Measurements of PAN, alkyl nitrates, ozone, and hydrocarbons during spring in interior Alaska

. Measurements of the atmospheric mixing ratios of ozone, peroxyacetylmtrate (PAN), hydrocarbons. and alk3q rotrates were made in a boreal forest ecosystem in the interior of Alaska from March 15 to May 14, 1993. During this period the mixing ratios of PAN, alkyl nitrates, and nonmethane hydrocarbons (NMHCs) generally decreased due to the influence of both meteorology and OH removal. Mean mixing ratios of ozone, PAN, C2 - C6 alk(cid:127)4 rotrates, and total C2 - C5 NMHC during southerly flow periods were 24.4 parts per billion (ppbv), 132.1 parts per trillion (pptv), 34 pptv, and 8.2 ppbCv, respectively. During a short period of northerly flow, mixing ratios of PAN and total NMHC were approximately 2 times the south-erly flow mixing ratios. PAN is correlated with ozone, and alk3q rotrates are correlated with alkanes. PAN and ozone mixing ratios exhibit similar diurnal variations on a number of days with an early morning mimmuln and afternoon maximum. This is likely due to a diurnal cycle in the boundar3 (cid:127) layer - free troposphere exchange and loss processes in the boundary layer for both 03 and PAN. Higher molecular weight (mw) hydrocarbons and alk34 rotrates are observed to decrease more quickly than the lower mxv hydrocarbons, consistera with removal by OH as the primal' loss process.

The Arctic atmosphere has some unusual tEatures relative to chemisto • in lower latitudes.Because the nitric acid reservoir is depleted during transport to the Arctic, organic nitrates are a major I}action of the NOy ( -= NOx + PAN + IlNO3 + RONO2 + N20,• +...) budget.PAN and alkyl nitrates have been reported to contribute 70-80% to NOy during spring in the high Arctic [Bottenheim et al., 1993;Muttmramu et aL, 1994].In addition to the odd nitrogen reservoir species, elevated levels of NMHC are present in the high latitudes during spring [Rasmussen et al., 1983;Blake cmd Rowland, 1985;Pe•ffett, 1993;dobson et al., 1994b].Furthermore, the decay of N(,)> during spring has been found to coincide with a pulse in NO mixing ratios [Honrath and daffe, 1992].The combination of enhanced N()x and NMHC from anthropogenic sources may influence the ozone budget in the Arctic troposphere during spring.
Surthce ozone mixing ratios observed at Barrow, Alaska, show a long-term increase of about 1%/year JOltmarts and LED.', 19941.•l]fis trend may reflect increasing NOx emissions and chemical processing of these anthropogenic pollutants.Hoxvever, the long-term ozone trends reported by Oltmans are most pronounced during summer, rather than in spring when ozone produced li'om processing of anthropogenic precursors might be expected.A possible explanation is that ozone produced during late spring is sufficiently long lived to contribute to ozone trends in the summer.Alternatively, regional sources of NOx may be responsible for the summer increase [daffe,

19931.
The broad objective of this work is to gain a better understanding of the sources and sinks of ozone and its precursors in the Arctic and, ultimately, to determine the primary factors affecting mixing ratios and trends of ozone in the Arctic troposphere.Here we present the measurements of trace gases related to oxidant chemistry and describe their trends and relationships during the winter-spring transition of 1993 at Poker Flat, Alaska.The trace gases we discuss are ozone, C2-Cs NMHC, PAN, and C2-C0 alk3'l nitrates.[Harris, 1982].After this period tmtil the middle of May, trajectories indicate transport Ikom the south and southeast.The unusual persistence of southerly tlow during this period contributed to the higher than usual ambient air telnperatures in Fairbanks.The daily mean temperature during the entire campaign (,5.8øC) was 4.8øC higher than the 30 year mean tbr this period.The data set is theretbre divided into three groups: days 82 to 86 (,Arctic flow') and the period befi,}re and after this time (,southerly flow).

Time
Statistics tbr the three difibrent groups are given in Tables la-lc.Measurements of alk31 nitrates and ozone did not start until days 89 and 100, respectively7 they are theretbre not included in Tables 1 a and lb  Alk.wl nitrates and NMHC mixing ratios decreased steadily during the measurement period.As expected t¾om their atmospheric litbtimes [Roberts, 19901. alkvl   During days when the surface temperature exceeded 10øC, growth in the bounda .rylayer mixes air with higher mixing ratios of both PAN and 03 from above.This boundary layerti'ee troposphere exchange apparently is less pronounced on cooler days.Table 2 lists the maximum temperature and the PAN/ozone slope for the 10 days when this slope was statistically significant at the 95% level.No clear picture emerges; however, part of the data seem to suggest that on days with highest temperatures PAN may be decomposing thermally to an increasing extent, leading to a small or even negative PAN/ozone slope.The maximum ratio during periods of concun'ent downward mixing was 21.5.The slope of 1.96 found when using all of the data as in Figure 3  Sounding data t¾om the Fairbanks international airport indicate median mixing heights during the afternoons of around 1400 m.Every night, low-level surface inversions formed.However, at the Poker Flat site, which is located about 50 km from Fairbanks and on a ridge, the situation is unclear.The nighttime loss of both PAN and 03 shown in Figure 4 suggest rapid deposition in a shallow inversion layer.Thus we conclude that in general the boundary, layer is a sink for both O3 and PAN.Deposition of both on snow surfaces is not likely to be a substantial loss process.In April, loss of both to vegetation probably became more important as the snow disappeared.[ Atkinson and Lloyd, 1984].This method ignores the retbrmation of PAN.However, it is probably an acceptable approach at Poker Flat, given that NOx mixing ratios are low.
The highest median PAN mixing ratio of about 170 pptv occurs at 1500 in the afternoon (Figure 4).The mixing ratio drops subsequently over the next 10 hours to 120 pptv.This drop is possible from thermal decomposition alone at about 3øC, using the approach discussed above.At temperatures of 10øC, which were frequently seen during our campaign, thermal decomposition alone would result in a drop I?om 170 pptv to about 50 pptv over 10 hours.Therefore for PAN, thermal decomposition in the boundary layer, when the temperature is above 0øC, is also increasingly important.
In spring, diurnal boundary layer variations are relatively large in interior Alaska.Daytime solar heating occurs during the many clear days, resulting in growth in the mixed layer.
At night, strong temperature inversions reform due to radiative cooling.Thus in the boundary layer we believe the observed mixing ratios result from the difference between the source from above and the boundary layer losses.Although the meteorological and chemical situation was rather different, a similar conclusion was also reached fi'om measurements taken during the summer of 1988 as part of the ABLE 3A campaign in Bethel, Alaska [Jacob et al., 1992].
Total alkyl nitrates correlate well with alkanes (Figure 5).The other NMHC show little or no correlation with alkyl nitrates.The slope of the linear regression is 47 (molecules alkanes/molecules alkyl nitrates).Alkyl nitrates are produced from alkanes when sufficient NOx is present.The primary removal process for alkanes is OH attack on the carbon chain, Ibr alkyl nitrates it is both photolysis and OH attack.The chemical litEtime of an alkyl nitrate is within 50% of that of the corresponding alkane, the differences are mainly due to different photolysis rates [Atlas et al., 1992].The correlation theretbre suggests that the ratio of pollutants in the source region is a controlling factor on the NMHC/alkyl nitrates ratio   et al., 1992].In other seasons a ratio between 3.5 and 10 was found at MLO, with the highest during spring 1992.The comparison suggests, in general, that higher ratios are found in more polluted environments.However, no clear picture emerges from this comparison.The primary removal for saturated NMHC is via reaction with the OH radical.Since higher molecular weight NMHCs have larger OH reaction rates, we would expect to see a faster falloff in their mixing ratio during the winter-spring transition at high latitudes.This was observed during this campaign for ethane, propane, and n-pentane (the bottom panel in Figure 2 shows ethane and propane only).Ethane mixing ratios drop by about 28% from March to May, whereas propane and npentane drop by approximately 73 and 85 %, respectively, over the same time period.For these hydrocarbons the percent decrease is consistent with their OH reactivity.I-butane and n-butane, however, decreased by only 63 and 70%, respectively, which compared to the other hydrocarbons is not in accord with their OH reactivity.This suggests that the butanes have additional sources on the transport path, or were influenced by local sources.
The measured species seem to fall into two groups: Ozone is very strongly correlated with PAN, and alk3'l nitrates with the alkanes.The correlation between the two groups is weaker but still statistically significant.The very different mixing ratios found for the short period of northerly flow supports the view that synoptic meteorology has a strong influence on pollution mixing ratios.
) are key species in tropospheric chemistry through their primar?• role in ozone production and inlluence on the liI•time of the hydrox34 radical [('rtttzen, 1979: Logan, 19851.Photochemistry in the troposphere oxidizes N()• to reservoir species such as HNO3, peroxyacctylnitrate (PAN).and other organic nitrates such as alk3'l nitrates (R()N()2).The ultimate sink tbr N()x is 1tNO3, which contributes to acid rain.Photolvsis or them•al degradation of these reservoir species can return N()x to the atmosphere.During the Arctic v¾inter, reduced temperature and/or insolation increases the atmospheric lil•times of PAN, alkyl nitrates, and nomnethanc hydrocarbons (,NMttCs), so that they may be transported over large distances ti•om midlatitudes into the Arctic.Subsequent warnting and increased insolation at the onset of spring can lead to the decomposition of these reservoir species and a return of NOx to the atmosphere.In this rammer, them•al decomposition of PAN may be the most important N()x source in the Arctic troposphere during the v• inter-spring transition [Isaksen et al., 1985' Penkett amt Brite, 19861 and thus may have an important effect on ozone production in the Arctic spring.
Measurements were made at Poker Flat Research Range, Alaska (,64ø11q'q, 147ø43'W, 501 m above mean sea level), t?om March 15 to May 14, 1993 (days of year 74 to 134) in a boreal tbrest ecosystem.A map of Alaska is shown in series tbr PAN, ozone, total NMHC, and alkwl nitrate mixing ratios show the general decline in 1nixing ratios during the winter-spring transition (,Figure 2).On two da.x,•s the isopentane mixing ratio was anomalously high.The measurements on these days appear to have been influenced b.x, • an unidentified source.Isopentane is theretbre not included in the sum of NMHC.Surface pressure, 850 mbar height, and isobaric back trajectories indicate primarily southerly flow with an Arctic influence on the site only once during this campaign.Days 82 to 86 show airflow tkOln the Arctic into the interior of Alaska.

Figure 3 .
Figure 3. Correlation of 10th percentiles of PAN and ozone.The data x•ere ordered by the ozone mixing ratios.•It•e line shows a linear regression, weighted by the standard deviation in the ordered ozone averages, with' a slope of 1.96 (pptv PAN/ppbv 03).
indicates that during most times, thermal decomposition during transport results in PAN lifetimes which are shorter than the ozone lifetime.That PAN and Os are correlated in both daily averages for the whole campaign and the hourly averages on many days is probably a result of several causes.First, the daily averages ibr both reflect the similar seasonal cycles, whereby both PAN and 03 show a recurrent April maximum [e.g., Barrie et al., 1989: Bottenheim et al., 1993: Oltmans atxl Levy, 1994].Whether the accumulated PAN reservoir causes the 03 maximum, as proposed by Isaksen et al. [1985] and Penkett and Brice [1986], is not clear from these data.The role of boundary layer exchange in transporting both PAN and ozone to the surface is apparent from the diurnal cycle on a number of days and the good correlation on those days.
observed at Poker Flat.Total NMHC and PAN are correlated only tbr the time of Arctic flow to the site.For the southerly flow regime no correlation is apparent, indicating different removal processes for PAN and alkanes, thermal decomposition, and OH attack, respectively.The ratio of PAN to alkyl nitrates and the variation of this ratio can suggest the processes affecting these organic nitrates during transport I?om a common source.Through o•dation of hydrocarbons in the presence of NOx, both PAN and alkT1 nitrates are associated with anthropogenic pollution.In addition, PAN and not alkyl nitrates may be produced from the oxidation of biogenic hydrocarbons or photolysis of acetone [Sit•h et al., 1994].Biogenic hydrocarbons were measured as early as April at Fraserdale, Ontario, [dobson et al., 1994a],which is in a similar ecosystem as Poker Flat.As already noted, the major PAN sink in the atmosphere is thermal decomposition, while alkyl nitrates have mainly photochemical sinks via photolysis and reaction with OH radicals[Roberts, 1990].Thus in principle, changes in the ratio of PAN to alkyl nitrates should reflect source variations and the relative lifetimes.The PAN/alkwl nitrate ratios increased steadily from about 2 to a maximum of around 6 on day 110, after that it