JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 108, NO. D16, 4495, doi: /2003jd003383, 2003

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 108, NO. D16, 4495, doi: /2003jd003383, 2003 An examination of the chemistry of peroxycarboxylic nitric anhydrides and related volatile organic compounds during Texas Air Quality Study 2000 using ground-based measurements James M. Roberts, 1 Bertram T. Jobson, 2 William Kuster, 1 Paul Goldan, 1 Paul Murphy, 1 Eric Williams, 1 Gregory Frost, 1 Daniel Riemer, 3 Eric Apel, 4 Craig Stroud 4 Christine Wiedinmyer, 4 and Fred Fehsenfeld 1 Received 7 January 2003; revised 31 March 2003; accepted 1 May 2003; published 19 August [1] Measurements of peroxycarboxylic nitric anhydrides (PANs) along with related volatile organic compounds (VOCs) were made at the La Porte super site during the TexAQS 2000 Houston study. The PAN mixing ratios ranged up to 6.5 ppbv and were broadly correlated with O 3, characteristic of a highly polluted urban environment. The anthropogenic PAN homologue concentrations were generally consistent with those found in other urban environments; peroxypropionic nitric anhydride (PPN) averaged 15%, and peroxyisobutyric nitric anhydride (PiBN) averaged 3% of PAN. Some periods were noted where local petrochemical sources resulted in anomalous PANs chemistry. This effect was especially noticeable in the case of peroxyacrylic nitric anhydride (APAN) where local sources of 1,3-butadiene and acrolein resulted in APAN as high as 30% of PAN. Peroxymethacrylic nitric anhydride (MPAN) was a fairly minor constituent of the PANs except for two periods on 4 and 5 September when air masses from high biogenic hydrocarbons (BHC) areas were observed. BHC chemistry was not a factor in the highest ozone pollution episodes in Houston but may have an impact on daily average ozone levels in some circumstances. INDEX TERMS: 0317 Atmospheric Composition and Structure: Chemical kinetic and photochemical properties; 0345 Atmospheric Composition and Structure: Pollution urban and regional (0305); 0365 Atmospheric Composition and Structure: Troposphere composition and chemistry; KEYWORDS: PAN, peroxyacetyl nitrate, urban air pollution Citation: Roberts, J. M., et al., An examination of the chemistry of peroxycarboxylic nitric anhydrides and related volatile organic compounds during Texas Air Quality Study 2000 using ground-based measurements, J. Geophys. Res., 108(D16), 4495, doi: /2003jd003383, Introduction [2] Urban and regional ozone air pollution is a complex problem that has resisted a complete solution for many years [National Research Council, 1991]. There is a general understanding that ozone is produced in these environments through photochemically initiated reactions involving oxides of nitrogen (NO x = nitric oxide, NO + nitrogen dioxide, NO 2 ) and volatile organic carbon compounds (VOCs). An accurate description of the chemistry is a necessary part of any numerical model on which policy decisions will be based. Such a model will need to be 1 Aeronomy Laboratory, NOAA/ERL, and Cooperative Institute for Research in the Environmental Sciences, University of Colorado, Boulder, Colorado, USA. 2 Atmospheric Sciences Division, Pacific Northwest National Laboratories, Richland, Washington, USA. 3 Rosentiel School of Marine and Atmospheric Science, University of Miami, Miami, Florida, USA. 4 Atmospheric Chemistry Division, National Center for Atmospheric Research, Boulder, Colorado, USA. Copyright 2003 by the American Geophysical Union /03/2003JD flexible and inclusive enough to describe the photochemistry pertinent to a wide variety of source contributions. The Houston, Texas air quality is among the worst in the country, and this urban area is a particular challenge because of the intense petrochemical sources associated with it [Kleinman et al., 2002; Ryerson et al., 2003; Wert et al., 2003]. While this air quality problem may seem to be of local interest, it is a model for other urban areas around the world that are, or may become, impacted by similar petrochemical sources. [3] The photochemistry associated with VOCs can be generally described as a series of oxidation steps initiated primarily by hydroxyl radical (OH) and involving NO x [Atkinson, 2000]. Primary emissions, hydrocarbons and oxygenated hydrocarbons, are thus converted to products, oxygenated hydrocarbons and organic nitrates. The peroxycarboxylic nitric anhydrides {RC(O)OONO 2 }, or PANs (PAN is commonly referred to by its misnomer, peroxyacetyl nitrate) are a major class of product species. These compounds are formed only in the atmosphere and are closely related to the O 3 formation process [Roberts, 1990, 1995]. The relative concentrations of these compounds can be used to discern the nature of the VOCs that ACH 4-1

2 ACH 4-2 ROBERTS ET AL.: PANS DURING TEXAQS 2000 contribute to O 3 formation [Williams et al., 1997; Nouaime et al., 1998; Roberts et al., 1998, 2002]. [4] Measurements of the PANs and associated VOCs were made during August and September 2000 at the La Porte super site as part of the Texas Air Quality Study 2000 (TexAQS 2000). The C 2 through C 6 hydrocarbons (HC) and oxygenates and C 2 C 4 PANs were measured around the clock at this site, which was close to the Houston Ship Channel, a large area of petrochemical complexes east of downtown Houston. Several publications have dealt with small parts of the PANs chemistry observed during TexAQS 2000 [Roberts et al., 2001a; 2001b] including the observation of peroxyacrylic nitric anhydride (APAN) [Roberts et al., 2001a], and the use of a simple sequential reaction model to assess PAN chemistry and sources [Roberts et al., 2001b]. This publication will describe the overall systematics of the measurements compared to data from other urban sites. Compounds specific to biogenic hydrocarbon or petrochemical sources are used to discern the contributions of those HC sources. In this paper some simple models are applied to the data to see how well the ambient HC and oxygenate data agree with kinetic parameters (rate constants, branching ratios) as currently understood. The chemistry observed in the Houston 2000 study will be contrasted with that observed in the Nashville 1999 study. 2. Site Description and Experimental Approach [5] The La Porte super site (LPA) was established at the La Porte Municipal Airport ( N, W), shown in Figure 1. The gas phase measurements were made at the northwest corner of the site with inlets that were situated atop a 10m scaffold. The airport is located in suburban southeast Houston, approximately 2 to 5 km south of the Ship Channel petrochemical complex. [6] Methods for the measurement of PANs in this study are described by Williams et al. [2000] and Roberts et al. [2002] and consisted of capillary gas chromatographic separation followed by electron capture detection. Methods for the measurement of VOCs made in this study are described by Roberts et al. [2001b] and Apel et al. [2002]. Measurements of NO x,no y,o 3, and CO are described by Williams et al. [1998]. The PANs were measured every 15 minutes, and the VOCs every 20 minutes or every hour depending on the particular instrument used. 3. Results [7] The C 2 C 6 VOC measurements of interest in this work are summarized in Table 1. These particular compounds were selected because they are either direct precursors of PAN compounds of interest, or are the major source of that precursor. Thus compounds such as ethene, which is often a major source of reactive VOC in Houston [Ryerson et al., 2003; Wert et al., 2003] were not listed in Table 1. Table 1 shows that the maximum concentrations observed at LPA were much larger than the median concentrations observed in the EPA 39 cities study (Seila et al. [1989] as summarized by Seinfeld [1989]). The 39 cities study was used for comparison here because it provides a wide survey of urban VOC data. A more thorough discussion of the VOC measurements at La Porte during TexAQS 2000 will be provided by Karl et al. [2003], B.T. Jobson et al. (manuscript in preparation, 2003), and W.C. Kuster (manuscript in preparation, 2003). Another thing to note in Table 1 is that while the median mixing ratios of the alkenes; propene, and 1-butene, which are precursors of PAN and PPN, respectively, are not particularly high at La Porte relative to other urban areas, the maximum observations are. The maximum isoprene is also higher than that observed in either forested or urban sites in North America. This observation consisted of only one point associated with a local petrochemical source and had not been photochemically reacted (see below). In general, the first-generation products of isoprene, methacrolein and methyl vinyl ketone, are lower at the LPA site than at other sites [Stroud et al., 2001, and references therein]. [8] A statistical summary of the PANs data set was given by Roberts et al. [2001a] and will not be repeated here. Additional information can be gleaned from the plots of individual PAN derivatives against PAN, shown in Figure 2 for the hours 1000 to 1900 LST. This time window is used because it is the period during which the ground data are most characteristic of the planetary boundary layer (PBL) as a whole. [9] Figure 2a shows the plot of PPN versus PAN wherein a high degree of correlation is seen, with the slope of the correlation of PPN with PAN for LPA (0.154 ± 0.002) in fair agreement with that observed in Nashville 1999 (0.137 ± 0.001) [Roberts et al., 2002]. Measurements of PAN and PPN made in March 1984 at a site 15 km WNW of La Porte were reported by Singh and Salas [1989] to have a ratio of average PPN to average PAN of However, PPN was assumed to have the same response as PAN in their system, an assumption that likely resulted in systematically low PPN measurements according to the work of Roumelis and Glavas [1989]. The trend of PPN/PAN observed at LaPorte of roughly 0.15 is consistent with a number of other studies in urban areas in North America [Roberts et al., 1998; Grosjean et al., 2001] and is reflective of a more or less consistent mixture of anthropogenic hydrocarbons (AHC) that arise mostly from mobile sources [Seinfeld, 1989; Parrish et al., 1998]. Two different groups of points in Figure 2a are denoted separately as exceptions; points observed from 1345 to 1515 LST on 30 August 2000 (open circles), and points observed from 1345 to 1730 LST on 31 August 2000 (open squares). It is hypothesized that these air masses reflect HC sources different from the North American urban average. [10] The correlation of APAN with PAN is shown in Figure 2b. To our knowledge only one other data set has been reported for this compound [Tanimoto, 2001; Tanimoto and Akimoto, 2001], which showed APAN/PAN between 0.01 (long-dashed line) and 0.04 (short-dashed line). Much of the LPA data fell roughly within this range, however a branch of points along the 0.17 line (solid line) was also observed. The highest APAN was 0.30 of PAN. It is not possible to conclude from this data set what a North American urban average APAN/PAN ratio is since; the Houston environment is recognized to be highly perturbed by sources of the two main APAN precursors, 1,3-butadiene and acrolein [Roberts et al., 2001a], and APAN probably has a significant reaction rate with OH radical (see below). Our previous GC/ECD analyses in Nashville during the

3 ROBERTS ET AL.: PANS DURING TEXAQS 2000 ACH 4-3 Figure 1. A map of the Houston-Galveston Bay region. Sites associated with the TEXAQS 2000 study are shown as crossed squares, and the NOx point sources operating in the region are shown as circles sized according to emission rate. The coastline and major highways are shown as solid lines, and the boundaries of minor bodies of waters such as the ship channel are shown as shaded lines campaign Roberts et al. [2002] shows the presence of APAN, but at a low level, 0.01 of PAN. [11] The correlation of PiBN with PAN is shown in Figure 2c. The solid line is the linear-least squares fit to the TexAQS 2000 data (0.03 ± ) and the dashed line is the fit from the Nashville 1999 data (0.024 ± ) [Roberts et al., 2002]. There are a number of points, unusually high in PiBN relative to PAN, that were observed at individual times, i.e., not during one continuous time period. The implication is that these points were derived from air masses that had been impacted by particular sources high in PiBN precursors, such as isobutane, relative to the other AHCs. Isobutane does indeed stand out in a number of instances from the profile expected of standard urban AHC (see below). Likely sources of these branchedchain alkanes, and therefore the PiBN anomaly, are liquefied petroleum gas (LPG) storage, handling, and transport, and natural gas production and processing. [12] The correlation of MPAN with PAN is shown in Figure 2d. In general MPAN mixing ratios were fairly low relative to PAN in comparison to higher biogenic hydrocarbon (BHC) areas such as Nashville. Exceptions to this were the afternoons of 9/4 and 9/5 during which the MPAN/ PAN were close to the range of , which has been found to be characteristic of air masses with a high degree of isoprene impact [Roberts et al., 1998, 2002]. The highest

4 ACH 4-4 ROBERTS ET AL.: PANS DURING TEXAQS 2000 Table 1. Summary of Selected VOC Compounds Measured at the La Porte Site Compound Max a Min Average Standard Deviation Median N b 39 Cities c Propene Butane Isobutane Isopentane Butene d Acetaldehyde Propanal ,3-Butadiene < Acrolein 6.81 < Isoprene Methacrolein Methyl vinyl ketone a Mixing Ratios in ppbv. b Number of observations. c Median from the 39 Cities Study of Seinfeld as discussed by Seila et al. [1989]. d Listed as the sum of 1-butene and 2-methyl propene. MPAN mixing ratios were just over 200 pptv whereas those measured around Nashville were well over 300 pptv [Williams et al., 1997; Nouaime et al., 1998; Roberts et al., 1998]. 4. Discussion [13] The concentrations of PANs that are measured at a given site are a reflection of the abundance of precursor VOCs and NO x, and the degree of photochemical processing of those precursors. This section will explore how variations in precursors affected the absolute and relative amounts of the different PAN compounds. Part of this analysis will involve comparing and contrasting the VOC chemistry observed in Houston with that observed in other urban areas, especially that of Nashville, TN. In the process of this discussion, an assessment will be made of the impact of biogenic isoprene on Houston regional air quality. Figure 2. Plots of (a) PPN, (b) APAN, (c) PiBN, and (d) MPAN versus PAN. The solid circles are data measured between 1000 and 1900 LST. In Figure 2a the open circles were from 1345 to 1515 LST on 30 August, the open squares were from 1345 to 1730 LST on 31 August, the solid line is the linear least squares fit to the TexAQS data, and the dashed line is the linear least squares fit to the Nashville 1999 data. In Figure 2b the solid line is drawn through the population of points that were highest in APAN and is 0.17, the short-dashed line is 0.04, and the long-dashed line is In Figure 2c the solid line is the linear least squares fit to the TexAQS 2000 data, and the dashed line is the linear least squares fit to the Nashville 1999 data. In Figure 2d the open triangles are from 4 and 5 September, and the shaded sector is the range MPAN/PAN.

5 ROBERTS ET AL.: PANS DURING TEXAQS 2000 ACH 4-5 Figure 3. A schematic representation of the VOC-NO x chemistry leading to the formation of PAN compounds. [14] A schematic representation of PAN formation chemistry is shown in Figure 3. The pathways can be broadly distinguished by those that proceed via the simple aldehyde precursor (RCHO), and those that proceed via dicarbonyls or difunctional radicals. An example of the former would be the OH-initiated photooxidation of ethane and an example of the latter would be the photolysis of methyl glyoxal. Another general aspect of PANs formation chemistry is that the larger and more complicated the organic backbone of the molecule, the fewer are the possible sources. There are many possible sources of peroxyacetic nitric anhydride both AHC and BHC (chiefly isoprene), but there are many fewer possible precursors for a compound such as MPAN. In fact methacrolein is probably the only significant one of any abundance in the atmosphere. While the presence of both organic and NO x precursors are necessary for production of the corresponding PAN compound, it is not sufficient. PAN compounds result from photochemical processing that by its nature produces O 3. As a consequence, PAN and O 3 are highly correlated in areas where this chemistry is active [Roberts, 1995; Roberts et al., 1998; 2001b; Williams et al., 1997]. [15] Every oxidation of NO to NO 2 by an organic radical shown in Figure 3 results in the formation of one O 3 molecule, thus it is clear that NO x has a prominent role in the production of O 3.NO x measurements at LaPorte showed a wide range, from below 1, to more than several hundred, ppbv. Local sources often raised the nighttime NOx levels to close to 100 ppbv, however, the mixing of the nighttime boundary layer upon sunrise resulted in daytime PBL mixing ratios generally in the range of 5 to 30 ppbv. The details of the ozone production chemistry are beyond the scope of this work and will be discussed in future articles (G. Frost manuscript in preparation, 2003). However, it is useful to note that the intense O 3 formation episodes at LaPorte fall into the urban/suburban O 3 production category described by Chameides et al. [1992] that is limited by the availability of reactive VOC. This feature will become important when the contribution of isoprene to O 3 formation is considered below. [16] The relationship between PPN and PAN shown in Figure 2a can be viewed in the context of the PANs formation chemistry. The simple aldehyde precursors of PAN and PPN will be considered as a starting point since the results of Roberts et al. [2001b] implied that AHC chemistry was dominant at La Porte during TexAQS A plot of propanal versus acetaldehyde is shown in Figure 4 divided by time of day, being characteristic of the well-mixed daytime boundary layer. There was not a large difference in day and night points in Figure 4 perhaps because these compounds have both primary and secondary sources. The points from the unusual time periods noted in Figure 2 are also highlighted. The data points in Figure 4 are clustered roughly about the 1:10 line. The periods of higher PPN:PAN correspond to higher propanal:acetaldehyde, conversely the periods of lower PPN:PAN correspond to lower propanal:acetaldehyde. [17] The formation of PANs from the simple aldehydes involves the reaction with OH to form the peroxyacyl radical [RC(O)OO], and both the formation and removal of the PANs are mediated by NO x as shown in Figure 3. This has been treated quantitatively by Roberts et al. [2001b] who used a simple sequential reaction model to relate the ratios of PPN:propanal to those of PAN:acetaldehyde. The La Porte data were found to agree with the model to within ±50% as shown in Figure 5. Figure 5 also shows a distinct difference between the day, LST, and night points, because the higher degree of photochemical processing happening in the daytime. These features strongly imply that the main sources of PPN and PAN in this environment were the simple aldehydes. The loss of PPN and PAN is primarily by thermal decomposition in this environment, and to the first approximation this happens at the same rate for both PANs. Consequently, the ratio of PPN:PAN should be a reflection of the formation chemistry. The ratio of propanal to acetaldehyde of roughly 0.10 (Figure 4), and three separate measurements of the rate

6 ACH 4-6 ROBERTS ET AL.: PANS DURING TEXAQS 2000 methacrolein and MPAN [Roberts et al., 2001b]. The rate of the first reaction, that of acrolein with OH, is well known [Ullerstam et al., 2001; Magneron et al., 2002]. The branching ratio (k 1a /(k 1a +k 1b ) between aldehyde-h abstraction and addition to the double bond as shown in Equations 1a and 1b; CH 2 ¼ CH CHO þ OH! CH 2 ¼ CH CO þ H 2 O ð1aþ CH 2 ¼ CH CHO þ OH þ O 2! HOCH 2 CðOOÞH CHO ð1bþ was recently estimated to be no more than 80% [Magneron et al., 2002]. However, a more definitive study by Orlando and Tyndall [2002] has determined it to be 68%. The hydrogen abstraction channel leads to APAN formation; CH 2 ¼ CH CO þ O 2! CH 2 ¼ CH CðOÞOO ð2þ Figure 4. The relationship between propanal and acetaldehyde measured at the La Porte site. The solid points are for the hours LST, the shaded points are for the hours LST, and the open circles and squares are for the same time periods specified in Figure 2a. The line shows the 1:10 relationship. constants of the OH reaction with propanal and acetaldehyde that gave ratios of 1.4 to 1.53 [Niki et al., 1978; Kerr and Sheppard, 1981; Semmes et al., 1985], would imply that the average ratio of ambient PPN to PAN should be 0.14 to 0.15, which is in reasonable agreement with the observations shown in Figure 2. [18] The populations of points that were unusual in Figures 2a and 2d are shown plotted on the correlation of PPN/propanal with PAN/acetaldehyde in Figure 5. The points corresponding to the time periods selected in Figure 2a lie in a group at the top right portion of the distribution, but otherwise there is no significant separation between the points observed on 30 August and those observed on 31 August. The points corresponding to the time periods selected in Figure 2d all lie on the lower right side of the model line, which is consistent with the expected impact of isoprene sources on this chemistry, but the effect is not nearly as large as that observed in the Nashville 1999 measurements made at Cornelia Fort Airpark (hereafter referred to as CFA 99) [Roberts et al., 2001b]. [19] The fractional abundance of APAN relative to PAN at La Porte was much larger at times than the data sets described by Tanimoto [2001] and Tanimoto and Akimoto [2001] due to the local sources of 1,3-butadiene and acrolein [Roberts et al., 2001a; Texas Natural Resource Conservation Commission (TNRCC), 2000]. It is interesting to see if the ratio of APAN to acrolein is correlated with PAN/acetaldehyde, and to what extent the distribution of those ratios agrees with known chemistry. The application of the sequential reaction model to acrolein and APAN requires knowledge of several reaction rate coefficients and branching ratios, in the manner directly analogous to CH 2 ¼ CH CðOÞOO þ NO 2! CH 2 ¼ CH CðOÞOONO 2 ð3aþ The loss of APAN occurs either through thermal decomposition; CH 2 ¼ CH CðOÞOONO 2! CH 2 ¼ CH CðOÞOO þ NO 2 ð3bþ Figure 5. The relationship between PPN/propanal ratios and the PAN/acetaldehyde ratios measured at the La Porte site. The solid points are for the hours LST, the shaded points are for the hours LST, and the open circles, squares, and triangles are for the same time periods specified in Figure 2. The solid squares are model calculations for the base case discussed by Roberts et al. [2001b], and the solid triangles are model calculations for the La Porte daytime average conditions also discussed by Roberts et al. [2001b].

7 ROBERTS ET AL.: PANS DURING TEXAQS 2000 ACH 4-7 Figure 6. The ratio APAN/acrolein plotted versus PAN/ acetaldehyde. The open circles are for the time period LST, and the shaded dots are for LST. The solid circles are for the model base case, and the solid triangles are for the La Porte average conditions both described by Roberts et al. [2001b]. or, because of the unsaturated nature of the compound, reaction with OH radical; not likely to be due to errors in the above outlined chemistry (i.e., rates and branching ratios), which are almost certainly not off by more than 50% or so. This disagreement between the modeled lines and measurements are probably due to the fact that one of the major model assumptions, that the parent compounds are of common origin, is not true in this case. This can be taken as further evidence of the importance of the major point sources of 1,3-butadiene and acrolein impacting this environment. [21] Another possible reason for the discrepancy in Figure 6 is that there are other sources of peroxyacrylyl (APA) radicals in this environment. It is not clear what those sources could be since they would have to have source strengths greater than 1,3-butadiene or acrolein, or be much more reactive. Moreover, the compound or compounds would have to produce APA radicals directly, i.e., not through the intermediate acrolein. Since 1,3-butadiene is close to the limit of reactivity, and there are not any observations or emissions inventory data that indicate large sources of possible precursors, we reject the hypothesis that there are other sources of APA radicals in this environment. [22] The PiBN versus PAN plot in Figure 2c implies that there are usually high sources of organics that produce PiBN operating in the Houston environment. The simplest source of PiBN would be isobutane, which is normally associated with vehicle exhaust in urban North America. A plot of isobutane versus n-butane is shown in Figure 7 for both the La Porte 2000 data set and for measurements made aboard the NOAA WP3 aircraft during the 1999 Middle Tennessee Ozone Study. The 1999 data are clustered along the line: isobutane = 0.45 n-butane, that has been proposed by Parrish et al. [1998] as characteristic of urban VOC CH 2 ¼CH CðOÞOONO 2 þ OH þ O 2! HOCH 2 CðOOÞH CðOÞOONO 2 ð4þ The thermal decomposition rate has been measured to be essentially the same as that of PAN at 298K, although the Arrhenius parameters obtained by Grosjean et al. [1994] appear incorrect. On the basis of the preponderance of evidence in the literature [Kirchner et al., 1999, and references therein], the thermal decomposition of APAN should have roughly the same Arrhenius parameters as that of PAN. To our knowledge the rate constant for the reaction of APAN with OH has not been measured. However, it is estimated here to be based on the rate constant for MPAN + OH, [Orlando et al., 2002], and the ratio of estimated rate constants for OH reaction with APAN and MPAN of 0.51, from the method of Kwok and Atkinson [1995]. This value for APAN + OH is in the middle of the range estimated by Orlando and Tyndall [2002]. [20] The aldehyde > PAN sequential model [Roberts et al., 2001b] can be applied to the acrolein and APAN data using the above kinetic data. Figure 6 shows the plot of APAN/acrolein versus PAN/acetaldehyde along with the sequential reaction model lines for the same conditions used in Figure 5. Both day and night periods are shown and both have a significant number of points that are factors of 10 to 20 away from the model line. This large deviation is Figure 7. The mixing ratios of isobutane versus those of n-butane measured both at La Porte (solid dots) and aboard the NOAA WP3 (shaded dots) during the Nashville 1999 intensive experiment. The square is the medians from the 39 cities study (see Table 1). The dashed line is the 0.45:1 line, and the solid line is the 1:1 line.

8 ACH 4-8 ROBERTS ET AL.: PANS DURING TEXAQS 2000 Table 2. Comparison and CFA and La Porte Data Sets for Isoprene and Related Products a CFA La Porte Compound Maximum Average Median Maximum Average Median Isoprene MVK MACR MPAN b a All mixing ratios in ppbv. b Daytime periods only were considered for MPAN; LST at CFA and LST at La Porte. sources, and is consistent with the medians from the 39 Cities Study, also shown in Figure 7. The La Porte data are higher than the 1999 data in absolute mixing ratios and in the ratio of isobutane to n-butane. The emissions inventories for the Houston-Galveston area list a number of isobutane sources near the La Porte site, mostly associated with liquefied natural gas handling and processing [TNRCC, 2000]. Other 2-methyl alkanes or methyl alkenes could also be precursors to PiBN, and we note that isopentane, for example, is also relatively high at La Porte at times (Table 1). It is not possible to demonstrate a definitive relationship between elevated branched alkane and higher PiBN/PAN because we are lacking a measure of photochemical processing and measurements of key intermediates, e.g., isobutanal. However it is consistent with our understanding of the chemistry of 2-methyl alkane chemistry that the Houston Figure 9. The ratio MPAN/MACR plotted versus PAN/ acetaldehyde for the time period LST (open circles). The solid circles are for the model base case, the solid triangles are for the La Porte average conditions, and the crosses denote the trend observed for the CFA99 data described by Roberts et al. [2001b]. Figure 8. The ratio MVK/isoprene versus MACR/isoprene measured at La Porte during the period (open circles) and the period (shaded circles). Also shown are the CFA (open diamonds) and SOS ROSE I (open squares) data sets [Stroud et al., 2001], plotted in eight bins, with error bars denoting the extent (max and min) of each bin on the horizontal scale, and the standard deviation of each bin on the vertical scale. The solid line is the result of the sequential reaction model. area would be higher in PiBN/PAN than the typical urban North American atmosphere. [23] The chemistry of MPAN has been the subject of some discussion since it is a unique product of isoprene photooxidation [Bertman and Roberts, 1991; Williams et al., 1997; Nouaime et al., 1998; Roberts et al., 1998] and isoprene is the most important biogenic hydrocarbon in many environments. The environment around Houston has some significant areas of biogenic isoprene emissions [Wiedinmyer et al., 2001] and it also has isolated petrochemical sources of isoprene. The relationships of isoprene and its first generation products, methacrolein (MACR) and methyl vinyl ketone (MVK) will be used below to examine the extent of photochemical processing that isoprene has undergone. [24] The maximum, average, and median mixing ratios of isoprene, MVK, MACR and MPAN are summarized in Table 2 for both the CFA and La Porte data sets. La Porte was lower than CFA for every compound in each category, with the exception of the maximum isoprene. Median isoprene concentrations were more than a factor of two lower, and the medians of the concentrations of the isoprene product species were factors of 3 to 4 lower at La Porte. [25] Isoprene and its first generation products were modeled by Stroud et al. [2001] using the sequential reaction model. Application of this analysis to the La Porte data is shown in Figure 8, along with the previous data sets. The agreement of the measurements with the model line is quite good for all the data sets, and in the case of the La Porte data confirms that the isoprene photochemistry in this environment matches that expected from reaction rate con-

9 ROBERTS ET AL.: PANS DURING TEXAQS 2000 ACH 4-9 Figure 10. Isentropic backtrajectories for air masses that arrived at the La Porte site between the hours of 1600 and 1900 LST on 5 September, plotted on a map of the Houston-Galveston are. Also shown are the biogenic isoprene area emissions estimated according to the method of Wiedinmyer et al. [2001]. stants and branching ratios. This also strongly indicates that there are no other significant sources high in either MACR or MVK operating in this environment. The single point in the lower left corner of Figure 8 corresponds to the isoprene plume observed at 0956 LST on 23 August The position of this point means that hardly any photooxidation had taken place in this plume. There is a slight systematic difference between the day ( LST) and night data points form La Porte, consistent with the expected effect of the slight processing of isoprene and products by O 3, and the absence of OH [Stroud, 2000]. A consistent picture emerges concerning the general differences in isoprene, MACR and MVK between the CFA and La Porte data sets. Petrochemical sources of isoprene while occasionally observed in Houston have a small impact the photochemistry at La Porte. Biogenic sources of isoprene at La Porte were less important in general, compared to those that impacted Nashville, as indicated by the CFA99 data. [26] The sequential reaction model can be applied to the MPAN-MACR system in the manner described above for APAN-ACR and described for the CFA99 data by Roberts et al. [2001b]. The resulting plot of MPAN/MACR versus PAN/Acetaldehyde is shown in Figure 9 along with the model lines for the same conditions used in Figures 5 and 6. The MPAN/MACR ratio has a much narrower range than the APAN/ACR plot, and the points are all within x3 of the model line. This in is contrast to the MPAN/MACR versus PAN/Acetaldehyde plot for the CFA 99 data set [Roberts et al., 2001b] in which there was significant deviation from the model line, especially at low ratios, due to PAN production from isoprene. If we assume that the poor agreement of the APAN/ACR ratio with the model line (x10 20 differences) is due the fact that ACR has local petrochemical point sources, then the rough agreement of the MPAN/MACR ratio with the model line implies that MPAN and MACR have mostly area sources as was the case in the CFA 99 analysis.

10 ACH 4-10 ROBERTS ET AL.: PANS DURING TEXAQS 2000 Figure 11. The timeline for PAN (open circles), MPAN (open diamonds), NO x (small open circles), and O 3 (line) measurements at La Porte during the 4 5 September 2000 period. Figure 12. The timeline for PAN (open circles), MPAN (open diamonds), NO x (small open circles), and O 3 (line) measurements at La Porte during the 30 August 2000 high O 3 episode.

11 ROBERTS ET AL.: PANS DURING TEXAQS 2000 ACH 4-11 [27] The observations that had the highest MPAN/PAN ratios (triangles Figure 2d) observed at La Porte corresponded to two periods on 4 and 5 September Backtrajectories for the 5 September period are shown in Figure 10 along with estimates of isoprene emissions. The trajectories indicate that the measured air masses passed over an area along the Trinity River that has a relatively high concentration of isoprene-emitting vegetation. Note that these trajectories largely bypass the Houston Ship Channel sources. The timeline of PAN and MPAN on 4 and 5 September is shown in Figure 11 along with the corresponding NOx and ozone mixing ratio. In general daytime NOx is in range of a few ppbv to a few tens of ppbv, corresponding to a situation where O 3 production will be sensitive to reactive VOC. Time periods when MPAN is 0.1 to 0.2 of PAN are shaded in Figure 11 and correspond to ozone mixing ratios in the range of 50 to 110 ppbv and the highest MPAN/PAN corresponds to ppbv O 3. While isoprene chemistry is certainly contributing to O 3 production in these air masses, the O 3 is less than the 1 hr air quality standard of 120 ppbv. A change to the 8-hour standard of 80 ppbv would mean that some of these isoprene-impacted periods such as that on 4 September would be close to violation. The most intense O 3 period at La Porte was observed on 30 August 2000 and is shown in Figure 12 along with the corresponding PAN and MPAN. During this period, PAN and O 3 are highly correlated, but MPAN was at most a few percent of PAN. We conclude that isoprene photochemistry is not the cause of the rapid and intense O 3 formation at La Porte. 5. Summary and Conclusions [28] The PAN compounds, PAN, APAN, PPN, PiBN, and MPAN were measured at La Porte, Texas, during August and September 2000, as part of the TexAQS 2000 experiment. PAN was broadly correlated with ozone due to their common photochemical origin. The compounds PPN, PiBN, and especially APAN showed some variation relative to typical North American urban values due to local petrochemical sources of VOCs. PPN values that were higher and lower relative to PAN were related to higher and lower concentrations of propanal, respectively, relative to a typical propanal:acetaldehyde of 1:10. Anomalously high PiBN values were observed at times probably due to higher than average isobutane/butane ratios, or related effects of local liquefied natural gas handling. The unusually high APAN values observed in this data set have been attributed to 1,3-butadiene and acrolein sources [Roberts et al., 2001a]. [29] The chemistry of MPAN in this environment was examined with respect to the possible role of isoprene in Houston ozone photochemistry. Although Houston has both biogenic and some petrochemical sources of isoprene, the former had only limited impact on the ozone observed at La Porte during this study and the later were observed on only one occasion in the form of a chemically-immature plume (i.e., a plume in which high isoprene but low product concentrations were observed). The limited periods when isoprene chemistry was most evident, identified using the ratio MPAN/PAN, did not correspond to violations of the 1 hour 120 ppbv air quality standard. However, isoprene sources close to Houston may be a significant factor in the ability of this urban area to meet the proposed 8 hour, 80 ppbv air quality standard. [30] Acknowledgments. We thank the City of La Porte, Texas for the use of their Municipal Airport. 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