Diurnal Cycles of Acrolein and Other Small Aldehydes in Regions

Aug 29, 2008 - Corresponding author phone: (602)543-6021; fax: (602) 543-6073; ... atmospheric concentrations of acrolein and several small aldehydes ...
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Environ. Sci. Technol. 2008, 42, 7084–7090

Diurnal Cycles of Acrolein and Other Small Aldehydes in Regions Impacted by Vehicle Emissions NICHOLAS SPADA,† ERIN FUJII,† AND T H O M A S M . C A H I L L * ,‡ DELTA Group, University of California, Davis, California 95616, and Department of Integrated Natural Sciences, Arizona State University at the West Campus, P.O. Box 37100, Phoenix, Arizona 85069

Received June 16, 2008. Revised manuscript received July 25, 2008. Accepted July 29, 2008.

This research determined the diurnal and seasonal differences in the ambient atmospheric concentrations of acrolein and several small aldehydes and attempted to link the chemicals to their potential sources. Two summertime and two wintertime sampling episodes were conducted in Roseville, CA at a site located near several busy roadways. One additional sampling episode was conducted at a remote site in the summer to estimate regional background concentrations of aldehydes. Each sampling episode consisted of duplicate samples collected every two hours around the clock for three days. Acrolein concentrations did not correlate with traffic density, ozone concentrations, or tracers of direct vehicle emissions, which argues against vehicles being a dominant source of ambient acrolein through primary emissions or secondary oxidation products. The results showed that wintertime acrolein concentrations correlated well with 2-furaldehyde, which is a tracer of biomass burning, thus suggesting that wood smoke isanimportantsourceofambientacrolein.Otherregularlydetected carbonyls were tentatively assigned to different source classes (direct vehicle emissions, photochemical oxidation, wood smoke or transport from the Sierra Nevada Mountains) based on time series patterns and correlations with indicators of potential sources (e.g., ozone, traffic density, etc.).

Introduction Acrolein (2-propenal) is a reactive unsaturated aldehyde that is considered to be among the greatest noncancer, organic chemical health hazards in outdoor ambient air quality (1-3) and could be associated with decreased respiratory function (4). Ambient acrolein concentrations are generally considered to have a large contribution from vehicle emissions (3), either by direct emissions (5-9) or through the oxidation of other emitted chemicals. However, acrolein is also formed from many incomplete combustion processes (10) such as wood smoke (11, 12), cooking (13-15), tobacco smoke (16-18), and incense (19). Therefore, it is unclear which sources are the greatest contributors to ambient acrolein concentrations. The study of ambient acrolein concentrations and its potential sources has been hampered by the lack of highly time-resolved concentration data. Most of the available * Corresponding author phone: (602)543-6021; fax: (602) 543-6073; e-mail: [email protected]. † University of California. ‡ Arizona State University. 7084

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ambient analytical methods rely on 24 h sample collection times, but this long sample collection time prevents the determination of diurnal cycles of acrolein that could provide insight into the sources of acrolein. The other main complication that limits the investigation of ambient acrolein concentrations is the scarcity of analytical methods that are capable of detecting low concentrations of acrolein. The most common carbonyl analytical method, which involves derivatization by DNPH, has been shown to be unreliable for acrolein due to the instability of the acrolein-DNPH derivatives (20). Other methods that rely on passive samplers or cartridges have low flow rates or adsorption rates that limit the amount of the carbonyls collected and thus the sensitivity of the analysis. Due to these problems, ambient acrolein concentrations are rarely reported. New analytical methods are becoming available for the detection of acrolein over short time periods (21-23) that can determine the shortterm fluctuation of acrolein. The objective of this research was to determine the diurnal concentrations of acrolein and other small aldehydes at a vehicle-impacted site to assess the contribution of vehicles to ambient acrolein concentrations. Two sample collection episodes were conducted in summer and two in winter at a site specifically designed to be impacted by vehicle emissions while a control site was sampled once for a comparison. Each sample collection episode consisted of collecting duplicate ambient samples every two hours for three straight days to determine the diurnal cycles of acrolein. The ambient concentration results were correlated with meteorological conditions (ozone, temperature, etc.) and tracer compounds to assess the potential source of acrolein and other small aldehydes.

Experimental Section Sampling Sites. The primary sampling site was the California Air Resources Board (CARB) site on North Sunrise Blvd in Roseville, California (N38 44.798 W121 15.861), which was on the roof of a single-story building in the greater Sacramento Metropolitan area. The site was located near several high traffic volume roadways including Interstate 80 (163 000 average daily travel), Douglas Blvd and Sunrise Blvd to observe local carbonyl enrichment resulting from vehicles. The site, which was approximately 460 m from Interstate 80, is not a “roadside” site and it may not observe the plume from a specific roadway (24). The amount of vehicle traffic in the east-bound lanes of Interstate 80 was counted for 30 s right before sample collection as an index of traffic density. The site also had standard air quality measuring equipment (O3, NOx, particulate matter) and meteorological equipment (temperature, RH and wind speed/direction) that was located on a 10 m tower on the roof. Lastly, CARB conducts canister sampling for several volatile chemicals including acrolein, so there can be comparison between the canister methodology and the mist chamber methodology. The second sampling site was a rural control site near Putah Creek, CA (N38 30.886 W122 03.491). This site was designed to determine the background concentrations of acrolein and other carbonyls in the absence of urban sources. In this case, the meteorological data and ozone concentrations were determined at ground level with portable instrumentation. Samples were collected during five sampling episodes. Two of these episodes were in Roseville in the summer of 2006 and two more were in the winter of 2006/2007. The different seasons were sampled to assess the importance of photochemical formation since the wintertime has much lower ozone concentrations, actinic fluxes, and temperatures. 10.1021/es801656e CCC: $40.75

 2008 American Chemical Society

Published on Web 08/29/2008

The last sampling episode was conducted at the control site during the summer of 2006 between the two summer Roseville sampling episodes. Each sampling episode consisted of duplicate sample collection every two hours for three consecutive days. The high time resolution was designed to determine diurnal concentration changes in response to traffic patterns or photochemical production. Each sampling episode started at 06:00 on Sunday and ended at 06:00 on the following Wednesday so each episode would encompass one weekend and two work days that may have different traffic patterns. The samples were collected and quantified using the mist chamber methodology described in (21). Briefly, 10 mL of a 0.1 M bisulfite solution was added to each of two mist chambers connected in series. The collection solution was spiked with acrolein-d4, benzaldehyde-d6, and acetaldehyded4 before sample collection. Air is then pulled through the mist chambers for 10 min at a flow rate of 13.8-19.2 L/min. The carbonyls partition into the aqueous phase and are then bound as sulfonate adducts. After sample collection, the bisulfite solution was transferred to a test tube containing hydrogen peroxide, which oxidizes the sulfite to sulfate and liberates the aldehydes, and pentafluorohydroxylamine (PFBHA), which derivatizes the aldehydes into a more stable and detectable form. The derivatives were extracted with hexane, concentrated by nitrogen evaporation, and analyzed by gas chromatography-negative chemical ionization mass spectrometry. In addition to the stationary site, a small series of “onroad” samples were collected where a sampler was placed in a vehicle with a 1.5 m PTFE inlet tube connecting it to the outside air. The vehicle was then driven through traffic on the major roads surrounding the Roseville site during the second summer sampling episode (see Section S-1 in the Supporting Information for more details). Four on-road samples were collected on freeways and four samples on the surface streets. These samples were collected on a breezy day (average velocity ) 2.8 m/s), so there was probably rapid air mixing occurring over the roadway. Lastly, simplistic source profiles were determined for three vehicles and two types of wood smoke to help strengthen the linkage between ambient chemical concentrations and their suspected sources. The sample collection details and results are presented in Section S-2 of the Supporting Information.

Results Meteorology. The meteorology showed that the sampling episodes were representative of the seasonal averages. The summertime temperatures in Roseville ranged from 19 to 39 °C, whereas the winter temperatures ranged from -1 to 15 °C (Figure S-2 in the Supporting Information). The control site had higher summer time temperatures (up to 45 °C) partly because the measurements were taken at ground level instead of a 10 m meteorological tower. The ozone concentrations ranged from 27 to 112 ppb in the summer and 2 to 30 ppb in winter (Figure S-3 in the Supporting Information). The wind direction at Roseville during the summer sampling events was largely from the south and northwest (Figure S-4 in the Supporting Information), which indicates that the site is downwind of the greater Sacramento Metropolitan area. In contrast, the winter sampling events were dominated by eastern winds with some periods from the northwest. The wind blew from the direction of Interstate 80 (which lies north to west of the site) about half the time in the second summer and the first winter sampling episodes. There was no major difference in average wind speed between summer (8.3 km/h) and winter (6.1 km/h) (Figure S-5 in the Supporting Information). Vehicle traffic density typically increased at 06:00-08:00 each weekday morning and then declined at about 20:00-22:00 in the evening, and it was consistent

between seasons (Figure S-6 in the Supporting Information). Traffic density increased later in the day (08:00-10:00) on Sundays. Chemical Concentrations. The analysis of the ambient air samples consisted of 54 carbonyl compounds, but only 20 were detected with enough regularity to report meaningful statistics (Table 1). The chemicals that were regularly detected and quantified were: acrolein, crotonaldehyde, 3-methyl2-butenal, 2-furaldehyde, glyoxal, methylglyoxal, m,o,ptolualdehydes, 2,3-butanedione, and pinonaldehyde (Table 1). Most of the analytes were detected during the study, but often not regularly enough to create a time series plot. The analytical method was most effective for aldehydes, whereas many ketones were not effectively measured by this method and are therefore not reported. In addition, no attempt was made to quantify formalydehyde or acetaldehyde due to extensive analyte blow-off and/or high blank concentrations. The ambient acrolein concentrations showed diurnal and seasonal differences (Figure 1). During the first sampling episode, acrolein was always detected with concentrations that ranged from 75 to 625 ng/m3, and the highest concentrations were observed in the early morning. Acrolein concentrations did not correlate with ozone (R2 ) 0.025), temperature (R2 ) 0.013), or local traffic activity (R2 ) 0.002). Acrolein was not detected in the second summer sampling episode, including the on-road samples. This is partly due to higher field blanks that caused the MDL to be 54 ng/m3, but even this detection limit would have been sufficient to detect acrolein if it was present in similar concentrations as the first sampling period. The reason for the acrolein differences between the two sampling periods is not known. Acrolein was only detected in 1/3 of the samples at the control site. After the first day of sampling, the period of stagnation ended and winds from the clean coast region blew inland. The wintertime acrolein concentrations showed a peak every evening from about 20:00 to 24:00 h. This cyclic peak in concentrations was observed in both winter sampling episodes and on both weekend and weekday nights. Comparisons with acrolein concentrations in the literature are complicated by the fact that there are very few reliable measurements of acrolein in ambient air samples. A review of the literature shows that the concentrations observed in this research are comparable or lower than previous research projects for California. The wintertime outdoor acrolein concentrations determined in Seaman et al. (25) in Placer County utilizing the same analytical methods as the current study averaged 200 ng/m3 in the morning and 350 ng/m3 in the evening. That research showed geographic difference in acrolein concentrations with ambient concentrations in the LA basin being 4-6 times higher than in Placer County. Summertime concentrations reported in Roseville, Salt Point, CA and Lassen National Park, CA were reported to be 290, 56, and 89 ng/m3, respectively, which are comparable to the range of values observed here (21). Destaillats et al. (26) determined that acrolein concentrations ranged from 31 to 140 ng/m3 at toll booths in the San Francisco Bay Area. A modeling study by Morello-Frosch et al. (3) predicted median ambient acrolein concentrations in California to be 360 ng/ m3. The California Air Resources Board’s Monitoring and Laboratory Division routinely determines acrolein concentrations at the same site using the EPA TO-15 canister method. The mean concentrations recorded at the site were 985 ng/ m3 in 2005 and 1240 ng/m3 in 2006 with a detection limit of 0.3 ppbv (690 ng/m3) (27). Most of the reported values are within a factor of 2 of the reported minimum detection limit where quantification is often difficult. Also, the reported acrolein concentrations from around the state had an average of 0.55 ppbv (1260 ng/m3) and a range from 0.45 to 0.75 ppb, which seems remarkably consistent despite varied sample locations. VOL. 42, NO. 19, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1. Median Concentration (ng/m3) and Range of Aldehydes Routinely Detected in Ambient Air Samplesa chemical acroleinc methacroleinb crotonaldehydeb,c 2-methyl-2-butenalb 3-methyl-2-butenalb 2-furaldehydeb 2-hexenal 2-heptenalc benzaldehydeb,c m,o-tolualdehydeb,d p-tolualdehydec 3,4-dimethylbenzaldehydeb,c glyoxalb 4-methoxybenzaldehydeb,c methylglyoxalb 3-phenyl-2-propenalb,c 2,3-butanedionec 2,3-pentanedione 3-hydroxybenzaldehydeb,c pinonaldehydeb,c

Roseville summer no. 1 (n ) 68)

Roseville summer no. 2 (n ) 88)

control site summer (n ) 82)

Roseville winter no. 1 (n ) 73)

Roseville winter no. 2 (n ) 74)

158 (72.8-624) 108 (23.5-352) 13.8 (5.50-68.1) MDL (0.0-14.8) 6.0 (0.0-49.2) 5.8 (0.0-23.8) MDL (0.0-30.0) 4.4 (0.0-25.1) MDL (0.0-1130) 14.5 (0.0-57.3) 7.5 (0.0-36.8) MDL (0.0-13.8) 731 (0.0-2050) 7.4 (0.0-15.7) 887 (172-2590) MDL (0.0-49.1) 66.8 (0.0-359) 11.1 (0.0-28.8) MDL (0.0-7.8) 75.3 (0.0-711)

no detections

MDL (0.0-152) 122 (0.0-1090) MDL (0.0-17.5) MDL (0.0-31.3) 7.8 (0.0-27.4) 12.1 (0.0-39.8) no detections

28.4 (0.0-235) 2.6 (0.0-20.3) MDL (0.0-113) 1.8 (0.0-12.6) 2.8 (0.0-9.9) 622 (76.6-5130) 1.6 (0.0-13.7) 2.3 (0.0-31.6) 65.3 (0.0-342) 9.3 (0.0-79.2) 5.3 (0.0-28.8) 2.4 (0.4-11.5) 147 (0.0-740) 1.2 (0.0-5.6) 134 (34.3-1590) MDL (0.0-572) 38.3 (0.0-258) 8.1 (0.0-58.0) 6.2 (0.0-63.6) 42.2 (0.0-162)

11.6 (0.0-324) MDL (0.0-17.0) MDL (0.0-174) MDL (0.0-12.8) 2.3 (0.0-9.5) 140 (14.0-2700) MDL (0.0-17.5) MDL (0.0-20.1) 30.2 (0.0-240) 6.4 (0.0-54.0) 3.6 (0.0-21.0) 1.0 (0.0-5.3) MDL (0.0-414) 2.1 (0.0-6.7) 113 (31.5-908) –

MDL (0.0-339) no detections 4.1 (0.0-14.8) 5.4 (0.0-35.2) 26.8 (6.8-194) MDL (0.0-44.7) MDL (0.0-7.4) MDL (0.0-498) MDL (0.0-14.1) 2.1 (0.0-8.5) MDL (0.0-6.8) 489 (0.0-2050) 6.9 (0.0-26.6) 972 (182-2340) 10.4 (0.0-191) 23.7 (0.0-147) MDL (0.0-21.5) MDL (0.0-12.2) 60.1 (0.0-374)

MDL (0.0-9.7) MDL (0.0-330) no detectionsd MDL (0.0-5.6) MDL (0.0-6.3) 227 (0.0-3040) 4.2 (0.0-12.3) 724 (0.0-3340) MDL (0.0-43.8) 108 (0.0-285) MDL (0.0-119) MDL (0.0-7.9) 98.1 (0.0-333)

45.1 (0.0-348) MDL (0.0-71.8) MDL (0.0-23.2) 38.9 (0.0-127)

a A median value reported as “MDL” indicates that the chemical was detected in less than half of the samples. Chemicals listed as “s” indicates quantification was not possible due to analytical problems, such as unacceptable calibration curves. b This chemical showed statistically significant seasonal differences (P < 0.01, Mann-Whitney test) between the summer and winter sampling episodes in Roseville. c This chemical showed statistically significant regional differences (P < 0.01, Mann-Whitney test) between the summer sampling episodes conducted at Roseville and the control site. d o,m-tolualdehyde was not detected at the control site, so statistical comparisons were not conducted.

The observed concentrations are generally lower than ambient acrolein concentrations reported from other geographic regions. Median outdoor acrolein concentration in the Eastern United States have been reported as 460 ng/m3 (28) but only 68% of the outdoor samples were above the detection limit of 140 ng/m3. A modeling study estimated that the median acrolein concentrations in the United States in rural and urban areas should be about 21 and 94 ng/m3, respectively (4). Mean outdoor concentrations of acrolein in Japan were reported as 33 ng/m3 (1). A review of ambient concentrations collected by older analytical methods generally showed concentrations in the low µg/m3 range (10). The concentrations of glyoxal showed a regular cycle of concentrations in the summer (Figure 2). The concentration of glyoxal reached its maximum in the middle to late afternoon each day in the summer at both Roseville and the control site. This cycle of concentrations correlates with the ambient ozone concentrations (R2 ) 0.52, n ) 322 from all episodes). The on-road samples showed similar glyoxal concentrations as the stationary site. In contrast, glyoxal concentrations were considerably lower in the winter sampling episodes. 7086

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The next chemical that showed prominent seasonal and diurnal cycles was 2-furaldehyde (Figure 3). The concentrations of 2-furaldehyde were very low during the summer. However, the concentrations were much higher in winter. There was a spike in concentrations every evening between 18:00 and 24:00 h. 2-furaldehyde was the most abundant carbonyl detected in the two wood smoke source samples (Supporting Information Section S-2, Table S-1), although formaldehyde and acetaldehyde were not measured. Several other chemicals also showed significant diurnal and seasonal cycles that could provide insight into the potential sources of acrolein. The time series plots for these compounds are presented in the Supporting Information due to space limitations. The concentrations of p-tolualdehyde and m,o-tolualdehydes (Figures S-7 and S-8 in the Supporting Information) showed two periods of increased concentration on each of the working days during winter. The first period of elevated concentrations centered around 08:00, and the second time corresponded to 20:00. These times roughly correspond to the work-day commuter traffic. None of the tolualdehydes were regularly detected at the control site. Also, the tolualdehydes were the only chemicals

FIGURE 1. Ambient acrolein concentrations (ng/m3) in Roseville, CA during the summer and winter. Each sampling episode consisted of duplicate samples collected every two hours for three consecutive days. The control site was sampled once in the summer for comparison. The dots represent the individual sample results and the line represents the average of the two field replicates at any given time. where the on-road samples showed appreciably higher concentrations (Mann-Whitney test, P ) 0.0051 for ptolualdehyde and P ) 0.064 for m,o-tolualdehydes where MDL values were used for nondetected samples) than the ambient samples collected at similar times (see section S-1 and Figure S-1 in the Supporting Information). Pinonaldehyde, which is an atmospheric oxidation product of R-pinene (29), showed higher concentrations in the summer than in the winter (Figure S-9 in the Supporting Information). The concentrations were the highest during

FIGURE 2. Ambient glyoxal concentrations (ng/m3) in Roseville, CA during the summer and winter. Each sampling episode consisted to duplicate samples collected every 2 hours for three consecutive days. The control site was sampled once in the summer for comparison. The dots represent the individual sample results and the line represents the average of the two field replicates at any given time. The open circles in the second Roseville sampling episode represents the “on-road” samples. the late night and early morning at both Roseville and the control site. The average concentrations were also similar between the two sites. Winter time concentrations of pinonaldehyde were very low and near the limit of detection. The concentrations of 3-methyl-2-butenal (Figure S-10 in the Supporting Information) showed many of the same patterns as pinonaldehyde except that the concentrations at the control site were considerably lower. VOL. 42, NO. 19, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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Discussion

FIGURE 3. Ambient 2-furaldehyde concentrations (ng/m3) in Roseville, CA during the summer and winter. The dots represent the individual sample results and the line represents the average of the two field replicates at any given time. The “on-road” samples were effectively the same as the stationary site, thus they are obscured by the solid circles. Methylglyoxal did not show the same clear diurnal cycle as glyoxal (Figure S-11 in the Supporting Information). The summer time samples frequently showed higher concentrations in the afternoon, but the concentrations during the second Roseville sampling episode showed the highest concentrations in the early morning. The winter cycles of methylglyoxal showed the same daily increase in concentrations in the early evening as was observed 2-furaldehyde and several other chemicals. However, the winter concentrations were still much lower than the summer concentrations. 7088

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The main objective of the project was to assess the relative importance of chemical emissions from vehicles and other common sources. The chemicals were assigned suspected sources by the following criteria: (1) the chemical correlated with a known tracer chemical of the source and the temporal trends showed similar behavior; (2) the chemical was present in relatively large quantities in the source samples presented in Section S-2 in the Supporting Information; (3)chemicals that showed higher concentrations in the “on-road” samples were presumed to have a direct vehicle emission; (4) chemicals regularly detected at the control site did not have local anthropogenic emissions; (5) diurnal and seasonal differences in chemical concentrations must intuitively match the suspected source. Chemicals could be assigned to more than one potential source group if the criteria are fulfilled for both types of sources. These assessments lead to the tentative identification of four groups of chemicals with similar sources. The first group was the photochemically derived chemicals, which included glyoxal and methylglyoxal (correlations with ozone over all sampling episodes were R2 ) 0.52 and 0.35, respectively). These were chemicals that showed a diurnal cycle in the summer with the highest concentrations in the afternoon when ozone concentrations were the highest. The lack of a strong diurnal cycle in winter, when ozone concentrations and actinic fluxes were low, also helps to identify these chemicals as dependent on atmospheric transformation rather than direct emission. The second group of chemicals appeared to be derived from wood smoke. These chemicals show a strong correlation with a biomass-burning tracer of 2-furaldehyde. Furthermore, these chemicals all showed a spike in concentrations in the evening of all the winter sampling day when fireplace use was expected to be the highest. Since there was no appreciable use of fireplaces/wood stoves in the summer, the lack of these chemicals in the summertime samples helps to confirm wood smoke as the most probable source. Chemicals that appear to have wintertime wood smoke sources included (with winter-time correlation coefficients with 2-furaldehyde) 2-furaldehyde, acrolein (R2 ) 0.88), methylglyoxal (R2 ) 0.53), 2,3-butanedione (R2 ) 0.44), m,o-tolualdehydes (R2 ) 0.48), and p-tolualdehyde (R2 ) 0.49). The third type of suspected chemical source was transport from the Sierra Nevada Mountains. These are chemicals that correlate with pinonaldehyde. This is a tracer for biogenic emissions from pine forests, which indicated that the air mass was over the Sierra Nevada Mountains. Correlation with pinonaldehyde does not mean that all the chemicals were of biogenic origin, but rather their source (biogenic or anthropogenic) was in the Sierra Nevada Mountains. Other than biogenic sources, the most likely source of air pollution from the Sierra Nevada in summer would be wood smoke from slash burning, campfires, etc. Unfortunately, most of the pine trees are dormant in winter and therefore not emitting pinene, thus this tracer was only useable for the summer sampling episodes. The presence of pinonaldehyde also indicates that chemicals in the air mass have been oxidized since ozone is needed to convert pinene to pinonaldehyde. The chemicals that appeared to follow this pattern were pinonaldehyde, acrolein (R2 ) 0.61 for summer sampling no. 1), 3-methyl-2-butenal (R2 ) 0.65), 2,3-butanedione (R2 ) 0.61), 4-methoxybenzaldehyde (R2 ) 0.27), and to a lesser extent, 2-furaldehyde (R2 ) 0.11). The last clearly defined group of chemicals appeared to arise from direct vehicle emissions. These chemicals were characterized by higher concentrations in the “on-road” samples, large mass fractions in the source samples, and time series plots that showed increases during the morning

and afternoon commute times. The absence of these chemicals at the control site further supports the chemicals classification as probably direct vehicle emission. Only the m,o,p-tolualdehydes fit all these criteria. The objective of this research was to determine the ambient acrolein concentrations at a site that was expected to be impacted by vehicles. The underlying expectation was the acrolein concentrations would be related to traffic patterns over the course of a day and between weekend days and week days. The results did not agree with these expectations. The results indicate that acrolein concentrations at Roseville were most likely due to wood smoke in the winter and by transport from the Sierra Nevada Mountains in the summer. The evidence that primary vehicle emissions are not the main source of ambient acrolein concentrations are (1) the acrolein concentrations did not follow the same temporal patterns as traffic levels; (2) the on-road samples did not show detectable concentrations of acrolein, which was the same as the ambient sampling site; (3) the highest acrolein concentrations in winter correlated with a biomass burning tracer in the evening. The results also suggest that acrolein is not the result of secondary oxidation of vehicle emissions since the acrolein concentrations did not correlate with ozone or other photooxidation products. However, acrolein formation by atmospheric oxidation could occur by other oxidants (e.g., NO3) that were not measured. Lastly, the winter time concentrations showed the highest concentrations in the early evening when after the sun had set and ozone concentrations were low. These results should not be misconstrued to mean that vehicles are not a source of acrolein; numerous studies (5-9), and the emission samples in this study show that acrolein is a primary pollutant from vehicles. The results do suggest that other acrolein sources, such as wood smoke, may be more important contributors to ambient acrolein concentrations than vehicles. This research also demonstrates the importance of collecting highly time-resolved ambient air samples when assessing the potential sources of chemicals. Many of the ambient air concentration measurements for organic chemicals are 24 h samples to match the regulatory requirements. However, day long sample collection obscures diurnal trends that help to identify sources of chemicals. Therefore, some highly timeresolved samples should be collected when an ambient chemical is near or exceeds a regulatory limit to help identify the source. Only by understanding the source of a chemical can effective mitigation measures be implemented.

Acknowledgments We thank the California Air Resources Board for both funding the project (Project No. 05-340) and for granting access to their sampling site and meteorological/air quality instrumentation. In particular, we would like to thank William Vance of CARB for his good organization and management of this project. The statements and conclusions in this paper are those of the authors and not necessarily those of CARB.

Supporting Information Available Two sections detailing the on-road and source sample methods and results. There are also time series plots for ozone concentrations, temperature, wind speed, wind direction, local traffic density, and the concentrations of p-tolualdehyde, m,o-tolualdehyde, pinonaldehyde, 3-methyl-2-butenal and methylglyoxal. This material is available free of charge via the Internet at http://pubs.acs.org.

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