Article pubs.acs.org/est
Ambient Acrolein Concentrations in Coastal, Remote, and Urban Regions in California Thomas M. Cahill* School of Mathematical and Natural Sciences, Arizona State University, West Campus, 4701 West Thunderbird Road, Glendale, Arizona 85306, United States S Supporting Information *
ABSTRACT: Acrolein (2-propenal) is a reactive chemical that is very toxic and has many sources. Acrolein is commonly detected in the atmosphere, but understanding the ambient concentrations of this compound has been hampered by analytical difficulties. The objective of this research was to utilize an analytical method specifically designed for acrolein to determine acrolein concentrations in remote regions. The purpose was to determine the natural background concentrations of acrolein which were simply lacking in the literature. In addition, rural and urban areas were sampled to determine the degree of anthropogenic enrichment in polluted environments. The results from the coastal and remote inland areas suggest that the median natural summertime background of acrolein was near 40 ng/m3, which was higher than the Environmental Protection Agency’s Reference Concentration (RfC) of 20 ng/m3. Acrolein concentrations in urban areas were approximately 3- to 8-fold higher than background concentrations, which was a lower degree of urban enrichment than expected. The results suggest that additional research is needed to understand the natural background concentrations of acrolein.
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Report published in 201324 stated on page 3−7 the following: “Although it is a NATTS MQO Core Analyte, acrolein was excluded from the preliminary risk-based screening process due to questions about the consistency and reliability of the measurements.” This suggested problems associated with the canister methodology for measuring acrolein that have not been well documented in the formal literature, although there are some details available on EPA Web pages. Other air quality monitoring groups, such as the California Air Resources Board, have adopted canister methods for acrolein determination, which means the results obtained by these monitoring groups may likewise be suspect. There are other ambient acrolein methods available in the literature, but they are infrequently used since they are not standard EPA methodologies. The implications of the questionable results from the two most widespread methods used to report acrolein concentrations are significant. The first implication is that much of the routine monitoring data for acrolein may be inaccurate, which may affect the risk assessment of acrolein. This may result in an overestimate of acrolein’s hazard assessment if the input data overestimated the ambient acrolein concentrations. The second implication is that our baseline understanding of acrolein in the environment may also be incomplete since there is scant data
INTRODUCTION Acrolein (2-propenal) is a common atmospheric chemical that has both many sources and well demonstrated toxic effects. Acrolein is acutely toxic and has been given an EPA inhalation Reference Concentration (RfC) of 20 ng/m3.1 The high toxicity of acrolein causes acrolein to appear near the top of noncancerous hazardous air pollutant assessments.2−4 Acrolein also has numerous sources that make it ubiquitous. Many of the sources are primary emissions from combustion sources, such as vehicles,5−8 wood smoke,9,10 tobacco smoke,11−13 and incense.14 However, it can also arise from sources such as heated cooking oils,15−17 building materials,18 and atmospheric oxidation of compounds. Acrolein’s high toxicity and numerous sources have resulted in considerable interest in ambient acrolein concentrations. Unfortunately, acrolein is a difficult compound to measure at ambient concentrations. The typical approach to measuring carbonyls, namely derivatization by 2,4-dinitrophenylhydrazine (DNPH) in a similar fashion to EPA method TO-11a, has been repeatedly shown to be inaccurate for acrolein.19−21 Furthermore, EPA method TO-11a does not list acrolein as a potential analyte,22 yet it continues to be used to determine acrolein concentrations along with other carbonyls. Currently, the EPA recommended analytical protocol for acrolein is canister sampling (e.g., method TO-1523 or equivalent). However, recent monitoring networks using the canister methodologies have also encountered problems with acrolein results. The most recent National Monitoring Programs Annual © 2014 American Chemical Society
Received: Revised: Accepted: Published: 8507
March 26, 2014 June 30, 2014 July 3, 2014 July 3, 2014 dx.doi.org/10.1021/es5014533 | Environ. Sci. Technol. 2014, 48, 8507−8513
Environmental Science & Technology
Article
reported by methods that do not have the quality control concerns of the DNPH and canister methods. In particular, an understanding of the natural background acrolein concentration is essential to determine if the EPA reference concentration of 20 ng/m3 is a realistic benchmark for a life-long, no detrimental effect level. The objective of this research was to determine ambient acrolein concentrations in remote regions to determine the natural background concentrations of acrolein and to compare these concentrations to urban areas with known anthropogenic air quality impacts. A total of 39 sampling sites were distributed around California to represent coastal, remote, intermediate, and urban regions. Acrolein concentrations were then determined with a mobile battery powered sampling mist chamber system that was specifically designed for acrolein determination. The results were then compared to recent literature data including data from questionable analytical methods.
remote high elevation sites were accessible. The late summer also avoided the high summer temperatures in the desert regions that might affect sampling. Unfortunately, late summer was wildfire season for California, and wildfires were present in the state during sampling. Since wood smoke is a well-known source of acrolein, sample sites were specifically situated to avoid the influence of wildfires so that no site was downwind of a wildfire or in an air basin impacted by a wildfire. The short sampling period was designed to characterize the ambient concentrations at a point in time, and it does not provide any information about the seasonal variation in concentrations. The rapid sampling schedule required samples being collected at different times during daylight hours, so the samples represent an average across any diurnal variation. Exact sampling location, time, and site conditions are provided in the Supporting Information as Table S1. The sites were distributed into four main categories. The first category was “coastal sites”. The coastal sites were designed to collect air that originated over the ocean with minimal risk of anthropogenic sources. The sites were situated away from urban areas that might contribute pollution during the prior day due to an “off shore” air flow, hence the majority of the sites were in Northern California where the prevailing wind brings oceanic air onto the coast. In all cases, the sites were located within 300 m of the seashore, and there were no anthropogenic sources between the sampling site and the ocean. The wind during sample collection was originated from the ocean. The second category was the “remote sites”. These were inland sites with minimal anthropogenic sources nearby, and the sites could not be in an air basin containing major metropolitan areas. The remote sites may have biogenic contributions to the ambient aldehyde concentrations. The remote sites were further divided into forested, desert, and mountain (>2000 m elevation) regions to investigate the effects of the local biotic communities on aldehyde concentrations. The next category was the “intermediate” sites that have light levels of human development (e.g., primarily rural agricultural areas) or the site resided in an air basin downwind of a major metropolitan area. These sites were expected to have a low to intermediate degree of anthropogenic sources into the ambient air quality. The “urban” sites represent the last category of sampling sites. These sites were located within the major metropolitan areas of San Francisco, Sacramento, Fresno, and Los Angeles. In all cases, these sites were expected to have considerable input from anthropogenic sources such as vehicles. The urban sites were further divided into “Northern California” (San Francisco Bay Area, Sacramento and Fresno) and “LA Basin” which consisted of four sites along a transect from Los Angeles to Ontario north of Interstate-10. Sites were located more than 300 m from major freeways to avoid roadway influences26 with a single exception that was approximately 230 m from a freeway. At each site, duplicate field samples were collected using the protocols published in refs 27 and 28 with the details provided in the Supporting Information. In short, the aldehyde samples were collected by a battery powered mist chamber system. The mist chamber method was selected since it can achieve sensitive results in a short period of time, so many sites can be sampled quickly. The ambient air was drawn through two mist chambers in series at a flow rate of approximately 10 L/min for 20 min. The mist chambers had a 0.1 M bisulfite solution that was enriched with four isotopically labeled aldehydes, namely
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MATERIALS AND METHODS The goal of the research was to sample a wide variety of geographic and developmental conditions over a short period of time (20 days) at the end of summer 2013. Prior to sampling, sites were identified and categorized based on expected meteorological conditions. The study design called for 10 sites in each of the categories, which were coastal, remote, intermediate, and urban. The categorization of the sites was revisited after field sampling with the HYSPLIT model.25 HYSPLIT, a Lagrangian trajectory model, was run in a backward isentropic mode to examine the path that the air parcel took to each site during the previous 24 h. This was used to ensure that the actual samples collected conform to the criteria for each category. Logistical issues during field sampling resulted in the removal of two intermediate sites and the addition of an urban site which resulted in the locations shown in Figure 1. Sample collection was conducted in late summer between September 17 and October 3, 2013. This time period was chosen since potential biogenic sources were still active and the
Figure 1. Locations of the sampling sites in California. Duplicate field samples were collected at each site. 8508
dx.doi.org/10.1021/es5014533 | Environ. Sci. Technol. 2014, 48, 8507−8513
chemical
8509
MDL−130b
MDL−470 MDL−30 MDL−280 MDL−150
* --* * 3.1 * * --------* * * * --*
41 19 17 8.5 2.5 38 46 2.0 2.2 3.1 52 43 3.7 240 78 77 10 MDL−25
MDL−61
MDL−18 MDL−9.7 MDL−11 MDL−44
MDL−570 MDL−1400 MDL−520 MDL−770
range
* * * *
median
120 150 200 320
MDL
* 93 --*
--*
* ------*
* * * * * *
* * * *
median
MDL−41
MDL−680b MDL−260
MDL−31
MDL−92
MDL−110
MDL−160 MDL−94 MDL−140 MDL−17 MDL−33 MDL−240
MDL−270 MDL−560 MDL−460 MDL−970
range
remote (n = 20)
* * --*
* 4.5
* * * --*
68 * * * 3.4 *
* * * *
median
MDL−55
MDL−710 MDL−220
MDL−55 MDL−35
MDL−873
MDL−73 MDL−12 MDL−8.5
MDL−110 MDL−85 MDL−34 MDL−19 MDL−26 MDL−56
MDL−390 MDL−530 MDL−240 MDL−750
range
intermediate (n = 15)
* * --*
* *
46 7.4 4.0 * ---
101 29 --* 6.2 *
* * * *
median
MDL−13
MDL−320 MDL−110
MDL−90 MDL−10
MDL−290 2.9−14 MDL−10 MDL−3.2
MDL−9.2 MDL−18 MDL−72
46−180 MDL−150
MDL−180 MDL−200 MDL−230 MDL−350
range
urban, Northern California (n = 14)
710 200 160 35
94 15
230 22 17 5.4 77
320 74 23 5.0 22 90
300 320 250 530
median
640−1100 150−290 MDL−230 MDL−43
MDL−240 MDL−46
160−320 18−30 14−22 3.8−6.9 MDL−140
230−410 40−99 MDL−34 MDL−19 MDL−42 MDL−260
170−570 190−900 MDL−400 330−680
range
urban, LA Basin (n = 8)
a
“---” denotes that the chemical was not detected in any samples from that region. “*” denotes that the chemical was detected in less than half of the samples, so a median value could not be calculated. An expanded table that includes another 9 compounds that were not regularly detected is presented in the Supporting Information as Table S2. bThe analyte was detected in exactly half of the samples so a meaningful median value could not be calculated.
n-Aldehydes: pentanal hexanal octanal nonanal Unsaturated aldehydes: acrolein methacrolein crotonaldehyde 3-methyl-2-butenal 2-hexenal 4-decenal Aromatic aldehydes: benzaldehyde o,m-tolualdehyde p-tolualdehyde 3,4-dimethylbenzaldehyde 3-hydroxybenzaldehyde Misc. aldehydes: 3-methylbutanal 2-furaldehyde Diones: 2,3-butanedione 2,3-pentanedione 2,4-pentanedione 2,3-hexanedione
coastal (n = 20)
Table 1. Median Concentrations (ng/m3) and Range of Carbonyls in the Different Regionsa
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dx.doi.org/10.1021/es5014533 | Environ. Sci. Technol. 2014, 48, 8507−8513
Environmental Science & Technology
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Table 2. Comparison of Ambient Outdoor Acrolein Concentrations between the Current Study and Prior Researcha citation
methodb
current study
PFBHA
41
31
PFBHA PFBHA
0.9 6.0 to 54
27
PFBHA
12 to 35
18
PFBHA PFPH DNSH CT
9.0 230 NR NR
35
CT
4
model model canister canister canister ---k DNPH
21 9.2 21 16 ----NR 690 115 ---k NR
DNPH DNPH DNPH DNPH
NR NR 90 NR
DNPH
NR
28
32 33 34
36 37 38 24 2 39
40 7 41 42
43
MDLc
remote
rural
MDL (MDL−130) MDL (MDL−160)
68 (MDL−110)
MDL (MDL−152) coastal: 56 remote: 89
21 (3.4−91)
urban 165 (46−410)d 32−100 MDL and 158 (MDL−624)d 11.6 and 28.4 (MDL−324)e 290d 600 (90−1700)e 210−600 460 (MDL−4580) 300 ± 210e 870 ± 780e,f spring: 160 (120−240)g summer: 140 (69−160)g fall: 210 (92−250)g winter: 280 (160−480)g 360 94 (17−440) 2,360 ± 800h 1,400 (MDL−9,200)i 847 (MDL−25,200)j 33 246.2 ± 3.8l
