Summertime distribution and relations of reactive odd nitrogen species and NOy in the troposphere over Canada

We report here large-scale features of the distribution of NO s, HNO 3, PAN, particle NO3', and NO y in the troposphere from 0.15 to 6 km altitude over central Canada. These measurements were conducted in July-August 1990 from the NASA Wallops Electra aircraft as part of the joint United States-Canadian Arctic Boundary Layer Expedition (ABLE) 3B-Northern Wetlands Study. Our findings show that this region is generally NO s limited, with NO s mixing ratios typically 20-30 parts per trillion by volume (pptv). We found little direct evidence for anthropogenic enhancement of mixing ratios of reactive odd nitrogen species and NOy above those in background air. Instead, it appears that enhancements in the mixing ratios of these species were primarily due to emissions from several day old or CO-rich-NOs-poor smoldering local biomass-burning fires. NO s mixing ratios in biomass-burning impacted air masses were usually <50 pptv, but those of HNO(cid:127) and PAN were typically 100-300 pptv representing a twofold-threefold enhancement over background air. During our study period, inputs of what appeared to be aged tropical air were a major factor influencing the distribution of reactive odd nitrogen in the midtroposphere over northeastern North America, These air masses were quite depleted in NOw (generally <150 pptv), and a frequent summertime occurrence of such air masses over this region would imply a significant influence on the nitrogen Our findings show that the chemical composition of aged air masses over subarctic Canada and those documented in the Arctic during ABLE 3A have strikingly similar chemistries, suggesting large-scale connection between the air masses influencing these regions.

HNO 3, PAN, p-NO3', and total reactive odd nitrogen (NOy = NO + NO 2 + NO 3 + N20• + HONO + HNO 3 + PAN + higher alkylperoxy nitrates such as PPN + RONO: + p-NO3-) in the low-to mid-troposphere over the Hudson Bay lowlands (northern Ontario) and the Schefferville region (northern Quebec and Laborador) of Canada. Then within specific air masses, we examine relationships between reactive odd nitrogen species and correlations with O 3 and CO. We were particularly interested in studying tropospheric chemistry over this region of North America since it is influenced intermittently by natural and anthropogenic emissions. Thus we examined reactive odd nitrogen chemistry in the unique variety of air mass compositions over central Canada that represent the mixing of "background" Arctic air with air influenced by natural and anthropogenic emissions from North America. Nitric oxide, NO•, and NO were measured simultaneously y with the Georgia Tech two-photon/laser-induced fluorescence (TP/LIF) instrument [Bradshaw et al., 1985;Sandholm et al., 1990]. This spectroscopically selective NO technique simultaneously determined NO, NO produced from the photolysis of NO 2, and NO produced from the reduction of NO compounds using a 300 ø C gold catalytic surface with 0.3• CO as a reducing agent. A 1-kW photolytic converter was operated with a photolysis passband of 350 nm < •, < 410 rim, a photolytic yield ranging from 30 to 60% and sample residence times ranging from 2 to 4.5 s. A porcelain-glass coated inlet was used to sample ambient air in an orientation perpendicular to the airstream. These NxOy measurements were reported using integration times of 90 and 180 s. Accuracy of the instrument calibration is estimated to be ñ16% for NO and +18% for NO• and NO at the 95% confly dence limit. Limits of detection for a 180-s signal integration were about 3 pptv for NO and about 10 pptv for NO 2 with a signal-to-noise ratio of 2'1. The typical measurement precision for NOy (at 95% confidence limit) was circa ñ10% at 500 pptv increasing to circa +20% at 200 pptv [Sandholm et al., 1992;Sandholm et al., this issue].
The NASA Ames PAN instxtu•ent provided measurements of this species using electron capture gas chromatography detection from a cryogenically enriched sample of ambient air [Singh and Salas, 1983;Gregory et al., 1990]. The system used an aft facing Teflon inlet with the instrument operated at constant pressure (1050 mbar) isolated from aircraft cabin pressure fluctuations. The sampling time of PAN was typically 90 s followed by an 8-min analysis time. In-flight calibration was accomplished using PAN synthesized in liquid n-tridecane [Gaffney et al., 1984]. The PAN measurements have an estimated accuracy of about +20% and a precision of +10%. Given the above conditions, the limit of detection for PAN was about 5 pptv.
Nitric acid and p-NO 3-measurements were performed with the University of New Hampshire (UNH) mist chamber instrument and shrouded atmospheric aerosol sampling systems respectively [Talbot et al., 1990[Talbot et al., , 1992

Atlas et al., An intercomparison of three H.NO 3 measurement techniques, unpublished data, 1990
]. In ABLE 3B the HNO 3 system employed a 40-mm ID inlet with a porcelain-glass coating. The Georgia Tech and UNH sampling systems used identical inlet compositions to ensure uniformity in passing HNO 3 through both instruments. Laboratory testing of a porcelain-coated inlet showed that in the 100-300 pptv range, HNO 3 was passed through the entire ~l.5-m length with 100+3% efficiency. The inlet was mounted perpendicular to the fuselage. This was done to minimize collection of aerosols, especially supermicron particles typically enriched in nitrate. Sampling times were usually 15 min for HNO 3, with a corresponding limit of detection of 10 pptv. The overall uncertainty is estimated to be +20% for mixing ratios >100 pptv increasing to + 30% for mixing ratios near the limit of detection. The atmospheric aerosol system and an assessment of its performance is described elsewhere [R. W. Talbot et al., Improvements in aerosol inlet performance in airborne applications, submitted to Journal of Geophysical Research, 1993]. The sampling system employed a curved-leading edge 8-mm nozzle housed inside a 150-mm ID shroud. The passing efficiency for submicron particles is believed to be virtually 100%. For supermicron particles the passing efficiency is unknown, but comparison of this inlet with one using a sharp-leading edge nozzle not housed in a shroud showed that p-NO 3-mixing ratios were an average of 150% higher with the current UNH system than those obtained with the other system. Potential sampling problems may still exist for p-NO3', but it is believed that the relative concentration trends between various air masses are substantially correct. Simultaneous sampling for p-NO 3-with two identical UNH systems indicates the precision was about +18% [Talbot et al., 1992].

Air Mass Categories
The GTE/ABLE 3A data for the North American Arctic and subarctic showed that CO was a good overall indicator of air mass history (see, for example, . We selected CO to serve as a proxy indicator of air mass composition based on our experience during ABLE 3 A, CO' s atmospheric chemical lifetime of several months, and CO's passiveness to removal by deposition processes. We use CO mixing ratios here primarily as a relative indicator of combustion inputs to air masses over central Canada. Thus we examined the chemistry of reactive odd nitrogen in air masses exhibiting various degrees of impact from combustion emissions. Hydrocarbon and halocarbon tracer species were used to distinguish biomass-burning from direct industrial inputs [Wofsy et al., this issue]. Air mass chemical compositions over northern Ontario were divided into two categories, "background" (CO 95-105 parts per billion by volume (ppbv)) and biomass-burning impacted (CO +110 ppbv). Air mass types over northern Quebec/Labrador were defined as tropical (CO 60-79 ppbv), mixed (CO 80-94 ppbv), "background" (CO 95-105 ppbv), and biomass-burning impacted (CO + 110 ppbv). A final air mass type with no geographic reference was defined as upper tropospheric/lower stratospheric influenced (CO +105 ppbv and dew point -25 ø to -47øC). Each air mass category was comprised of its own discrete data set. The tropical and mixed air mass types were not observed over the Ontario region. The classification "background" air is consistent with findings from GTE/ABLE 3A where CO mixing ratios of 95-105 ppbv were present in Arctic "background" air masses . Mixing ratios of 106-109 ppbv were observed for CO, but data associated with this range in CO were not used in this analysis. At 110 ppbv of CO it was clear that combustion contamination had affected the overall chemistry. We chose to omit this small group of data (i.e., CO 106-109 ppbv) from our analysis rather than assign a specific air mass type to it. The "tropical" category appears to represent aged marine air with CO mixing ratios ranging from 60 to 79 ppbv [Anderson et al., this issue]. The overall chemistry of this air mass, especially the hydrocarbon distribution, suggests a low-latitude source, most likely over the Pacific Ocean. Isentropic back-trajectory analysis also supports a tropical origin for this air mass, possibly related to outflow from Typhoon Steve [Shipham et al., this issue].
Over Quebec/Labrador air masses with CO mixing ratios of 80-94 ppbv were frequently encountered. A combination of air mass types appear to have been "mixed" to produce these mixing ratios of CO. For the upper tropospheric/lower stratospheric air mass category, data were included from time periods where CO _•105 ppbv and the dew point temperature was between -25 ø and -47øC. Restriction of CO to _•105 ppbv removed the obvious cases of fresh biomassburning influence. The remotely sensed O 3 and aerosol distributions were used to help identify intrusions of upper tropospheric/lower stratospheric air [Browell et al., this issue].
The highest mixing ratios of CO appeared to be associated with emissions from biomass-burning. It is important to note that the fires observed within the study areas were a combination of peat and forest combustion yielding significant smoke.
The few minutes of data that were obtained directly in a biomass-burning plume, close to its source during mission 9, was removed from the northern Ontario biomass-burning group so that the mixing ratios reported in Table 1 better represent "average" conditions. Mixing ratios for the species of interest in our analysis of the ABLE 3B data were merged into the appropriate air mass categories, defined above, for examination of the large-scale distribution of reactive odd nitrogen associated with the gradient in CO mixing ratio. The majority of the CO data collected below 1 km altitude was obtained in fast response mode (10 Hz). This facilitated flux calculation objectives but resulted in insufficient data for our analysis of the reactive odd nitrogen distribution. Fortunately, slow response (~1 Hz) CO data were available for ascents and descents within the boundary layer and at the very start and end of the low-level (0.15 km altitude) flight legs. These data were used together with the associated mixing ratios of other species primarily NO NOy, C2C1 n, selected fluorocarbons (F-12, F-13, F-113), and the hydrocarbons ethene, ethyne, ethane, and propane, to assess the "cleanliness" of the boundary layer air. The air mass distinction was usually quite obvious, since the air masses at 0.15 km altitude were either impacted by biomass-burning or represented "background" air. In fresh biomass-burning emissions, ethene was elevated by an order of magnitude, or more, whereas ethyne, ethane, and propane were enhanced twofold to fivefold [Blake et al., this issue]. We admit that the <l-km altitude air mass divisions are subjective, but there was no other alternative except to entirely ignore the boundary layer data. We believe our divisions are substantially correct, but caution should be exercised in the interpretation of the <lkm reactive odd nitrogen data. Of the other air mass categories, only the mixed air masses were observed at <1 km altitude. Data collected at <1 km altitude was only incorporated into the mixed grouping when there was simultaneous CO measurements. The data used in this paper represent about 80% of the measurement time intervals during the ABLE 3B flight series.
To summarize reactive odd nitrogen chemistry in various air masses over central Canada, the highest resolution measurements were used for individual species' 90-s intervals for NO, NO2, and NO; 90-s intervals every 8 min for PAN; 15-min intervals for I-fNO3; and 20-to 30-min intervals for p-NO3'. For consistency with NO and NO mixing ratios, 90-

Occurrence of Air Mass Types Over the Study Region
This section provides a qualitative time series scenario of the occurrence o f air mass types sampled o ver central Canada during ABLE 3B. During the study period we observed more diversity in air mass types over Quebec/Labrador than over  Ontario. The apparent constant mixing of various air mass types over northern Quebec/Labrador provided a convenient gradient in CO mixing ratio for examining the distribution of reactive odd nitrogen.
Missions 2-9 were conducted over northern Ontario/ Manitoba during July 1990. During missions 2-5 we sampled largely "background" air intercepting a few biomass-burning plumes on missions 4 and 5. Missions 6-9 were apparently dominated by emissions from biomass-burning in Ontario and long-range transport from fires in western Canada and Alaska [Shipham et al., this issue]. The air masses in the entire 0.15-to 6-km tropospheric column over the study region during missions 6, 8, and 9 were severely affected by biomass-burning emissions. Over Ontario we encountered predominantly either "background" air or air masses apparently influenced predominantly by biomass-burning emissions. Additionally, in the 3-to 6-km altitude range, pockets of dry air (dew point -25 ø to -47øC) were sampled intermittently on all missions except 6. These dry air masses are believed to have their origins in the upper troposphere/lower stratosphere and, subsequently, subsided to our highest flight altitudes.
Missions 11-19 were centered over northern Quebec/ Labrador during August 1990. In this region, air masses were sampled which had a variety of CO mixing ratios ranging from 60 to nearly 400 ppbv. Missions 11 and 13 appeared to be strongly influenced by emissions from biomass burning. What appears to have been tropical marine air strongly influenced the chemistry in the 2-to 6-km altitude . We find little direct evidence over the Canadian study regions for a major impact of industrial inputs enhancing the mixing ratios of reactive odd nitrogen species above their mean values in "background" air.

Reactive Odd-Nitrogen Distribution Over a Gradient in CO Mixing Ratios
Tables 1-4 summarize mixing ratio statistics for reactive odd nitrogen species, NOy, 0 3, CO, and CH 4 in the air mass    The median HNO• mixing ratio in upper tropospheric/lower stratospherically influenced air was 32 pptv, which was identical to that in "background" air over Ontario (29 pptv at 3-6 km) but twofold larger than "background" air values found over Quebec/Labrador (16 pptv).

PAN. The vertical distribution of PAN generally had its familiar increase in mixing ratio with increasing altitude.
Like HNO3, PAN mixing ratios were variable, even in "background" air. In the boundary layer over Ontario, PAN mixing ratios (median = 27 pptv) were about twofold smaller in "background" air than those in similar air masses over Quebec/Labrador (median = 65 pptv). In the 3-to 6-km altitude range, PAN mixing ratios were increased fivefold to sixfold over boundary layer air values. Over Ontario, PAN mixing ratio values within this altitude range in "background" air were about half (median = 162 pptv) of those observed over Quebec/Labrador (median = 321 pptv). The tropical air masses over Quebec/Labrador exhibited the smallest mixing ratios of PAN at 3-to 6-km altitudes (median = 128 pptv). PAN mixing ratios measured coincident with the smallest values of CO in the tropical air were only 30-40 pptv (at 3 km altitude). Such small values of PAN are typical of tropical Pacific air [Ridley et al., 1990a, b;Singh et al., 1990]. Air masses over Ontario influenced by biomassburning emissions showed a factor of 2-3 enhancement in PAN mixing ratios at all altitudes compared to those in "background" air. In contrast, the only perceivable influence ofbiomass-burning on PAN mixing ratios over Quebec/ Labrador was a twofold increase above the "background" air values (median of 72 versus 140 pptv) in the 1-to 3-km altitude range. The PAN mixing ratios in air masses influenced by the composition of upper tropospheric/lower stratospheric air (median = 297 pptv) were nearly twofold larger than values in "background" air over Ontario (median = 162 pptv) but were similar to values in "background" air over Quebec/Labrador (median = 321 pptv). p-NOs-. Median mixing ratios of p-NO 3-were less than 10 pptv in all air masses except those impacted by biomassburning emissions. Particle-NO 3-mixing ratios were almost always less than coincident vapor phase mixing ratios of HNO 3. In air masses influenced by biomass-burning emissions, p-NO 3-mixing ratios were typically twofold larger than those observed in "background" air. In the 3-to 6-km altitude range over Ontario we observed the largest enhancements in p-NO3'. The increases observed were consistently at least an order of magnitude larger than values in "background" air.
NOy. In the boundary layer over both Canadian study areas, NOy mixing ratios were approximately 170 pptv in "background" air masses. Increased altitude in these air masses resulted in NO mixing ratios that were similar over both study regions (-2Y40 pptv at 1-3 km). However, in the 3-to 6-km altitude range significantly larger values were found over Ontario (median = 522 pptv) than over Quebec/ Labrador (median = 321 pptv). The smallest NOy mixing ratios in the boundary layer were associated with the mixed air masses over Quebec/Labrador (median = 75 pptv). At altitudes of 3-to 6-km the tropical air masses exhibited the smallest NOy mixing ratios (median = 135 pptv) of any air mass type. Biomass-burning emissions apparently produced larger enhancements in NOy mixing ratios over central Canada compared to eastern Canada. Below 3 km in altitude, NO mixing ratios in biomass-burning influenced air over Ontario were enhanced about twofold (medians of 348 and 577 pptv) compared to those in "background" air (medians of 165 and 263 pptv). This enhancement dropped to around 50% in the 3-to 6-km altitude range (median of 522 versus 769 pptv). Similar comparisons between biomass-burning impacted and "background" air masses over Quebec/Labrador showed NO enhancements of <50% below 3 km but approaching a factor of 2 in the 3-to 6-km altitude range. Air masses influenced by the composition of the upper troposphere/lower stratosphere had NOy mixing ratios (median = 569 pptv) that were similar to those in "background" air over Ontario (median = 522 pptv at 3-6 kin) but were almost twofold larger than "background" air values observed over Quebec/Labrador (median 313 pptv at 3-to 6-kin).

Correlation Among Species
To illustrate the relationship between CO and selected species in the troposphere over central Canada, a series of plots are presented in Figure 3. These plots show data obtained in various air mass categories over the Quebec/ Labrador region. Although there were a few mixing ratios of CO between 150-400 ppbv (-15 data points for the biomassburning category), we have selectively plotted the data below 150 ppbv CO in Figure 3  unclear. Definitively determining whether the NOy source is due to long-range transport of natural (e.g., biomass-burning, stratosphere/troposphere exchange or lightning) or anthropogenic inputs, or even the result of high-altitude aircraft emissions is probably beyond the interpretive understanding that can be achieved with the present data set.

Air Masses
The chemical composition of the mixed air over Quebec/ Labrador suggests that it may have originated from the mixing of "background" and tropical air masses. The mixed air masses were quite depleted in HNO 3 and concomitantly NO at low altitudes (Table 3). This suggests possible removal of HNO3 by precipitating frontal systems moving through the Quebec/Labrador region at this time [Ship ham et al., this issue]. It seems likely that cloud processing of the mixed air masses was responsible for the very small NO mixing ratios (below 50 pptv) observed in these air masses at altitudes below 3 km. Interestingly, the NO• mixing ratios in the mixed air masses were not significantly different than those in the other air mass types sampled in this study. This may be indicative of PAN thermal decomposition constantly replenishing NO x, as this mechanism appears to provide an important source of NOx in the summer Arctic troposphere [Jacob et al., 1992;Singh et al., 1992].
Air masses corresponding to the "mixed" category were not observed in the troposphere over Ontario. We did encounter CO mixing ratios of 80-94 ppbv, but these were only in association with pockets of dry air that are believed   (Tables 1 and 3). On the average the boundary layer air temperatures were about 10øC warmer over Ontario (~22øC) than Quebec/Labrador (~12øC). This difference in air temperature, combined together with PAN's thermal stability, may explain the observed regional PAN distributions.

values of the ratio PAN/NO• in air masses impacted by biomass-burning over Ontario had essentially the same values as those in "background" air (Figure 4a). This was also the case over the Quebec/Labrador region, even though the ratio values were about a factor of 2 larger than those over Ontario. This geographic difference in ratio values was caused by the twofold larger mixing ratios of PAN at low altitudes over Quebec/Labrador compared to over Ontario
The small NO• mixing ratios suggest that we routinely sampled air masses that were aged a few days or had primarily smoldering combustion inputs. It appears that in these air masses the PAN mixing ratio, and thus the value of the ratio PAN/NOx, was controlled mainly by PAN's thermal stability [e.g., Wunderli and Gehrig, 1991]. In the range of tropospheric temperatures measured over Canada, a change of a few degrees could shift the PAN half-life by twofold . Only in the 3-to 6-km altitude range, where PAN is much more thermally stable were very high values of the ratio PAN/NO• (15-30) observed. As in the high-latitude atmosphere over Alaska [Sandholm et al., 1992 Although in the boundary layer, PAN/NO, ratios in "background" air were very similar over both Canadian study areas, free tropospheric ratio values were significantly larger (p=0.05) over Ontario (median = 9.6) than those over the Quebec/Labrador region (median = 6.7). Isentropic trajectories show that during the ABLE 3B experiment the Ontario region received substantially more direct inputs of highlatitude air masses than did the Quebec/Labrador area [Shipham et al., this issue]. The frequent occurrence of air masses over Ontario with "background" Arctic air CO mixing ratios supports the trajectory analysis. "Background" air was observed on every aircraft mission over Ontario, except 8 and 9, where biomass-burning inputs apparently dominated the chemical signature. The relatively large mixing ratios of PAN observed in these high-latitude NOx-limited "background" air masses [Singh et al., 1992]   The largest values of the ratio HNO3/NO • were found in biomass-burning impacted air masses. With reference to "background" air there were marginal increases in HNO3/  1 and 3, Figures 6 and 7). However, the NO, mixing ratios were somewhat larger, while those of HNO 3, p-NO3', and PAN were significantly smaller in these dry pockets of air compared to those in biomassburning impacted air masses at 3-to 6-km altitudes. The relatively small CO and CH 4 mixing ratios in the dry pockets of air are inconsistent with the idea that with biomassburning and possibly industrial emissions influenced their chemical composition. In addition, mixing ratios of C2C14 and selected fluorocarbons and hydrocarbons in the upper tropospheric/lower stratospheric air masses were similar to those observed in the "background" and tropical air masses over Canada (Table 7). The fact that the median mixing ratios of selected hydrocarbons, C2C1 n, and F-12 (Table 7)    We still need to address the confounding situation in the upper tropospheric/lower stratospheric air masses where relatively small mixing ratios of almost all species discussed in this paper are needed in combination with apparent NO x and NO• sources. It appears that other sources of NO x and NO• in the middle to upper troposphere, such as lightning and high-efficiency turbine jet aircraft [Ehhalt et al., 1992], are required to explain our observations. We believe that we have sampled the latter on several occasions, seeing large NOx and NO• enhancements relative to CO. However, perhaps a more satisfying explanation for the composition of the upper tropospheric/lower stratospheric air masses is described in a companion paper [Sandholm et al.,  tion could lead to enhanced production of PAN and other organic-nitrogen containing compounds, especially in the midtroposphere where hydrocarbon precursors might be more abundant than at higher altitudes. To better clarify the source(s) of reactive odd nitrogen in the pockets of dry air over eastern North America, it would seem desirable to sample this same region again using an aircraft with a much higher altitude ceiling than the NASA Electra.

Comparison of Arctic Boundary Layer Expedition (ABLE) 3A and ABLE 3B Air Mass Compositions
The chemical composition of the upper tropospheric/ lower stratospheric air masses over Canada is strikingly similar to what was observed at 3-to 6-km altitudes in the high Arctic during ABLE 3A (Tables 4 and 8). As previously pointed out, the "background" air over Ontario exhibited chemical characteristics suggestive of a high-latitude source (Table 1)     The chemical signatures of these three air masses exhibited median CO mixing ratios in the range of 90 -105 ppbv and are very chemically similar to those air masses that were classified as "background" over the Canadian study regions.
The air masses indicated as originating from the interior of Siberia are believed to have been influenced primarily by biomass-burning emissions [Sandholm et al., 1992], even though anthropogenic emissions may also contribute a significantportion of both the CO and NO budgets for latitudes > 60øN over Alaska [Jacob et al.,19•2]. The Siberian air masses that were observed in the 3-to 6-km altitude range over Alaska possess chemical signatures remarkably similar to those air masses observed over Canada that were characterized as having been significantly influenced by biomassburning emissions. Certainly there are many intriguing aspects and potential connections of the ABLE 3A and ABLE 3B data sets which need to be explored in more detail.

CONCLUSIONS
We have described the distribution of reactive odd nitrogen in the troposphere over central Canada. Various air mass categories were defined by bracketing specific CO mixing ratios over a gradient from 60 to several hundred parts per billion by volume (ppbv). Over Ontario we observed only air masses classified as "background" (CO, 95-105 ppbv) or biomass-burning impacted (CO>105 ppbv). Pockets of dry air were frequently encountered in the 3-to 6km altitude range, presumably due to subsidence of upper tropospheric/lower stratospheric air. During our observational period the atmospheric structure was more complex over Quebec/Labrador than what was observed over Ontario. The intrusion of tropical air (CO, 60-79 ppbv) at altitudes of 2-6 km over Quebec/Labrador had a major influence on the chemistry there. Additional marine air may have been injected into the midtroposphere over North America due to the passage of Hurricane Bertha off the east coast of Canada/United States in early August. Besides "background" and biomass-burning impacted air masses, a mixed air mass was identified over Quebec/Labrador with CO mixing ratios of 80-94 ppbv. Frequent incursions of upper tropospheric/lower stratospheric air also influenced this region.
"Background" air in the Ontario area had chemical characteristics very similar to what was observed in the highlatitude atmosphere over Alaska during ABLE 3A. This did not seem to be the case over Quebec/Labrador. The ABLE 3B meteorological analysis supports this contention, indicating frequent intrusion of continental polar air over Ontario and a predominance of westerly flow with long fetches over the Northwest Territories and Alaska influencing the Quebec/ Labrador area. We find little direct evidence over central Canada for enhancement of mixing ratios of reactive odd nitrogen species and NOy above those in "background" air by anthropogenic emissions. Generally, this region appears to be dominated by NOz-limited aged air masses. Even when the chemistry was apparently driven primarily by biomassburning impacts, NOz concentrations were typically <50 pptv. Biomass-burning emissions influencing the Canadian study regions appear to be several days old or from CO-rich-NO•-poor smoldering local fires. Within the boundary layer NO• appears to be efficiently converted to HNO 3, but in the free troposphere, NO• seems to be controlled by the large PAN reservoir which may be derived, to some extent, from biomass-burning emissions.
Our analysis focused on synthesizing the ABLE 3B reactive odd nitrogen data set in a systematic manner to provide a referable baseline for subsequent interpretation of the measurements. Indeed, for this region of the troposphere over North America it appears that CO mixing ratio provides one reasonable basis for formulating air mass categories. This approach was chosen specifically to facilitate examination of the large-scale processes influencing the distribution of reactive odd nitrogen species and NOy. Our analysis has uncovered several seemingly unexplained important features that appear to significantly influence the large-scale tropospheric chemistry over northeastern North America. These features are posed below in the following form of questions'