An examination of the chemistry of ... - Wiley Online Library

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 108, NO. D16, 4495, doi:10.1029/2003JD003383, 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 Stroud4 Christine Wiedinmyer,4 and Fred Fehsenfeld1 Received 7 January 2003; revised 31 March 2003; accepted 1 May 2003; published 19 August 2003.

[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 O3, 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 INDEX TERMS: 0317 Atmospheric Composition and ozone levels in some circumstances. 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:10.1029/2003JD003383, 2003.

1. 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 (NOx = nitric oxide, NO + nitrogen dioxide, NO2) 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. 0148-0227/03/2003JD003383

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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 NOx [Atkinson, 2000]. Primary emissions, hydrocarbons and oxygenated hydrocarbons, are thus converted to products, oxygenated hydrocarbons and organic nitrates. The peroxycarboxylic nitric anhydrides {RC(O)OONO2}, 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 O3 formation process [Roberts, 1990, 1995]. The relative concentrations of these compounds can be used to discern the nature of the VOCs that

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ROBERTS ET AL.: PANS DURING TEXAQS 2000

contribute to O3 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 C2 through C6 hydrocarbons (HC) and oxygenates and C2 – C4 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 (29.669°N, 95.064°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 NOx, NOy , O3, 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 C2 – C6 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 0.06. 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


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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.

1999 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 ± 0.0005) and the dashed line is the fit from the Nashville 1999 data (0.024 ± 0.0003) [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 0.1 –0.2, which has been found to be characteristic of air masses with a high degree of isoprene impact [Roberts et al., 1998, 2002]. The highest

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Table 1. Summary of Selected VOC Compounds Measured at the La Porte Site Compound

Maxa

Min

Average

Standard Deviation

Median

Nb

39 Citiesc

Propene Butane Isobutane Isopentane 1-Butene Acetaldehyde Propanal 1,3-Butadiene Acrolein Isoprene Methacrolein Methyl vinyl ketone

111 18.4 71.6 30.7 41.7 22.2 4.2 16.67 6.81 33.9 1.16 2.26

0.012 0.079 0.074 0.060 0.002 0.49 0.041