) must be attained before ozone sink reactions gain dominance over
ozone production.
DAVID P., DO YOU WANT TO ADD MORE TO THIS DISCUSSION?
8
3.5 Transport Event Case Study
In Figure 8a six weeks of data for four NMHC during spring 2005 are shown (note the logarithmic concentration scale). NMHC
concentrations are highly variable, close to 10-fold increases were observed several times during this observation window. The high
correlation, with concurrent minima and maxima of these four individual NMHC is noteworthy. The amplitudes of relative mixing
ratio increases are highest for the shorter-lived compounds. Underneath these variable data, the springtime decline in the NMHC
mixing ratios can be discerned. The ln [propane]/[ethane] and ln [butane]/[ethane] analysis for the same data (Figure 8b) can be used
to investigate the short-term changes in the inferred photochemical age of air reaching PICO-NARE. Again, a high variability is
found, with high ln [NMHCi]/[ethane] ratios (indicating ‘fresh’, e.g. little processed air) coinciding with periods of enhanced absolute
NMHC mixing ratios and low ln [NMHCi]/[ethane] ratios (indicating ‘old’, e.g. well processed air) coinciding with periods of low
absolute NMHC mixing ratios.
Three periods where air switched from a ‘fresh’ signature to an ‘old’ signature and back to a ‘fresh’ character were subjected to a
closer investigation. These three, ~ 1-day intervals are indicated by the circles in Figure 8b. The corresponding data points are
marked by the same colors and compared with all data during this April-May period in the ln [butane]/[ethane] versus ln
[propane]/[ethane] plot in Figure 9A. Here, we derived the line constraining the mixing boundary by assuming that air, influenced
with recent emissions, was not mixed with zero-concentration background air (as done for Figure 5) but instead, more realistically and
as suggested by McKeen and Liu (1993) was mixed with background (B) air that had inferred seasonal ethane, propane and butane
mixing ratios of 900, 60, and 54 pptv, respectively. These modifications result in the shaped form of the mixing boundary that
constrains these data. 24-hour [OH] was estimated at 2 x 106 molecules cm-3 (according to xxx et al., for …. DAVID, GIVE
PARTICULAR CONDITIONS). Starting values for ethane, propane and butane, representing typical conditions for the levels and
ratios of these compounds in source regions were set to 10, 5, and 4 ppbv (ref., DAVID, ANY REFERENCE WE CAN CITE FOR
THAT?). The concentration change of these NMHC and their ratios can than be calculated according to
model:
Background:
0d
only
With K being the mixing OH
constant
(0.1 s-1, as suggested by ref (DAVID??) and k = 2.04 x 10-13, 9.77 x 10-13 and 2.29 x 10-12 molecules
ethane=900
cm-3s-1 for ethane, propane and n-butane, respectively. Integration of this equation and consideration of hydrocarbon pairs will yield
Mixing and OH
propane=60
4/17 16:00 to 4/19 1:00
butane=4
4/19 17:00 to 4/20 15:30
5d
K=0.01 h-1
9
3/26 to 5/14
OH=2E6
Mixing Only
DAVID T. and DAVID P: CAN YOU GIVE EQUATION THAT ACTUALLY WAS USED TO CALCUALTE THE
THEORETICAL ‘TIME’ LINE IN FIGURE 9A?
This equation, using the above parameters, allows putting a hypothetical average photochemical aging, respectively transport time
from the occurrence of fresh emissions to arrival at Pico. Under these assumptions, the data of the three episodes marked in Figure 8b
are defined with average ages of 9-13, 17-20, and x-x (ADD RESULTS FROM THIRD PERIOD) days respectively.
DAVID, CAN YOU CHECK THE [OH] YOU USED FOR THE MODEL. WOULD THERE BE A reasonable argument FOR
POSSIBLY USING A SOMEWHAT HIGHER OH. IT LOOKS LIKE HIGHER OH WOULD IMPROVE THE AGREEMENT
BETWEEEN THE NMHC AGE CALCULATION AND THE TRAJECTORIES?
Back trajectories for these three episodes are shown in Figure 9. First, during 4/17-19 air sampled at Pico had previously been
traveled ansported across the Northern Pacific and then been rapidly transported over California, the Midwestern U.S. and Northern
Canada. In contrast, air sampled during 4/19-20 had resided within the mid-Atlantic region for a minimum of 10 days. Air sampled
during 4/21-23 had passed over xxxxx RICHARD, CAN YOU ADD THIS? RICHARD, IS THERE ANYTHING YOU CAN ADD
ABOUT THE HEIGHTS OF THESE BACKTRAJECTORIES?
The comparison of the NMHC interpretations with these trajectory analysis imply that periods identified with ‘photochemically fresh’
air coincided with air transport over populated, U.S. continental regions, where, most likely, an injection of recent anthropogenic
emissions had occurred. In contrast, the period that was identified as ‘photochemically old’ was attributed to conditions where air had
resided over the Atlantic ocean for an extended (> 10 days) period of time.
These conclusions are further supported by FLEXPART results. Both of the above discussed events with increased [NMHCi]/[ethane]
were apparent in the FLEXPART simulation, with emission and outflow considered from the entire US. This simulation produced an
estimated total enhancement of ~10 ppb of CO for the first event at Pico and ~30-50 ppb for the second (Figure 8c). For the first event,
slightly less than 6 pbb of CO were attributed to the emissions from the western US, with about 1/3 of that being 5 days old, slightly
more than 1/3 being 6 days old, and the remainder being distributed between the remaining age classes from 7-10 days. For the
second event, total CO contributions and age distributions vary more widely. Here, the source region is 1/4 from the western U.S.
and 3/4 from the eastern U.S. The average ages steadily increase from ~7 days at the start to 13 days at the end of the period. The
average FLEXPART ages for these two events were 7-9 days and 6-10 days respectively. FLEXPART indicated only a slight CO
10
enhancement for Pico for the period between the two events (~00 UTC on 4/19 to ~18 UTC on 4/20), when low levels of CO (and
other species) were observed at the station, all of which was emitted from the US 10 or more days earlier.
The FLEXPART results present a strong case for the relative ages of the CO present during first event and the relatively low period
following. The small age distribution for the first event is consistent with the transport of a fairly coherent air mass, as indicated by the
uniformity of the trajectories. The age of the CO also coincides with times that the trajectories are over potential source regions over
the US. Likewise, in the low-CO period following the first event, the older ages and lower CO during the period following the first
event are consistent with expectations for background air. RICHARD/CHRIS; THIS LATER PARAGRAPHY MAY BE
SOMEWHAT REDUNDANT AFTER THE PREVIOUS PARAGRAPH. DO YOU WANT TO MODIFY?
Conclusively, both the back-trajectory analysis and FLEXPART calculations yield reasonable confirmation of the aforementioned
interpretations from observed concentrations of NMHC and their photochemical processing and mixing during transport.
4. Summary and Conclusions
Air sampled at PICO-NARE shows high variability in NMHC and their ratios during all times of the year. This observation is
indicative of the variable atmospheric transport conditions that bring in air with variable flow and with much different origin and
photochemical history. Overall, concentrations of NMHC at PICO-NARE are higher than at MLO, which reflects the higher
influence of the adjacent continents to air composition in the central Atlantic region in comparison to the Northern Mid-Pacific.
Short-chain NMHC remain elevated in air plumes that have been influenced by either anthropogenic injections or biomass burning
after time scales in excess of 1 week during their transport to the PICO-NARE station. Isoprene data convincingly describe
summertime (mostly buoyant) upslope flow occurrences. Isoprene was found as the best of all chemical tracers to identify upslope
flow.
A good correlation was determined between seasonally differentiated NMHC variability and the NMHC OH lifetimes. Regression
analysis of the lnx=A-b relationship for these data yields a b-value of 0.xx, which confirms the remote, marine island character of the
Pico site and the lack of major local influences on NMHC levels.
Summertime ozone/(ln [propane]/[ethane]) correlations show higher variability, indicating more variability in photochemical
conditions than during wintertime. Net ozone destruction typically only occurs, after photochemical processing has reduced ln
[propane]/[ethane] to values < -2.5; a conditions only observed during the summer.
11
The reasonable agreement that was found between indirectly derived photochemical ages of NMHC in air plumes sampled at Pico and
back-trajectory and FLEXPART analysis suggests that assumptions that went into the model calculations were reasonable and that
interpretations of NMHC ratios provide a meaningfull tool for deciphering photochemical age and transport of air sampled at Pico.
Acknowledgments
We thank P. Goldan, NOAA Aeronomy Laboratory, Boulder, CO for the reference analysis of the primary NMHC standard prior and
after its use at Pico. M. Dziobak and M. Val Martin helped with GC instrument maintenance tasks at the Pico. This research was
funded by a grant from the NOAA Office of Global Programs (award # NA03OAR4310072).
DOES FLEXPART USE NEED TO BE ACKNOWLEDGED?
References
Blake D.R. and F.S. Rowland (1995) Urban leakage of liquefied petroleum gas and its impact on Mexico City air quality. Science 269, 953-956.
Bottenheim J.W. and M.F. Shepherd (1995) C2-C6 hydrocarbon measurements at four rural locations across Canada. Atmos. Environ. 29, 647-664.
Bottenheim J.W., P.C. Brickell, T.F. Dann, D.K. Wang, F. Hopper, A.J. Gallant, K.G. Anlauf and H.A. Wiebe (1997) Non-methane hydrocarbons and CO during
Pacific ’93. Atmos. Environ. 14, 2079-2087.
Ehhalt D.H., F. Rohrer, A. Wahner, M.J. Prather and D.R. Blake (1998) On the use of hydrocarbons for the determination of tropospheric OH concentrations. J.
Geophys. Res. 103, 18981-18,997.
Gautrois M., T. Brauers, R. Koppmann, F. Rohrer, O. Stein and J. Rudolph (2003) Seasonal variability and trends of volatile organic compounds in the lower
polar troposphere. J. Geophys. Res. 108, 4393, doi:10.1029/2002JD002765.
Goldstein A.H., S.C. Wofsy, and C.M. Spivakovsky (1995) Seasonal variations of nonmethane hydrocarbons in rural New England: constraints on OH
concentrations in northern midlatitudes. J. Geophys. Res. 100, 21023-21033.
Greenberg, J.P., D. Helmig, and P.R. Zimmerman, Seasonal measurements of nonmethane hydrocarbons and carbon monoxide at the Mauna Loa Observatory
during the Mauna Loa Observatory Photochemical Experiment 2 (1996), J. Geophys. Res., 101, 14581-14598.
Honrath R.E., R.C. Owen, M. Val Martin, J.S. Reid, K. Lapina, P. Fialho, M.P. Dziobak, J. Kleissl, and D.L. Westphal (2004), TITLE, J. Geophys. Res., 109,
D24310, doi;10.1029/2004JD005147.
Jobson B.T., D.D. Parrish, P. Goldan, W. Kuster, F.C. Fehsenfeld, D.R. Blake, N.J. Blake and H. Niki (1998) Spatial and temporal variability of nonmethane
hydrocarbon mixing ratios and their relation to photochemical lifetime. J. Geophys. Res. 103, 13557-13567.
Jobson B.T., S.A. McKeen, D.D. Parrish, F.C. Fehsenfeld, D.R. Blake, A.H. Goldstein, S.M. Schauffler, and J.W. Elkins (1999) Trace gas mixing ratio
variability versus lifetime in the troposphere and stratosphere: Observations. J. Geophys. Res. 104, 16091-16113.
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Jobson B.T., C.M. Berkowitz, W.C. Kuster, P.D. Goldan, E.J. Williams, F.C. Fehsenfeld, E.C. Apel, T. Karl, W.A. Lonneman and D. Riemer (2004)
Hydrocarbon source signatures in Houston, Texas: Influence of the petrochemical industry. J. Geophys. Res. 109, D24305, doi:10.1029/2004JD004887.
Kleissl J., R.E. Honrath, M.P. Dziobak, D. Tanner, M. Val Martin, R.C. Owen, and D. Helmig (2006) The occurrence of upslope flows at the Pico mountaintop
observatory: A case study of orographic flows on a small, volcanic island. J. Geophys. Res., submitted for publication.
McKeen S.A. and S.C. Liu (1993) Hydrocarbon ratios and photochemical history of air masses. Geophys. Res. Let. 20, 2363-2366.
Parrish D.D., C.J. Hahn, E.J. Williams, R.B. Norton, F.C. Fehsenfeld, H.B. Singh, J.D. Shetter, B.W. Gandrud, and B.A. Ridley (1992) Indications of
photochemical histories of Pacific air masses from measurements of atmospheric trace species at Point Arena, California. J. Geophys. Res. 97, 15,883-15,901.
Parrish D.D., M. Trainer, V. Young, P.D. Goldan, W.C. Kuster, B.T. Jobson, F.C. Fehsenfeld, W.A. Lonneman, R.D. Zika, C.T. Farmer, D.D. Riemer and M.O.
Rodgers (1998) Internal consistency tests for evaluation of measurements of anthropogenic hydrocarbons in the troposphere. J. Geophys. Res. 103, 22339-22359.
Parrish D.D., E.J. Dunlea, E.L. Atlas, S. Schauffler, S. Donnelly, V. Stroud, A.H. Goldstein, D.B. Millet, M. McKay, D.A. Jaffe, H.U. Price, P.G. Hess, F.
Flocke, and J.M. Roberts (2004) Changes in the photochemical environment of the temperate North Pacific troposphere in response to increased Asian emissions.
J. Geophys. Res. 109, D23S18, doi:10.1029/2004JD004978.
Penkett S.A., N.J. Blake, P. Lightman, A.R.W. Marsh, P. Awyl, and G. Butcher (1993) The seasonal variation of nonmethane hydrocarbons in the free
troposphere over the North Atlantic Ocean: Possible evidence for extensive reaction of hydrocarbons with the nitrate radical. J. Geophys. Res. 98, 2865-2885.
Rudolph J. (1995) The tropospheric distribution and budget of ethane. J. Geophys. Res. 100, 11369-11381.
Stohl A., Hittenberger M. and Wotawa G. (1998) Validation of the Lagrangian particle dispersion model FLEXPART against large scale tracer experiments.
Atmos. Environ. 32, 4245-4264.
Stohl A., Forster C., Frank A., Seibert P., and Wotawa G. (2005) Technical Note : The Lagrangian particle dispersion model FLEXPART version 6.2. Atmos.
Chem. Phys. 5, 2461-2474.
Tanner D., D. Helmig, J. Hueber and P. Goldan (2006) Gas chromatography system for the automated, unattended, and cryogen-free monitoring of C2 to C6
non-methane hydrocarbons in the remote troposphere. J. Chrom., 1111, 76-88.
Val Martin M., R.E. Honrath, R.C. Owen, G. Pfister, P. Fialho and F. Barata, Significant enhancements of nitrogen oxides, black carbon and ozone in the North
Atlantic free troposphere resulting from North American boreal wildfires. J. Geophys. Res., submitted for publication.
Williams J., V. Gros, B. Bonsang and V. Kazan (2001) HO cycle in 1997 and 1998 over the southern Indian Ocean derived from CO, radon, and hydrocarbon
measurements made at Amsterdam Island. J. Geophys. Res. 106, 12719-12725.
Williams J., H. Fisher, G.W. Harris, P.J. Crutzen, P. Hoor, A. Hansel, R. Holzinger, C. Warneke, W. Lindinger, B.. Scheeren and J. Lelieveld (2000) Variabilitylifetime relationship for organic trace gases: A novel aid to compound identification and estimation of HO concentrations. J. Geophys. Res. 105, 20473-20486.
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