Gaseous and particulate species from two prescribed fires were sampled in-situ, to better characterize prescribed burn emissions. Measurements included gaseous and fine particulate matter (PM2.5) species, particle number concentration, particulate organic carbon (POC) speciation, water-soluble organic carbon (WSOC) and water-soluble iron. Major PM2.5 components included OC (∼57%), EC (∼10%), chloride (∼1.6%), potassium (∼0.7%) and nitrate (∼0.9%). Major gaseous species include carbon dioxide, carbon monoxide, methane, ethane, methanol and ethylene. Particulate organic tracers of biomass burning, such as levoglucosan, dehydroabietic acid and retene, increased significantly during the burns. Water-soluble organic carbon (WSOC) also increased significantly during the fire and levels are highly correlated with total potassium (K) (R2 = 0.93) and levoglucosan (R2 = 0.98). The average WSOC/OC ratio was 0.51 ± 0.03 and did not change significantly from background levels. Thus, the WSOC/OC ratio may not be a good indicator of secondary organic aerosol (SOA) in regions that are expected to be impacted by biomass burning. Results using a biomass burning source profile derived from this work further indicate that source apportionment is sensitive to levels of potassium in biomass burning source profiles. This underscores the importance of quantifying local biomass burning source profiles.
[1] The seasonal and spatial variations of source contributions of 112 composite fine particulate matter (PM2.5) samples collected in the Southeastern Aerosol Research and Characterization Study (SEARCH) monitoring network during 2001–2005 using molecular marker-based chemical mass balance (CMB-MM) model were determined. The lowest PM2.5 concentration occurs in January with higher values in warm months (maxima in July at four inland sites versus October at the coastal sites). Sulfate shows a similar pattern and plays a primary role in PM2.5 seasonality. Carbonaceous material (organic matter plus EC) exhibits less seasonality, but more spatial variations between the inland and coastal sites. Compared with the data at coastal sites, source attributions of diesel exhaust, gasoline exhaust, other organic matter (other OM), secondary sulfate, nitrate, and ammonium in PM2.5 mass at inland sites are higher. The difference in source attributions of wood combustion, meat cooking, vegetative detritus, and road dust among the eight sites is not significant. Contributions of eight primary sources to fine OC are wood burning (17 ± 19%), diesel exhaust (9 ± 4%), gasoline exhaust (5 ± 7%), meat cooking (5 ± 5%), road dust (2 ± 3%), vegetative detritus (2 ± 2%), cigarette smoke (2 ± 2% at four urban sites), and coke production (2 ± 1% only at BHM). Primary and secondary sources explain 82–100% of measured PM2.5 mass at the eight sites, including secondary ionic species (SO42−, NH4+, and NO3−; 41.4 ± 5.7%), identified OM (24.9 ± 11.3%), “other OM” (unexplained OM, 23.3 ± 10.3%), and “other mass” (11.4 ± 9.6%). Vehicle exhaust from both diesel and gasoline contributes the lowest fraction to PM2.5 mass in July and higher fractions at BHM and JST than other sites. Wood combustion, in contrast, contributes significantly to a larger fraction in winter than in summer. Road dust shows relatively high levels in July and April across the eight sites, while minor sources such as meat cooking and other sources (e.g., vegetative detritus, coke production, and cigarette smoke) show relatively small seasonal and spatial variations in the SEARCH monitoring network.
Secondary organic aerosol (SOA) in the southeastern US is investigated by analyzing the spatial-temporal distribution of water-soluble organic carbon (WSOC) and other PM2.5 components from 900 archived 24-h Teflon filters collected at 15 urban or rural EPA Federal Reference Method (FRM) network sites throughout 2007. Online measurements of WSOC at an urban/rural-paired site in Georgia in the summer of 2008 are contrasted to the filter data. Based on FRM filters, excluding biomass-burning events (levoglucosan < 50 ng m 3), WSOC and sulfate were highly correlated with PM2.5 mass (r2~0.7). Both components comprised a large mass fraction of PM2.5 (13% and 31%, respectively, or ~25% and 50% for WSOM and ammonium sulfate). Sulfate and WSOC both tracked ambient temperature throughout the year, suggesting the temperature effects were mainly linked to faster photochemistry and/or synoptic meteorology and less due to enhanced biogenic hydrocarbon emissions. FRM WSOC, and to a lesser extent sulfate, were spatially homogeneous throughout the region, yet WSOC was moderately enhanced (27%) in locations of greater predicted isoprene emissions in summer. A Positive Matrix Factorization (PMF) analysis identified two major source types for the summer WSOC; 22% of the WSOC were associated with ammonium sulfate, and 56% of the WSOC were associated with brown carbon and oxalate. A small urban excess of FRM WSOC (10%) was observed in the summer of 2007, however, comparisons of online WSOC measurements at one urban/rural pair (Atlanta/Yorkville) in August 2008 showed substantially greater difference in WSOC (31%) relative to the FRM data, suggesting a low bias for urban filters. The measured Atlanta urban excess, combined with the estimated boundary layer heights, gave an estimated Atlanta daily WSOC production rate in August of 0.55 mgC m 2 h 1 between mid-morning and mid-afternoon. This study characterizes the regional nature of fine particles in the southeastern US, confirming the importance of SOA and the roles of both biogenic and anthropogenic emissions.
[1] Fine particles (PM2.5) were collected using filter-based high-volume samplers during summer-winter 2008 at a rural site in the central Pearl River Delta (PRD), south China, to determine typical secondary organic aerosol (SOA) tracers from significant biogenic (isoprene, monoterpenes, and sesquiterpenes) and anthropogenic (aromatics) precursors. Average isoprene SOA tracers were significantly higher during summer (126 ng m−3) than during fall-winter (25.1 ng m−3), owing largely to the higher isoprene emission and reaction rates in summer. Average monoterpene SOA tracers during summer (11.6 ng m−3) and fall-winter (16.4 ng m−3) showed much less difference compared to isoprene SOA tracers, probably resulting from the counteracting effects of temperature on the precursor emission/tracer formation and on gas/particle partitioning. The concentrations of the aromatics' SOA tracer (2,3-dihydroxy-4-oxopentanoic acid) ranged from 1.70 to 52.0 ng m−3 with an average of 15.1 ng m−3, which was the highest reported in ambient air. The secondary organic carbon (SOC) estimated by the SOA-tracer method averaged 3.07 μg C m−3 in summer and 2.00 μg C m−3 in fall-winter, contributing 38.4% and 8.7% to OC, respectively. During summer, aromatics-SOC and isoprene-SOC reached 2.25 ± 1.5 μg C m−3 and 0.64 ± 0.7 μg C m−3 and accounted for 76% and 18% of the estimated SOC, respectively, while during fall-winter, aromatics-SOC (1.64 ± 1.4 μg C m−3) was dominant with a share of 79% in total estimated SOC. These results indicated that anthropogenic aromatics were dominant SOC precursors in the highly industrialized and urbanized PRD region. During summer, SOC levels estimated by elemental carbon (EC) tracer method were not only consistent with but also correlated well with those by SOA-tracer method. During fall-winter, however, SOC by SOA-tracer method was only about one third of that by EC-tracer method. Their gaps were significantly correlated with the biomass burning tracer levoglucosan, indicating that input from biomass burning emission with very high ratios of OC/EC during fall-winter would result in an overestimate of SOC by EC-tracer method. Therefore cautions should be taken when estimating SOC by EC-tracer method, especially when biomass burning exhibits significant influences.
At a rural site in the central Pearl River Delta (PRD) region in south China, fine particle (PM2.5) samples were collected during fall-winter 2007 to measure biogenic secondary organic aerosol (SOA) tracers, including isoprene SOA tracers (3-methyl-2,3,4-trihydroxy-1-butene, 2-methylglyceric acid, 2-methylthreitol and 2-methylerythritol), α-pinene SOA tracers (cis-pinonic acid, pinic acid, 3-methyl-1,2,3-butanetricarboxylic acid, 3-hydroxyglutaric acid and 3-hydroxy-4,4-dimethylglutaric acid) and a sesquiterpene SOA tracer (β-caryophyllinic acid). The isoprene-, α-pinene- and sesquiterpene-SOA tracers averaged 30.8±15.9, 6.61±4.39, and 0.54±0.56ngm−3, respectively; and 2-methyltetrols (sum of 2-methylthreitol and 2-methylerythritol, 27.6±15.1ngm−3) and cis-pinonic acid (3.60±3.76ngm−3) were the dominant isoprene- and α-pinene-SOA tracers, respectively. 2-Methyltetrols exhibited significantly positive correlations (p<0.05) with ambient temperature, probably resulting from the enhanced isoprene emission strength and tracer formation rate under higher temperature. The significantly positive correlation (p<0.05) between 2-methyltetrols and the estimated aerosol acidity with a slope of 59.4±13.4ngm−3 per μmol [H+] m−3 reflected the enhancement of isoprene SOA formation by aerosol acidity, and acid-catalyzed heterogeneous reaction was probably the major formation pathway for 2-methyltetrols in the PRD region. 2-Methylglyceric acid showed poor correlations with both temperature and aerosol acidity. The α-pinene SOA tracers showed poor correlations with temperature, probably due to the counteraction between temperature effects on the precursor emission/tracer formation and gas/particle partitioning. Among the α-pinene SOA tracers, only cis-pinonic acid and pinic acid exhibited significant correlations with aerosol acidity with slopes of −11.7±3.7 and −2.2±0.8ngm−3 per μmol [H+] m−3, respectively. The negative correlations observed for α-pinene SOA tracers might result from their transfer from particle to gas phase with the increase of aerosol acidity. The ratio of cis-pinonic acid plus pinic acid to 3-methyl-1,2,3-butanetricarboxylic acid (MBTCA) ranged from 0.28 to 28.9 with a mean of 7.19, indicating the relatively fresh α-pinene SOA tracers during our campaign.
n intercomparison of organic carbon (OC) and elemental carbon (EC) measurements was conducted based on ambient aerosol samples collected during four seasons in Beijing, China. Dependence of OC and EC values on the temperature protocol and the charring correction method is presented and influences of aerosol composition are investigated. EC was found to decrease with the peak inert mode temperature (Tpeak) such that EC determined by the IMPROVE (the Interagency Monitoring of Protected Visual Environments)-A protocol (Tpeak was 580 °C) was 2.85 ± 1.31 and 3.83 ± 2.58 times that measured by an alternative protocol with a Tpeak of 850 °C when using the transmittance and reflectance correction, respectively. It was also found that reflectance correction tends to classify more carbon as EC compared with transmittance; results from the IMPROVE-A protocol showed that the ratio of EC defined by reflectance correction (ECR) to that based on transmittance (ECT) averaged 1.50 ± 0.42. Moreover, it was demonstrated that emissions from biomass burning would increase the discrepancy between EC values determined by different temperature protocols. On the other hand, the discrepancy between ECR and ECT was strongly associated with secondary organic aerosol (SOA) which was shown to be an important source of the organics that pyrolyze during the inert mode of thermal–optical analysis.
The coasts of Bohai Sea (BS) and Yellow Sea (YS) in China support almost one-quarter of its population and provide more than one-third of the national GDP. BS and YS are downwind of the Asian continental outflow in spring and winter as influenced by the East Asian monsoon. This makes the two seas important sinks of land-based pollutants associated with the Asian continental outflow. The sixteen U.S. EPA proposed priority polycyclic aromatic hydrocarbons (PAHs) in 130 surface sediment samples collected from BS and YS were measured. Combined with our previous PAH data of 90 PM2.5 samples from the upwind areas, the sources of the PAHs in BS and YS were apportioned using positive matrix factorization (PMF) modeling. Four sources were identified: petroleum residue, vehicular emissions, coal combustion and biomass burning. Petroleum residue was the dominant contributor of PAHs in the coast of the Bohai Bay probably due to Haihe River runoff, oil leakage from ships and offshore oil fields. The contribution of vehicular emissions in BS was higher than that in YS, and the reverse was true for coal combustion and biomass burning. This difference in the source patterns in the sediments of the two seas could be attributed to the different PAH emission features of the upwind area related to demographic and economic conditions, as well as the marine geography. The ratios of selected 4–6 ring PAHs in the sediments compared well with those of the PM2.5 of the upwind areas, implicating that the particle phase PAHs in the atmosphere play an important role in the source to sink process of the pyrogenic PAHs in the region.