Fraction and composition of NO y transported in air masses lofted from the North American continental boundary layer

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 109,, doi: /2003jd004226, 2004 Fraction and composition of NO y transported in air masses lofted from the North American continental boundary layer D. D. Parrish, T. B. Ryerson, J. S. Holloway, 1 J. A. Neuman, 1 J. M. Roberts, J. Williams, 2 C. A. Stroud, 3 G. J. Frost, 1 M. Trainer, G. Hübler, 1 and F. C. Fehsenfeld 1 NOAA Aeronomy Laboratory, Boulder, Colorado, USA F. Flocke and A. J. Weinheimer Atmospheric Chemistry Division, National Center for Atmospheric Research, Boulder, Colorado, USA Received 7 October 2003; revised 25 January 2004; accepted 11 February 2004; published 6 May [1] Five field studies have included research aircraft flights over the continental United States and the western North Atlantic Ocean from 1996 through 2000 in spring, summer, and fall seasons. The major source of NO x in this region is fossil fuel combustion, which is localized within the continental boundary layer (CBL). We use CO as a tracer of these anthropogenic emissions to estimate the fraction of the emitted NO x that is exported to the free troposphere (FT), either as NO x itself or as its oxidation products. This export was identified as plumes enhanced in CO above an estimated background by at least 30 ppbv, which account for 20 31% of the air parcels sampled in the FT during the five field studies. These plumes were encountered throughout the FT up to the 8 km ceiling of the aircraft but were primarily located just above the CBL with average altitudes of km above ground level. In the summer over the continent, only 20 ± 5% of the originally emitted nitrogen oxides was transported in those plumes. This fraction is in reasonable accord with model results, but the models include only deep convection and not the shallow CBL venting mechanisms responsible for the observed plumes. During the two field studies in the early fall and in the spring over the western North Atlantic, we find that 9 ± 4% of the NO y was transported, although Li et al. [2004] suggest that this is an underestimate and that 15 ± 11% is more accurate. Both of these numbers indicate that model results in the literature overestimate the amount of NO y transported from the CBL to the FT. In these five field studies, HNO 3 generally accounted for one-half to two-thirds of the NO y, which is in contrast to the dominance by NO x and organic nitrates suggested by models. Over the North Atlantic, this difference is likely due to further photochemical processing of the NO y species within the FT and over the continent due to the different transport mechanism considered in the models. INDEX TERMS: 0322 Atmospheric Composition and Structure: Constituent sources and sinks; 0345 Atmospheric Composition and Structure: Pollution urban and regional (0305); 0368 Atmospheric Composition and Structure: Troposphere constituent transport and chemistry; KEYWORDS: nitrogen oxides, transport, removal Citation: Parrish, D. D., et al. (2004), Fraction and composition of NO y transported in air masses lofted from the North American continental boundary layer, J. Geophys. Res., 109,, doi: /2003jd Introduction [2] In the northern temperate latitudes fossil fuel burning and other industrial processes account for approximately three-fourths of the nitrogen oxides (NO x =NO+NO 2 ) 1 Also at Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, Colorado, USA. 2 Now at Max Planck Institut für Chemie, Mainz, Germany. 3 Now at National Center for Atmospheric Research, Boulder, Colorado, USA. Copyright 2004 by the American Geophysical Union /04/2003JD emitted to the atmosphere (Figure 1). These emissions profoundly affect tropospheric photochemistry in this region of the globe. In particular, as concentrations of NO x rise above approximately 40 pptv the overall photochemical effect moves from net destruction to net production of ozone [Fishman et al., 1979]. Since the lifetime of NO x in the lower troposphere is of the order of 1 day [Liu et al., 1987], these anthropogenic emissions would be large enough to maintain the entire troposphere at northern temperate latitudes above the critical NO x level (see Figure 1) if zonal and vertical transport and mixing were instantaneous. [3] Fossil fuel burning and other industrial sources (Figure 1b) are primarily localized in the continental bound- 1of18

2 Figure 1. Spatial distribution of NO x emissions given in units of concentration increase per day, assuming that the emissions are uniformly mixed zonally and vertically through the total column of the troposphere. (a) Latitudinal distribution of the global emissions segregated by source. For clarity, the aircraft emissions are also shown multiplied by a factor of 20. The abscissa is chosen to be linear in surface area. (b) Longitudinal distribution of fossil fuel NO x emissions at northern temperate latitudes (23 68 N). The global totals of the sources are given in Figure 1a. The emissions are from the EDGAR-2 emission inventory as included in the MOZART model (C. Granier, private communication, 2000), which do not include emissions from oceangoing vessels. ary layer (CBL), which limits their impact on ozone photochemistry in the troposphere as a whole. (The CBL represents approximately 10% of the mass of the global troposphere.) The large NO x emissions located in the northern temperate CBL make it a region of strong ozone production in the summer [Chameides et al., 1992]. However, if a large fraction of the emitted NO x were transported to and dispersed within the rest of the troposphere, its influence on the tropospheric ozone budget would be greatly enhanced. This follows from the much larger number of O 3 molecules produced from each NO x molecule at the lower NO x levels of the free troposphere (FT) [Liu et al., 1987]. [4] In this work we will utilize CO as a tracer for NO x emissions. Many of the anthropogenic sources that emit NO x also emit carbon monoxide (CO), but CO has a much longer lifetime than NO x, on the order of one to three months depending on season [Novelli et al., 1992]. Over the United States and immediately downwind of the continent, well-developed emission inventories that help define the emission ratio of CO to NO x heighten this utility of CO. However, there are at least two difficulties in using CO as a tracer for NO x emissions. First, the CO to NO x emission ratio varies substantially among different sources (e.g., automobiles versus power plants, diesel versus gasoline engines). Here we will simply use a nation-wide, annual average emission ratio. We assume that the sampled pollution plumes transport a representative sample of the variable emission ratios. Second, CO is not quantitatively conserved over the timescales of transport for the plumes sampled here. Throughout the analysis in section 3 we will assume quantitative conservation of CO, and in the discussion in section 4 we will make an approximate correction for the photochemical production and loss of CO. [5] Our goal in this paper has two parts. First, we use elevated CO levels measured in the FT to identify pollution plumes there. Second, the observed ratio of CO to NO x and its oxidation products are compared with the average emission ratio of CO to NO x. This comparison allows us to estimate the fraction of the NO x emissions that have been transported to and remain in the FT with the CO at the time of measurement, and allows the investigation of the relative contribution of each oxidized nitrogen species to the total transported. [6] We examine data sets from five field studies conducted over and downwind from North America. Figure 2 shows the study flight tracks. The flight plans were designed for a variety of science objectives under widely varying meteorological conditions. The two North Atlantic Regional Experiments (NARE) were conducted in early spring 1996 and late summer-early fall 1997 [Parrish, 2001, and references therein]. The other three studies were conducted in the summer: Stratospheric-Tropospheric Experiment: Radiation, Aerosols, and Ozone (STERAO) in 1996 [Dye et al., 2000], the Southern Oxidant Study (SOS) in 1999, and the Texas Air Quality Study (TexAQS) in The latter two field studies were concentrated in the CBL, but during all field studies the aircraft spent a significant fraction (19 62%) of flight time in the FT. Flights were primarily over the continental source regions, but two studies (NARE 1996 and 1997) probed well out over the North Atlantic Ocean. Table 1 gives some statistics for the data sets. [7] NO x and its oxidation products are referred to collectively as NO y. The five field studies included separate measurements of total NO y as well as its principal individual components: NO, NO 2, peroxyacetyl nitrate (PAN) and nitric acid (HNO 3 ). (HNO 3 was accurately measured in the 2of18

3 Table 1. Numbers of Measurements in Data Sets Field Study Total CO, NO y Data CO, NO y FT Data a,b CO, NO y FT Plume Data b,c NO x Plume Data c PAN Plume Data c HNO 3 Plume Data c NARE 1996, 10 s average 19,159 11,217 (59%) 3426 (31%) STERAO 1996, 10 s average 19, (47%) 2466 (26%) NARE 1997, 10 s average 32,855 20,522 (62%) 5311 (26%) SOS 1999, 5 s average 49, (19%) 2159 (23%) TexAQS 2000, 5 s average 39,407 d 8777 d (22%) 1761 d (20%) 1656 d 40 d 1294 d a FT indicates data collected in the FT. b Percentages are relative to numbers of data in preceding column. c Plume data criteria are defined in the text. d Excludes three flights with significant forest fire plume influence. FT only in the final field study.) The chemical form of the oxidized nitrogen species exported to the FT is important, since NO x is the only form that is active in catalyzing the photochemical production of ozone. PAN can dissociate to release NO x, so it acts as a NO x reservoir. HNO 3 is unreactive, releasing NO x only slowly (on the order of weeks) through photolysis and hydroxyl radical attack. In addition HNO 3 is the only soluble species, so precipitation, absorption by aerosols, and other deposition processes effectively remove it from the atmosphere, making HNO 3 the primary sink for NO x in the troposphere. [8] Stohl et al. [2002] have examined the extent and timescale of NO y removal for two (NARE 1996 and 1997) of the five field studies. They used a trajectory based tracer model to determine the history of sampled air parcels. Here we use a simpler analysis that gives no information concerning the timescale of the removal. We compare our results to those of Stohl et al. [2002] to show that they are consistent. We also compare our results to the chemical transport model calculations of Horowitz et al. [1998] and Liang et al. [1998], who examined the export of NO y from the North American CBL, as well as its chemical speciation. This latter comparison provides a test for the model treatment of the exchange between the CBL and the FT. [9] Section 2 of this paper describes the measurement techniques. Section 3 presents the data analysis and examples of the analysis for some of the data sets, particularly from the TexAQS 2000 study. Sections 4 and 5 discuss the results for all five studies, compare our results to others, and present the conclusions. 2. Instrumentation [10] The NOAA WP-3D aircraft served the first four field studies, while the NCAR Electra operated in the final one. The aircraft were equipped to measure concentrations of primary pollutants, ozone and other photochemical products, and the standard suite of meteorological and aircraft parameters. Specific descriptions of the instruments relevant to the current discussion are included here. NO, NO 2, HNO 3, NO y, CO, SO 2, CO 2 and ozone (O 3 ) data were collected as 1-s averages. The data presented here are averages over 10-s (first three field studies) or 5-s (last two field studies) periods. At the nominal air speed of the aircraft (100 m s 1 ) each average corresponds to about 1 or 0.5 km of flight path. (At maximum altitude, the true speed of the aircraft is about 50% greater). PAN was measured at discrete times at much lower frequencies. Table 2 gives the accuracies that have been estimated by the respective principal investigators for all of the measurements that are quantitatively utilized in the present work; they refer to the respective integration periods. [11] CO was measured by a vacuum UV fluorescence instrument similar to that described by Gerbig et al. [1996, 1999]. Holloway et al. [2000] describe the operation of the instrument including the calibration and zeroing procedures. They also show that the instrument used in the present study compared well with a tunable diode laser absorption instrument and with the vacuum UV instrument of Gerbig et al. [1999]. [12] NO, NO 2 and NO y were measured simultaneously with a four-channel O 3 -NO chemiluminescence instrument. The NO y species were converted to NO by Au catalyzed reduction by CO at 300 C. Ryerson et al. [1999] describe the zeroing, calibration and data reduction procedures. During the NARE 1996 and STERAO 1996 missions, the NO y sample was taken through an inlet that later tests demonstrated altered the HNO 3 concentration in the sample. The inlet partially absorbed HNO 3 and released a continuous but variable background that produced an NO y signal. Beginning with the NARE 1997 field mission, an improved inlet [Ryerson et al., 1999] was utilized. These inlet characteristics account for the varying estimated accuracy of the NO y measurement shown in Table 2. These limitations of the NO y data were included in the propagation of errors that yields the uncertainties of the final results. They are a major contributor to the total uncertainty of the results from the earlier field studies. [13] NO and NO 2 were measured by two separate channels of the chemiluminescence instrument. NO was directly measured, and NO 2 was measured by photolytic conversion Table 2. Estimated Accuracies for 5 or 10 s Integrated Measurements in the FT Species Field Study Accuracy, % CO all ±5 NO y NARE 1996, STERAO 1996 ±40 NARE to +5 SOS 1999, TexAQS 2000 ±10 NO x NARE 1996, STERAO 1996 ±30 NARE 1997 ±50 a SOS 1999, TexAQS 2000 ±10 PAN NARE 1996, STERAO 1996 ±20 NARE 1997 ±15 SOS 1999, TexAQS 2000 ±10 HNO 3 TexAQS 2000 ±10 a NARE 1997 NO x accuracy is degraded by uncertainties in the radiometer data used in the photostationary state calculation of NO 2. 3of18

4 Figure 2. Flight tracks for the five field studies. The flight segments in the FT are shown in colors denoting the respective field studies. to NO. Photostationary state calculations also gave NO 2 concentrations from measured NO and O 3 and the photolysis rate of NO 2 derived from onboard radiometer data. Ryerson et al. [1998] give more details of the NO and NO 2 measurement and the photostationary state derivation of NO 2. Comparison of measured and photostationary state derived NO 2 in the FT indicated that the latter was more accurate and precise for the first four field studies due to uncertainties in the zero of the NO 2 channel and poor sensitivity; these more accurate data are used in the analysis. During the last field study (TexAQS 2000), the NO 2 measurement system was improved [Ryerson et al., 2000] and the directly measured NO 2 is utilized. [14] PAN compounds were measured at 5 min or shorter intervals by an onboard gas chromatograph with electron capture detection. Williams et al. [2000] give details of the system that was used in the first three field studies. A similar system with improved time response, sensitivity, stability and calibration accuracy was used in the last two studies. In the TexAQS 2000 field study measurements included several other PAN-type species: peroxypropionyl nitrate (PPN), peroxyisobutyl nitrate (PiBN), peroxymethacrylyl nitrate (MPAN) and peroxyacrylyl nitrate (APAN). For only this one study, the PAN values reported are the sum of the five measured species. When PAN is compared with a more frequently measured species, the concentration of that species is interpolated from the start time of the 5 or 10 s average data to the start of the 2 s PAN sampling time. [15] HNO 3 was measured by an onboard chemical ionization mass spectrometer. Neuman et al. [2002] give a complete description of the instrument and its performance, and Huey et al. [1998] described a ground-based version of the instrument. The instrument was first operated in the 1997 study, but reliable FT data were collected only during the TexAQS 2000 study. [16] Ozone was measured by a NO-O 3 chemiluminescence instrument similar to that described by Ridley et al. [1992]. During the preflight period the chemiluminescence instrument was calibrated with a commercial UV absorption instrument (TECO Model 49) in prepared mixtures of O 3 in zero air. During flight both instruments measured ambient O 3. Ryerson et al. [1998] give more details of the O 3 measurement. [17] Modified, commercial, UV pulsed-fluorescence instruments (TECO Models 43S for the first three and 43C for the last two field studies) were used to measure SO 2. Ryerson et al. [1998] give more details of the SO 2 measurement. [18] CO 2 was measured by a commercial (LI-COR Model LI-6262) dual cell, nondispersive infrared absorption instrument that was enclosed in a temperature stabilized housing. An inlet system dried the sample flow with a commercial (Perma Pure Products Model PD SS) hygroscopic, ion exchange membrane drier and held the measurement cell at constant pressure. The measurement was made as a differential between the dried ambient sample in the measurement cell and a standard mixture of CO 2 in air in the reference cell. This standard mixture had CO 2 mixing ratios ( ppmv) near ambient levels, and was calibrated by the NOAA Climate Monitoring and Diagnostics Laboratory. The zero of the differential measurement was determined by passing the standard mixture through both the reference and measurement cells. The sensitivity of the instrument was determined by standard addition of a small flow of a calibrated, high concentration mixture of CO 2 in air to either the reference mixture or ambient air sample flow. 3. Analysis [19] Here we use the relationships between measured NO y, CO, SO 2,O 3 and CO 2 levels to identify and quantify the stratospheric and anthropogenic influences that determine these levels. We follow four steps in the analysis. First, elevated CO levels observed in the FT are taken as a marker for transported plumes of combustion emissions. Second, the relationship between CO and CO 2 is used to discriminate against the biomass burning plumes encountered in one study. Third, the amount of NO y present with the CO, compared to that in the original emissions, allows the determination of the fraction of the emitted NO y that still 4of18

5 PARRISH ET AL.: NOy LOFTED FROM NORTH AMERICA remains in the plume. Fourth, the chemical form of the NOy in the FT is determined from separate, simultaneous measurements of NOx, PAN and (for TexAQS only) HNO3. The following five subsections present this analysis through discussion of (1) the NOy-CO relationships in general, (2) a simple model to approximately understand these relationships, (3) the independence of anthropogenic and stratospheric influences, (4) the quantification of the amount and speciation of the NOy exported from the CBL, and (5) the uncertainties in the analysis. Figure 3. Relationship between NOy and CO for (a) SOS 1999 and (b) NARE The dots give all of the simultaneous 5 or 10 s averages collected. Where data are available, the dots are color-coded according to the simultaneously measured SO2 as indicated in the color bar, which also shows the upper range of the SO2 measurements. The black line labeled 100% gives the relationship expected from equation (1) for assumed background air (at the intersection of all the lines) with varying amounts of added anthropogenic emissions. Other black lines indicate background air plus anthropogenic emissions with only a fraction (indicated by percent labels) of the originally emitted NOx remaining in the air mass as NOy. The blue line represents the relationship for the mixing of background air with stratospheric air NOy-CO Relationships [20] In this section, we examine the relationship between NOy and CO both for all data, which are dominated by the CBL measurements, and for the FT data separately. The CBL relationships reflect the emission patterns and the removal of NOy species within the CBL. The contrast of the FT data with the CBL data illustrates the changes in the relationships that occur during and after transport of air from the CBL to the FT. To select FT data we generally take the measurements made at altitudes greater than 2 km above ground level (AGL) as measured by an aircraft radar altimeter. An exception is the TexAQS 2000 study, when examination of the vertical potential temperature profiles suggested that the CBL reached 2.2 km AGL at times. Therefore, for this study, the higher altitude limit was applied. [21] Figure 3 shows the relationship between NOy and CO for the complete data sets from two of the field studies. The data points are color-coded according to the SO2 concentration. For our purposes here, all of the measurements of each field mission are considered as a whole, and are assumed to be representative of the region and season of the flights. We make no differentiation with respect to location or meteorological condition. The SOS 1999 results (Figure 3a) exemplify the measurements taken over source regions. NOy, CO and SO2 reach much higher levels here than in the NARE 1996 data (Figure 3b), which were collected predominately downwind of the source regions (Figure 2). [22] In North America CO is emitted primarily from gasoline-powered vehicles and SO2 from point sources especially coal fired power plants. NOy is emitted by both of these sources. All three of these primary pollutants are qualitatively correlated in Figure 3a; in general high concentrations of one species are found with high concentrations of the other two. However, there is great variability. The points at very high NOy and SO2 levels with only modest CO levels represent plumes from power plants located in rural areas. Some of the points at high CO levels have only modest SO2 concentrations; these data represent urban plumes dominated by vehicular emissions. In Figure 3b the same general correlation is present, but the variability is considerably reduced. This reduced variability leads us to assume that, downwind from the source regions, the sampled air masses are approximately representative mixtures of the species emitted in the source regions and processed during transport Simple Mixing Model [23] A very simple model approximately explains the relationship between the measured mixing ratios of NOy 5 of 18

6 Table 3. Dates and Estimated Background Concentrations for the Field Studies Field Study Dates and CO. We consider a sampled air parcel to be background tropospheric air mixed with variable amounts of either anthropogenic pollution or stratospheric air. For simplicity we treat pollution and stratospheric additions separately. The term background is not a well-defined term. For our purposes here we assume that background tropospheric air is well-mixed air at the season and latitude of the study region, without recent anthropogenic or stratospheric influence. [24] For injection of anthropogenic emissions into a background air parcel the relationship between the measured NO y and CO concentrations can be approximated as DNO y ¼ f DCO=R emiss : Here D indicates the increase of the concentration of the indicated species above the background concentration, f gives the fraction of the NO x injected into an air parcel that remains in that parcel as any form of NO y at the time of measurement, and R emiss is the molar ratio of CO to NO y in the emissions. The black lines in Figure 3 indicate this relationship for various values of f given as percentages. The background concentrations correspond to the intersection of these lines. The relationship in equation (1) assumes CO is an inert tracer for the anthropogenic emissions. The validity of this assumption will be discussed in section 4. [25] For mixing of stratospheric air with background air, DCO ¼ c S DCO strat CO, bkgd ppbv NO y, bkgd ppbv NARE March to 11 April STERAO June to 17 July NARE August to 2 October SOS June to 19 July TexAQS August to 13 September ð1þ ð2þ DNO y ¼ c S DNO strat y ; ð3þ where c S indicates the fraction of stratospheric air in the sampled air parcel and DCO strat strat and DNO y represent the difference in concentration of CO and NO y between the stratosphere and the background troposphere. The blue line in each panel of Figure 3 shows the relationship between CO and NO y determined from equations (2) and (3). [26] The background mixing ratios used in the analysis of the data for the five field studies are given in Table 3. For CO the background levels, CO bkgd, are estimated from observations in unpolluted surface air in the western United States adjusted for the season and the year of the study. Other workers have selected slightly different CO background values for these same field studies. Stohl et al. [2002] use the same value for NARE 1996, but select 70 (rather than 75) ppbv for NARE Cooper et al. [2002a] use 115 ppbv and 79 ppbv for NARE 1996 and NARE 1997, respectively. These latter values were derived from the average CO value measured in the FT, minus one standard deviation. In assigning these values there is significant uncertainty, which we consider in assigning confidence limits to the final results. [27] For NO y the background levels, NO y bkgd, are set to be approximately equal to the lowest NO y levels observed in each study. Stohl et al. [2002] use the same value for the NARE 1996 and NARE 1997 studies. For the first 2 field studies these lowest values may be significantly affected by the artifact associated with the NO y inlet as discussed above. However, our analysis is insensitive to the particular value selected. [28] Koike et al. [2003] utilized a different background treatment when they investigated the transport of NO y species from the east Asian CBL. They noted that the lowest observed CO, CO 2 and NO y mixing ratios were a strong function of altitude at the lower flight levels, which resulted from multiple upwind injections of anthropogenic emissions as the air mass was transported over Europe and western Asia. To focus on only east Asian emission, these workers determined backgrounds of the respective species for each sampled air mass using the relationships between the minimum measured CO and CO 2. For the five flight campaigns analyzed in the present study, we are considering North America as a whole, and we observe no strong altitude dependence of the lowest measured CO and NO y levels, particularly in the western United States. Under these conditions our utilization of a single background value (with acknowledged uncertainties) for all air masses sampled in each field study is appropriate. [29] The stratospheric mixing ratios are taken as those observed in FT air with strong stratospheric influence. Danielsen et al. [1987] report measurements in tropopause folds. At the maximum measured O 3 of 320 ppbv, the CO was 30 ppbv. Murphy et al. [1993] show that at this same O 3 concentration NOy 1.2 ppbv. We take these values as characteristic of the stratospheric air mixed into the sampled tropospheric air parcels. These O 3 and NO y values are certainly small compared to values higher in the stratosphere Murphy et al. [1993], but they represent air with a stratospheric influence larger (based upon observed O 3 levels) than any sampled in the present field studies. Hence they can represent the linear influence of an admixture of stratospheric air that is represented by equations (2) and (3). In this study we use these values in only a qualitative manner; they demonstrate that the stratosphere is depleted in CO and enriched in NO y (as well as highly enriched in O 3 ) compared to the background troposphere. [30] The anthropogenic emission ratios are taken as the ratio of average U.S. emissions of CO to NO x. These can be estimated from emission inventories, such as the tabulations of each year s estimated emissions published by the U.S. Environmental Protection Agency [U.S. Environmental Protection Agency (USEPA), 2000a]. However, Parrish et al. [2002] show that the ratio of CO to NO y in U.S. vehicle emissions decreased by nearly a factor of three from 1987 to 1999, a rate of decrease underestimated by the U.S. EPA tabulations. This decrease is largely a result of the improved catalytic converter performance as the U.S. vehicle fleet has modernized. We estimate the average U.S. anthropogenic molar emission ratio for the year of each study through the following strategy. First, for all except on-road sources we take a recent U.S. EPA emission inventory downloaded 6of18

7 Table 4. Estimated Emission Inventories (Tg NO 2 yr 1 ) and Anthropogenic Molar Emission Ratios Year Source On-road emissions CO EPA a NO x Total emissions CO EPA a NO x On-road emissions CO this work NO x Total emissions CO this work NO x Emission ratio EPA a Emission ratio this work a From USEPA [2000a]. from the U.S. EPA Web site on 20 March 2002 that gave emissions through 1999; they are included in Table 4. This document is an update of a recent published emission report [USEPA, 2000a]. For 2000, we assume that the nonroad emissions have remained constant at the 1999 levels. Second, we take the Global Emissions Inventory Activity (GEIA) and Emission Database for Global Atmospheric Research (EDGAR 2.0) emission inventories [Olivier et al., 1999] estimate for annual U.S. on-road vehicle emissions of 64.8 Tg CO in Third, following the discussion of Parrish et al. [2002], we assume that the vehicle emissions of CO have decreased by 5.2% per year and that the CO to NO y ratio in vehicle emissions has decreased by 8.8% per year. The on-road emissions for the year of each field study are extrapolated from 1990 using these rates of change. Table 4 summarizes these emission calculations. The resulting average CO to NO y molar ratio in total U.S. emissions decreases from 5.9 to 5.1 over the four-year span of the studies. For comparison, U.S. EPA estimates for the ratios are 8 19% higher from 1996 to [31] Other workers have used somewhat different ratios. Parrish et al. [1991] used values of 4.3 and 6.7 for the eastern and western United States, respectively. They derived those values from the 1988 National Acid Precipitation Assessment Program (NAPAP) inventory [Saeger et al., 1989]. However, the vehicle emissions in this inventory were derived from techniques that underestimated CO emissions compared to NO x emissions [Fujita et al., 1992]. Jaeglé etal.[1998] use a ratio of about 10 from an earlier U.S. EPA tabulation [USEPA, 1995] to interpret 1996 measurements. Stohl et al. [2002] use a ratio of 6.3, also from a U.S. EPA tabulation [USEPA, 2000b] for both the NARE 1996 and 1997 studies Anthropogenic and Stratospheric Influences [32] In Figure 3, the points with CO significantly higher than background represent air with significant anthropogenic influence. The majority of the points with strong anthropogenic influence lie below the line representing 100% of NO y transported. These points, at least in an average sense, represent air parcels that lost a significant fraction of the NO y emitted into them before they were sampled. This conclusion agrees with those of Parrish et al. [1991], and Stohl et al. [2002], who also found that a large fraction of the NO y is removed within the CBL of North America. In the following we examine this conclusion quantitatively for air parcels in the FT, both with regard to total NO y and to the individual NO y components. [33] Figure 4 shows those data of Figure 3 that were collected in the FT, but now color-coded according to O 3 concentration. In both field studies, most of the low CO measurements are included in this subset of the data. In NARE 1996 (Figure 4b) the two groups of points that parallel the blue stratospheric influence line do indeed exhibit strong stratospheric influence. The minima in CO (56 and 67 ppbv) correspond to large maxima in O 3 (243 and 283 ppbv, respectively) as expected for large added fractions of stratospheric air. In both field studies of Figure 4, many of the points with CO lower than background values correspond to relatively enhanced O 3 levels, indicating significant stratospheric influence. [34] It is clear that the simple mixing model cannot quantitatively explain many points in both panels of Figure 4. Points that lie above and to the right of the stratospheric line, but to the left of the 100% anthropogenic line often represent air parcels with a mixture of stratospheric and anthropogenic influence. However, other points have low CO and low O 3 levels that are not consistent with stratospheric influence; they likely represent air masses that had been highly processed photochemically. Most of these low CO and low O 3 points have NO y levels that are moderately elevated above the background. These NO y elevations may represent moderate anthropogenic influence from CO poor sources, NO y production by lightning, or simply the difficulty of defining single CO and NO y background values. The following discussion is limited to only those data with significant anthropogenic influence as indicated by elevated CO levels. Any stratospheric or lightning influence on the corresponding NO y levels is neglected Fraction and Chemical Form of Anthropogenic NO y in the Free Troposphere [35] Figure 5a presents all of the data from the TexAQS 2000 study, and Figure 5b shows the fraction of those data collected in the FT. These data exhibit patterns similar to the SOS data in Figures 3a and 4a. The large majority of the 7of18

8 Figure 4. Relationship between NO y and CO in the FT. The format is the same as in Figure 3 with expanded axes, except that the dots are color-coded according to the logarithm of the simultaneously measured O 3 as indicated in the color bar, which also indicates the range of observed O 3 values. The data have been sorted so that the higher O 3 values are more visible in the plot. total data set in Figure 5a again falls between the 10 and 50% lines, indicating that in this region also, a large fraction of the emitted NO x is deposited within the CBL. There are fewer points characteristic of power plant plumes, which reflects the different emphases of the field missions. [36] In Figure 5b the data with elevated CO levels fall between the 5 and 50% lines, which indicates that between 50 and 95% of the originally emitted NO x was lost from the air parcel before measurement in the FT. This is generally true for all five studies. Some plumes that were transported through cyclonic systems [Cooper et al., 2001] had lost as much as 97% of the NO y. The color-coding in Figure 5b indicates that very few of the air parcels sampled in the FT had SO 2 levels above 1 ppbv. This pattern is found in the other four field studies as well. Thus a large fraction of the emitted SO 2 is also lost before measurement, either through deposition processes or conversion to sulfate, which was not measured. [37] The possible influence of forest fires complicates the quantitative interpretation of the TexAQS 2000 measurements in the context of the simple mixing model developed above. Satellite imagery examined in the field during the study showed that plumes from large forest fires in the Montana-Idaho region of the western United States were episodically transported to the study area in Texas. Since our objective is to investigate the transport of anthropogenic emissions, we must ensure that we remove any possible influence from forest fire plumes in the FT data that we will quantitatively evaluate. Forest fires and anthropogenic sources differ in the emission ratio of CO to CO 2. Figure 6 shows the relationship between CO and CO 2 in the FT for the data of Figure 5b. For significantly elevated CO, the data fall into two groups. Here we can use a qualitative argument to identify one group as forest fire plumes and the other as anthropogenic pollution. Finlayson-Pitts and Pitts [2000] give an average CO:CO 2 molar ratio of 0.10 for biomass burning, and Kean et al. [2000] find an average ratio of for motor vehicle exhaust. The dashed lines in Figure 6 indicate the effect of adding variable amounts of emissions with these two ratios to air with background levels of CO (Table 3) and CO 2. The CO 2 background is taken as the August September 2000 average of the measurements made by the NOAA Climate Monitoring and Diagnostics Laboratory at their baseline observatory at Point Barrow, Alaska (T. Conway, private communication, September 2001). The black points indicate all of the FT data collected on three flights: 18, 20 and 27 August. Since the data from these three flights approximately parallel the forest fire relationship, we exclude them from further consideration. The data from the remaining flights lie predominately to the right of the vehicle emission curve at high CO values. This is expected because emissions from other anthropogenic sources (e.g., power plants) have lower CO to CO 2 ratios compared to the emissions from motor vehicles. These data are treated further as representing anthropogenic emissions. [38] To isolate anthropogenic pollution plumes in the FT, we consider only the FT data with CO at least 30 ppbv higher than CO bkgd. This criterion ensures that the selected data represent at least modestly polluted air parcels. These data lie to the right of the vertical dashed line in Figure 7. In Figure 7a the points are color coded according to the 8of18

9 PARRISH ET AL.: NOy LOFTED FROM NORTH AMERICA (23 ± 10%) and especially HNO3 (69 ± 19%) were the primary forms of NOy. [39] We can now quantify the relative amount of the originally emitted NOx that remained in these FT pollution plumes at the time of measurement by solving equation (1) for f: Remiss =DCO: f ¼ NOy NObkgd y ð4þ Similarly, we can calculate the fraction of the originally emitted NOx transported in each chemical form of NOy from f NOx ¼ NOx Remiss =DCO ð5þ fpan ¼ PAN Remiss =DCO ð6þ Remiss =DCO: fhno3 ¼ HNO3 NObkgd y ð7þ In equations (5) (7) NObkgd is accounted for by assuming y that it is 100% HNO3. Figure 8 shows the histograms that result from equations (4) (7) applied to each colored data point lying to the right of the dashed lines in Figure 7. On average 16.7% of the originally emitted NOx was transported from the CBL and measured in the FT as some form of NOy. The average fraction still present as NOx represented only 2.6% of the original emissions, and the Figure 5. Relationship between NOy and CO for the TexAQS 2000 study in the same format as Figure 3. All data are included in Figure 5a and are limited to those collected in the FT in Figure 5b with expanded axes. fraction of NOy still present as NOx. Most plumes lie between the 10 and 20% lines, which indicates the fraction of the originally emitted NOx that remained in the air parcels. Of that, only a small fraction, 14 ± 8% (mean ± std. dev), was in its original chemical form (i.e., NOx) at the time of measurement. Figures 7b and 7c show that PAN Figure 6. Relationship between CO and CO2 in the FT for the TexAQS 2000 study. The black points indicate days that we identify as influenced by forest fire plumes. (See discussion in text.) The open circle gives the assumed background levels of CO and CO2. The two dashed lines indicate the relationships from literature reports for forest fire and vehicle emissions. 9 of 18

10 Figure 8. Histograms of percent of originally emitted NO x that was present in pollution plumes encountered in the FT during the TexAQS 2000 study. The histograms are colorcoded, and the average percentages are indicated, for each NO y species in the figure annotations. These values have been corrected for nonconservation of CO. corresponding averages for PAN and HNO 3 were 4.3% and 11.1%, respectively. The sum of the average fractions of the 3 separately measured NO y species (18.0%) agrees reasonably with the average fraction of NO y transported (16.7%), which indicates that the independent measurements of NO y and its component species are consistent with each other, and that the treatment outlined by equations (4) (7) is internally consistent. The same analysis has been applied to the other 4 field studies. Figure 9 and Table 5 give the average of the derived fractions of emitted NO x transported as total NO y, and in the form of the individually measured NO y species in the plumes in the FT. [40] The assumption that NO y bkgd is 100% HNO 3 is reasonable from the point of view that NO y in the free tropospheric plumes is predominately HNO 3. However, as Figure 7 show, in the FT NO y near background levels is primarily in the form of NO x and PAN with little HNO 3. Thus it can be argued that NO y bkgd should be subtracted from NO x or PAN in equation (5) or (6). Alternatively, a fraction of NO y bkgd could be subtracted from each NO y component in equations (5) (7), and those fractions calculated from the average NO y composition at background levels. Our choice gives upper limits for f NOx and f PAN and lower limits for f HNO3, but it should be noted that the choice has only a weak effect upon the final results. For example, the average f PAN for TexAQS drops from 4.3% to 3.1% 10 of 18 Figure 7. Relationship between NO y and CO in anthropogenic pollution plumes in the FT for the TexAQS 2000 study. The format is the same as in Figure 5b with the vertical scale expanded, except that the dots are color-coded according to (a) NO x /NO y, (b) PAN/NO y, and (c) HNO 3 / NO y as indicated in the color bars. In Figure 7b the relatively few PAN measurements are indicated by larger symbols.

11 Figure 9. Summary of percent of originally emitted NO x that is exported to the FT, corrected for nonconservation of CO. The unshaded bars with confidence limits indicate the values derived from the measurements in the five field studies sorted by season. The shaded bars give the model results of Liang et al. [1998] for the respective season. when the NO bkgd y is assumed to be 50% (i.e., 0.1 ppbv) PAN Uncertainties [41] The data, simple mixing model, and associated analysis presented above are subject to uncertainties, so it is desirable to quantify approximate confidence limits for the results. In this section we estimate contributions to these confidence limits from two major sources: possible measurement uncertainties and the ambiguity of assigning single values for the backgrounds of CO and NO y for an entire field study. [42] The estimated instrumental accuracies given in Table 2 provide a starting point for assigning the confidence limits for the measurement uncertainty. However, we recognize that the measurement of these trace species from aircraft is difficult, and that measurement of NO y and its component species has proven to be a particularly challenging task for atmospheric chemists [e.g., Bradshaw et al., 1998]. The NASA Global Troposphere Experiment carried out a series of aircraft instrument comparisons [Gregory et al., 1990a, 1990b, 1990c, 1990d] to help quantify the uncertainties in the reactive nitrogen measurements. Here we review our measurements of these species to ensure that we derive conservative confidence limits. [43] The carbon monoxide measurement has been formally compared with two other instruments [Holloway et al., 2000]. These comparisons, plus other informal comparisons during several of these field programs [e.g., Nicks et al., 2003], indicate that the accuracy quoted for CO measurements in Table 2 provides a realistic confidence limit for that measurement. [44] When NO x, PAN, HNO 3 and NO y measurements are made simultaneously, they provide an internal consistency test of their accuracy; i.e., the sum of the three major component species should approximate NO y. At least near source regions, other NOy species make only minor contributions [see, e.g., Parrish et al., 1993] and the sum NO x + PAN + HNO 3 is expected to agree with NO y to within the respective uncertainties. Only during the TexAQS 2000 study were all of the three major components measured along with NO y in the FT. Ryerson et al. [2003] show that the three major components plus the measured C 1 -C 5 alkyl nitrates agreed with NO y to within 2.5%. Neuman et al. [2002] show that 90% of the NO x oxidation products (NO y - NO x ) are accounted for by HNO 3 + PAN + PPN. The C 1 -C 5 alkyl nitrates account for another 2%. However, these comparisons are dominated by CBL data, and the focus of this paper is on the FT. For the TexAQS 2000 FT data, we find that PANs + HNO 3 = (1.01 ± 0.08) * (NO y NO x )+ (0.02 ± 0.09) ppbv with an r 2 = 0.82, where 95% confidence limits are given. The agreement in all of these tests is well within the uncertainty expected from propagation of the measurement accuracies given in Table 2, and indicates that those uncertainties are realistic confidence limits for TexAQS Table 5. Estimated Percentages With Estimated Confidence Limits of NO x Emitted From Surface Fossil Fuel Sources Transported to the FT as Various Forms of NO y TEXAQS 2000, % SOS 1999, % STERAO 1996, % NARE 1997, % NARE 1996, % NO y 17 ± 5 27 ± 8 19 ± 11 9 ± 5 11 ± 6 SNO y /NO y 108 ± 20 NO x 2.6 ± ± ± ± ± 0.4 NO x /NO y 16 ± 2 12 ± 1 19 ± 8 8 ± 4 11 ± 4 PAN 4.3 ± ± ± ± ± 2.6 PAN/NO y 26 ± 3 27 ± 3 58 ± ± ± 21 HNO 3 11 ± 3 HNO 3 /NO y 67 ± 7 (61) a (23) a (57) a (47) a a Values in parentheses estimated by subtracting NO x and PAN contribution from 100%. 11 of 18

12 [45] For the SOS 1999 study there are HNO 3 data available only for the CBL. For these data we find the linear regression PAN + HNO 3 = (0.82 ± 0.03) * (NO y NO x ) (0.1 ± 0.2) ppbv with an r 2 = Higher PAN type compounds and alkyl nitrates account for another 2% of the NO x oxidation products. This agreement is again well within the uncertainty expected from propagation of the measurement accuracies. Although this test is for CBL data only, it does indicate that there are no large, unrecognized systematic measurement errors, and that the SOS 1999 measurement uncertainties also provide realistic confidence limits. [46] Nitric acid measurements are not available from the 1996 and 1997 studies, so such internal consistency tests are not possible for the first three field studies. We have informally compared the NO x and NO y measurements during some of these field studies. Problems were identified [Ryerson et al., 1998] and corrected [Ryerson et al., 1999, 2000] as a result, and we judge that the relatively large uncertainties given in Table 2 for these measurements in the 1996 and 1997 studies are realistic confidence limits. For the PAN measurements there are no independent tests, and Williams et al. [2000] report that no instrument comparisons are available. Lacking confirming tests for the relatively small measurement accuracies quoted in Table 2, we deem it prudent to consider the possibility that there might be unrecognized systematic errors in these measurements to assure that we reach defensible conclusions. In discussing the final conclusions regarding the PAN contribution to the NO y export, we will consider the implications of confidence limits of ±50% for PAN in the 1996 and 1997 studies. [47] The assignment of single values for the backgrounds of CO and NO y for an entire field study also contributes to the confidence limits that we can assign to the fractions derived in equations (4) (7). To quantify this contribution we recalculate for each field study the average fractions varying CO and NO y backgrounds by our estimates of their uncertainties: ±10 ppbv and ±0.1 ppbv, respectively. This calculation is done using the same points above the limits selected to define the plumes as indicated in Figure 7 (i.e., DCO > 30 ppbv). [48] We propagate the maximum fractional deviations found from this variation with the confidence limits for the two measurements appearing in each of equations (4) (7). The error bars in Figure 9 indicate the results, which we take as overall confidence limits on the percent of the originally emitted NO x exported to the FT in each chemical form. The major contributors are the uncertainty in the CO background and the measurement confidence limits, particularly for the larger values. [49] We have not addressed the additional uncertainty that comes from using a single, national average emission ratio for each field study. The national average emission ratio may have significant errors [see, e.g., Parrish et al., 2002], there are important temporal and regional differences in the average emission ratios [see, e.g., Marr et al., 2002], and the plumes encountered in any field study may constitute a biased sample of the regional emissions. We know of no defensible method to quantify these uncertainties, and have neglected them. Our only justification for this neglect is that the uncertainties arising from the two contributions discussed above are judged to be large enough to encompass the additional uncertainties arising from our use of average emission ratios. However, revisions of emission inventories may necessitate revisions of the quantitative results. For example, the most recent U.S. EPA tabulation available through their Web site ( trends/) gives total CO and NO x emissions for 1999 that are 20% larger and 7% lower, respectively, than those used in this work. [50] Finally, it is clear that the treatment presented here is highly simplified. This simplification potentially introduces unknown errors. For example, we concentrate only on plumes of transported pollutants that we identify by CO elevations of at least 30 ppbv above background, but we have not demonstrated that these relatively concentrated plumes really represent the average fate of CBL emissions. The real utility of our treatment here may lie in its guidance for examining relationships in the data and in providing a framework for comparing model results with measurements. A related paper [Li et al., 2004] discusses some of these comparisons. 4. Discussion 4.1. Estimates of NO y Exported From the CBL [51] For five field studies we have quantified the fraction of the originally emitted nitrogen oxides remaining in plumes of anthropogenic emissions that have been lofted to the FT. The lower histogram in Figure 9 and Table 5 show these results for the plumes sampled in each of five field studies. The seasonal fractions from the model of Horowitz et al. [1998] and Liang et al. [1998] are included for comparison in Figure 9. In each of the five field studies, the air parcels that we identify as plumes account for a significant fraction of the FT data collected: 20 31%. [52] In the analysis presented in the preceding section, we treated CO as an inert, conserved tracer. However, as Chin et al. [1994] discuss, CO has significant chemical sources and chemical sinks. They show that the net source of CO in the summertime CBL of the eastern United States is 18% higher than the emission flux. In the results presented in Figures 8 and 9, in Table 5, and in the quantitative discussion that follows, the derived values have been increased by 18% to approximately account for the nonconserved nature of CO. We have applied the same correction to the fall and springtime studies even though we have not modeled the seasonal variation of the net CO source. It is a relatively small correction, so ignoring the seasonal variation is not expected to introduce a large error. [53] Estimates for the fraction of the emitted NO x that is transported to the FT in any form of NO y ranged from 9% to 27% in the five field studies. The results from the three summertime studies, all conducted over the North American continent, agree within their estimated uncertainties (range 17 27%; weighted average ± 1 sigma confidence interval = 20 ± 5%). The fractions derived from the fall and spring NARE studies are smaller (9 ± 4%), but these two studies were primarily conducted over the Atlantic Ocean further removed from the source regions (Figure 2). This relative remoteness and the correspondingly different lofting mechanisms rather than the seasonal difference likely lead to the smaller fractions of transported NO y. Our next task is to consider these 12 of 18