ARIZONA DEPARTMENT OF ENVIRONMENTAL QUALITY

Transcription

1 ARIZONA DEPARTMENT OF ENVIRONMENTAL QUALITY MEMORANDUM AQD: AAS:EVAL:PGH: TO: FROM: RE: Mike George, Manager, Assessment Section Peter Hyde Organic compounds in the ambient air of Douglas/Agua Prieta DATE: July 6, 2000 cc Ira Domsky Tom Moore Randy Sedlacek Dan Catlin Summary Gaseous and semi-volatile particulate organic compounds -- consisting of nonmethane hydrocarbons (NMHC), oxygenated hydrocarbons (OHC), (the sum of which is termed volatile organic compounds (VOC), halogenated hydrocarbons (HHC), and polycyclic aromatic hydrocarbons (PAH) -- were sampled every sixth day for 24 hours from January 1999 to February 2000 at five sites in Douglas, Arizona and Agua Prieta, Sonora. The most complete organic compound data set ever collected in Arizona, these concentrations in the two border cities, especially Agua Prieta, showed moderately to extremely elevated levels above the background concentrations. 1

2 Nonmethane hydrocarbons (NMHC) Introduction Nonmethane hydrocarbons consisted of about 150 hydrogen-carbon compounds with carbon numbers two to 12, all sampled for 24 hours starting at 1100 hours in canisters every sixth day, and analyzed by gas chromatography. This section examines their site-to-site, seasonal, and day-to-day variability in total NMHC concentration and in their composition for the period of January mid-february Their concentrations are related to probable emission sources through target compounds and compound ratios and are compared with NMHC concentrations from other Arizona studies. The monitoring sites, described fully in the study protocol, are given in Table 1. Table 1. Douglas/Agua Prieta air monitoring sites name description and remarks cfe Agua Prieta -- electric plant southwestern corner of the city, near dump, shanty town, and subject to emissions from insulated wire burning; receptor for the entire two cities emissions under winds from the north fire Agua Prieta -- fire station close to the border and border crossing, in the heart of the commercial district, close to the malquiladora manufacturing zone cem Douglas -- cemetery close to the border, centrally located, subject to emissions from the dragging of the unpaved road parallel to the border in the U.S. adot Douglas -- ADOT yard about two miles north of the border, at the north edge of Douglas vortac Douglas -- Vortac background site, at Douglas airport, 10 miles north of the border Long-term averages, time series, seasonal variation, and composition Average long-term concentrations of NMHC -- only those identified by the chemical analysis (a portion of the total NMHC is not identified) -- and other statistics are presented in Table 2. 2

3 Table 2. Identified NMHC in Douglas/Agua Prieta (Units are ppbc) fire cfe cem adot vortac avg std min max n coef var These long-term averages point out that the concentrations are highly variable by site, with the fire site elevated 30 times above background; cfe, 20 times; cem, 6 times, and adot, 5 times. The relatively high coefficients of variation (the standard deviation divided by the average) suggest high sample-to-sample variability. The wide ranges (difference between the minimum and maximum) reveal that this airshed has both low and extremely high NMHC concentrations. These NMHC concentrations, which can be divided into their three principal structural groups, are shown as Figure 1.: paraffins: straight, branched chain, and cyclic compounds with only single bonds, such as ethane, propane, 2,2-dimethylpentane olefins:straight, branched chain, and cyclic compounds with one or more double bonds, such as ethene, 1,3-butadiene, and isoprene, and aromatics: compounds with a benzene ring, such as benzene, toluene, and xylene. 3

4 p p b C olefi para arom Figure 1. Long-term averages of the three principal groups of nonmethane hydrocarbons 0 In contrast to their variable magnitudes, the three structural groups composed nearly equal percentages of the total at the five sites: paraffins to 65%; olefins, 17 to 20%; and aromatics, 20 to 24%. fire cfe cem adot vortac With the exception of the background site, NMHC concentrations varied considerably from sample day to sample day. By far the highest concentrations were recorded at the two Agua Prieta sites on June 29 and November 26: over 4000 ppbc at the fire site and over 1500 ppbc at the cfe site (Figure 2). 4

5 p p b C Figure 2. 0 Time series of nonmethane hydrocarbons at the five sites 01/06/99 02/17/99 03/31/99 05/12/99 06/23/99 08/10/99 09/21/99 11/02/99 12/14/99 adot cfe cem fire vo 5

6 p p b C Figure 3. Seasonal variation of nonmethane hydrocarbons The seasonal variation 0 of NMHC was similar at the four urban sites, while the background site of vortac varied hardly at all. For the coldest months, November through January, the concentrations ranged from 60 to 90% above the February - April concentrations. The May - July and August - October periods were also higher than the February - April concentrations at all sites, except for the cemetary in May - July (Figure 3). adot cfe cem fire vo 11,12,1 2,3,4 5,6,7 8,9,1 The contribution of individual compounds to the total NMHC was similar at the four urban sites, with the following compounds contributing from 56 to 62% of the total: propane, toluene, butane, isopentane, acetylene, m-p-xylene, pentane, ethene, and ethane. In Table 3, the top ten contributing compounds at the fire site are given, with their corresponding ranking at the other three urban sites. 6

7 Table 3. The ten most abundant species at the Agua Prieta fire site, with corresponding rankings at the other three urban sites compound fire cfe cem adot n-propane toluene n-butane isopentane acetylene m-p-xylene limonene pentane ethene methylpentane cfe: #5, 1-decene, #8, ethane, #9, 2,3,5-trimethylhexane cem: #5, ethane, #10, benzene adot: #5, ethane, #9, hexane As this table demonstrates, the most important compounds at the highest concentration site, the fire site, are common to the other three urban sites, with the exception of limonene, and, to a lesser extent, 2-methylpentane. While ethane is the fifth or the eighth highest contributor at the other three sites, at the fire site it ranks twelfth. This commonality of the most abundant species among sites applies to the top 25 compounds as well, as indicated in Figure 4, with the exceptions being alpha-pinene, ethane, isobutane, limonene, and hexane. 7

8 o r d e r o f a b u n d a n c e wide range: importance differs widely by site narrow range: compound of near equal importance at all sites 20 Figure Rank order of compounds at the four urban Douglas/Agua Prieta sites 0 a_pine acetyl bz124m etbz benze ethane Emissions and indicators ethene hexa2m hexa3m Emissions of some of these key compounds come from the following sources (Stoeckenius, 1994) (Table 4). In addition, ratios of ambient concentrations of certain compounds can be instructive (Fujita, 1997). The ratio of xylenes to benzene is indicative of the net fractional loss of the reactive compounds (xylene), in comparison with nonreactive compounds (benzene). The ratio of ethane to acetylene has been measured since the 1980's and is indicative of the degree of catalyst-equipped vehicles in the fleet. Ambient ratios of 1,2,4-trimethylbenzene to undecane and decane, and decane to undecane ratios can be compared with the ratios in diesel exhaust, vehicular exhaust dominated by gasoline vehicles, and other sources. Some of these ratios for Douglas/Agua Prieta are given in Table 5, along with their reported values. i_buta ipenta limon m_etol mcypna mp_xyl n_buta n_hex n_pent n_prop o_xyl pa224m pen23m prope pena2m tolue pena3m 8

9 Table 4. Emission sources of key compounds compound major sources comments acetylene mobile sources and combustion tracer for vehicle exhaust ethane mobile sources tracer for vehicle exhaust ethane natural gas non-reactive propane liquid propane and natural gas relatively non-reactive isobutane consumer products, gasoline evaporative emissions replacement of CFCs in consumer products butane gasoline evaporative emissions tracer of gasoline use benzene motor vehicle exhaust and combustion tracer for motor vehicle exhaust toluene solvent use and mobile sources one of the most abundant species in urban air internal olefins gasoline evaporative emissions reactive such as propene, butene xylene solvent use and mobile sources reactive Table 5. Indicator ratios of NMHC and their Douglas/Agua Prieta values ratio indicator value Douglas/Agua Prieta average value fire cfe cem adot xylene/benzene higher values mean less reactivity well-maintained catalyst vehicles ethane/acetylene 1,2,4-trimethylbenzene/unde cane decane/undecane 1,2,4-trimethylbenzene/deca ne non-catalyst fuel-rich or open loop conditions << normal diesel % gasoline, 10% diesel diesel % gasoline, 10% diesel 0.99 diesel % gasoline, 10% diesel Examining these ratios suggests that for the duration of the 24-hour sample, the Agua Prieta sites have more reactive hydrocarbons than the Douglas sites (the background site of vortac had the lowest xylene/benzene ratio of 2.42). The four sites had nearly equal ethane/acetylene ratios -- all well below 1.0, indicating that vehicular emissions were dominated by non-catalyst vehicles 9

10 and/or catalyst vehicles operating in fuel-rich or open loop conditions. The value of this ratio for Los Angeles in 1995 was 0.9; in Phoenix in 1996, it varied from 0.8 to 1.0. The three ratios involving decane or undecane -- with their higher values at the two Agua Prieta sites -- suggest that diesel exhaust contributes less to the ambient air of Agua Prieta than of Douglas. Inferences about emission patterns can also be drawn from the percentage abundances of the key source compounds in Table 4. These percentages, given in Table 6, suggest that natural gas emissions, indicated by ethane, are a larger share of the inventory in Douglas and liquid propane emissions, indicated by propane, contribute more to the total in Agua Prieta -- consistent with the lack of natural gas in the latter city. Consumer product emissions, whose surrogate is isobutane, appear to be a larger contributor to the total inventory in Douglas. Mobile source emissions as indicated by acetylene, ethane, and benzene, appear to contribute a larger share to the Douglas inventory, especially if the adot site -- two miles north of the border and hence less susceptible to transport from Agua Prieta than the cem site -- is considered. With toluene as an indicator of solvent use, only the cfe site stands out from the others as having a smaller contribution. With xylene indicating solvent use, the adot site appears to have the largest contribution. These are the kind of qualitative checks that ambient NMHC concentrations can provide on the emissions inventory. More quantitative ones can be forthcoming through receptor modeling. Table 6. Emission sources of key compounds and their abundance at the Douglas/Agua Prieta NMHC sites compound major sources percentage abundance fire cfe cem adot acetylene mobile sources and combustion ethane mobile sources ethane natural gas propane liquid propane and natural gas isobutane consumer products, gasoline evaporative emissions butane gasoline evaporative emissions benzene motor vehicle exhaust and combustion toluene solvent use and mobile sources xylene solvent use and mobile sources Comparison with other studies Other monitoring studies of nonmethane hydrocarbons in Arizona -- at least those with a comparable suite of compounds analyzed -- have been conducted in metropolitan Phoenix as a 10

11 part of intensive ozone field studies. Earlier monitoring work performed as part of the Arizona Hazardous Air Pollutant (HAPS) study included about 70 of the 150 NMHC compounds analyzed for the ozone studies and in Douglas/Agua Prieta. For those specific compounds common to all studies, the concentrations of the individual species will be compared. For the total of nonmethane hydrocarbons, the Douglas and Agua Prieta concentrations will be compared with the comparable samples from the Phoenix ozone field studies. The comparisons are somewhat compromised by the difference in the sample and study duration. In Douglas and Agua Prieta, 24-hour samples were collected the year round, while in the Phoenix ozone studies, the samples were of four hours duration and collected only during the hotter months. As the seasonal variation in Douglas/Agua Prieta indicates, because the winter months have higher concentrations than the other seasons, the long-term averages would be expected to be elevated over the summer-only studies. It s unclear how the 24-hour average compares with the four-hour averages of nonmethane hydrocarbons, since the diurnal variation of hydrocarbons has not been determined in either Douglas/Agua Prieta or Phoenix. Presented in Table 7, these data show the remarkable result that the total nonmethane hydrocarbon concentrations in Agua Prieta are generally much higher than the central-city Phoenix concentrations. The two Douglas sites are about 25% lower than the Phoenix concentrations. Only a single value of the 106 recorded in urbanized Phoenix, that is, at Supersite, South Scottsdale, or Bank One, that of 4,307 ppbc at Supersite in 1997, ranks with the highest of the Agua Prieta concentrations. The second-highest concentration from Supersite in 1997 was , with an average of ppbc, more in line with the other Phoenix urban values. As the background concentrations north of Douglas are consistent with the background values near Phoenix, this means that in a city one thirtieth the population of Phoenix, the hydrocarbons are nearly as high or higher than in the larger city with its considerably greater emissions and presumable greater emission densities. 11

12 Table 7. Nonmethane hydrocarbons at Douglas/Agua Prieta and from other studies avg std min max n dur(hrs) months fire all cfe all Super * Super Scot Bank Super cem all adot all JR SR PV UM vortac all PV PV Super Phoenix SuperSite, central Phoenix Scot South Scottsdale (Thomas Rd and Miller Rd) Bank Bank One, dowtown Phoenix JR Jackrabbit road near McDowell Rd, far west Phoenix SR Salt River Pima Indian Reservation Palo Verde Nuclear Generating Station (50 miles west of Phoenix) UM Usery Pass, far eastern part of metropolitan Phoenix * second high at Supersite in 1997 was ppbc For the comparisons of the Douglas/Agua Prieta results with the HAPS studies conducted in Phoenix, Tucson, Payson, Casa Grande, Hillside, and Nogales, individual species concentrations can be examined, as well as the total NMHC concentrations of the compounds common to all studies. Although this value must be considered a partial NMHC concentration, since fewer than one half of the 150 compounds of the Douglas/Agua Prieta work are common to all the studies, it can still serve as a surrogate for total NMHC. In the HAPS studies, about 25 compounds of interest were identified, based on the combination of their concentrations and toxicity. Of these compounds of interest, the five common to all studies are presented in Table 8. These long-term average concentration data reveal a number of facets of the Douglas/Agua Prieta data: The Douglas background site of vortac has, with the exception of n-hexane, concentrations consistent with the other background sites. The Nogales Bombers site three to four times as much benzene, 1,3-butadiene, and toluene as any other site. The Agua Prieta fire site has concentrations roughly equal to urban Phoenix, except for toluene, which is twice as high in the smaller city. 12

13 The two Douglas sites have much lower concentrations than the Agua Prieta sites, and are comfortably in the middle for most of the compounds with Tucson, Payson, and most of the urbanized Phoenix sites from the ozone field studies. All urban sites exceed the Arizona Ambient Air Quality Guideline (AAAQG) for benzene and 1,3-butadiene; several sites have exceeded the 24-hour AAAQG for these compounds; no sites have exceeded the 24-hour AAAGG for hexane, naphthalene, and toluene. 13

14 Table 8. Long-term average concentrations of five compounds of interest from Arizona HAPS studies and from Douglas/Agua Prieta (Units are ppbc) compound benzene 1,3-butadiene n-hexane napthalene toluene annual AAAQG none none none 24-hour AAAQG N-bombers* N-PO* Phoenix** AP-fire*** N-Technica* AP-cfe*** N-carrillo* Phx-Super 97**** Tucson** Payson** urbanized Phx 96**** D-cem*** D-adot*** Phx-Super 98**** Casa Grande** Salt River Pima **** D-vortac*** Hillside** Usery Mtn 98**** Palo Verde 97**** Palo Verde 98**** * Ambos Nogales study, year-round, every sixth day, 24-hour samples ** Arizona Hazardous Air Pollutant Study, year-round, every sixth day, 24-hour samples *** Douglas/Agua Prieta, year-round, every sixth day, 24-hour samples **** Phoenix area ozone field studies, late spring, summer, 4-hour samples; urbanized Phoenix 96 is the average of the Supersite, South Scottsdale, and Bank One 14