| Airborne fine particulate matter (PM2.5) has been linked to adverse human health effects, reduced visibility, climate change, and other air quality concerns. Typically, the major contributors of PM2.5 include mobile source emissions, biomass burning, and secondary sources with anthropogenic and biogenic nature in origin. The metropolitan Atlanta, GA area, located in the southeastern U.S and populated by over 5.4 million residents, is of particular interest for air quality study and air pollutant control due to high emissions of mobile sources, biomass burning, coal-fired power plants and biogenic volatile organic compounds (VOC), with vigorous photochemical processes occurring as well.;Effective control strategies for air pollutants require a detailed investigation of chemical composition of airborne PM2.5 in this area as well as quantitative identification of specific source impacts on ambient air quality. In this research, various airborne PM2.5 samples were collected and analyzed, which are directly impacted or dominated by onroad mobile and other typical urban emissions, regional transport sources, prescribed burning plumes, wildfire plumes, as well as secondary sources with anthropogenic and biogenic nature in origin. Day-night, seasonal and spatial variations of PM 2.5 characterization were also studied. The impacts or contributions of major sources were identified quantitatively through the receptor source apportionment models. These modeling results, especially on-road mobile source contributions and secondary organic carbon (SOC) were assessed by multiple approaches to provide additional information for PM2.5 control strategies. Furthermore, season- and location-specific source profiles were developed in this research to reflect real-world and representative local emission characterizations of on-road mobile sources, aged prescribed burning plumes, and wildfire plumes. Secondary organic aerosol (SOA), a major component of PM2.5 in the summer, was also explored for contribution and individual sources.;To investigate on-road emissions, regional transport and SOA effects, 12-hr and 24-hr PM2.5 filter samples were collected in summer 2005 and winter 2006 from three sites: two from urban Atlanta (one site adjacent to a freeway and another 400 m away), and one at a rural site. Detailed PM 2.5 chemical speciation was conducted, including organic carbon (OC), elemental carbon (EC), water-soluble OC (WSOC), ionic species, tens of trace metals, and over one hundred of solvent-extractable organic compounds. In particular, this research focused on primary and secondary organic molecular markers in airborne PM2.5, which were identified and quantified by the gas chromatography/mass spectrometry (GC/MS) method, as well as source apportionment for fine OC and PM2.5.;Our results show that organic matter, sulfate and ammonium are major components of PM2.5 in both seasons, with significantly higher levels found in the summer; whereas nitrate is important only in the winter. Sulfate dominates PM2.5 in the summer, particularly on haze days. Homogeneous distributions of WSOC reflect impacts from SOA in the summer and from biomass burning emissions in the winter. Primary organic compounds usually exhibit different attributes of day vs. night, whereas secondary organic tracers vary little. Much higher concentrations of automotive-related primary species, especially EC and some primary organic compounds are observed at the roadside site. Season-specific on-road mobile source profiles were developed by using differences in chemical species concentrations between the roadside site and the nearby campus site. Calculated on-road source profiles differ from mobile source profiles measured in laboratory elsewhere. Significant seasonal differences are observed for 2- methyltetrols, cis-pinonic acid and pinic acid, organic tracers of biogenic SOA. Little correlation is found between 2-methyltetrols with cis-pinonic or pinic acid, whereas cispinonic and pinic acids are strongly correlated with each other. In addition, particulate organic matter (OM) was estimated through mass balance analysis of gravimetric PM2.5, and the OM/OC ratio was found to depend on season and location.;Source apportionment of PM2.5 and organic carbon were performed using the molecular marker-based chemical mass balance (CMB-MM) model. Contributions of major primary sources were calculated, including diesel vehicle exhaust, gasoline vehicle exhaust, wood combustion, meat cooking, road dust, and vegetative detritus. CMB-MM modeled roadway-related source contributions (i.e., diesel vehicle exhaust, gasoline vehicle exhaust and road dust) were evaluated at the roadside site by comparing to differences of total OC measurements between the roadside site and the nearby campus site. As a particular focus of this active research, SOC contribution was estimated by four different approaches: (1) the CMB-MM model; (2) the EC tracer method; (3) the WSOC method; (4) the secondary organic tracer method. Finally, fraction boundaries of SOC in total OC were estimated at the roadside, the campus and the rural sites for the summer and the winter. Results suggest that SOC fractions in total OC usually distributed in a range with WSOC-estimated SOC as the lower bound and with CMB-MM-estimated SOC as the upper bound.;To better understand the processes impacting the aging of prescribed fire and wildfire plumes, a detailed chemical speciation of PM2.5 and carbonaceous aerosols was conducted by GC/MS analysis. Ambient concentrations of many organic species (levoglucosan, resin acids, retene, n-alkanes, and n-alkanoic acids) associated with wood burning emission were significantly elevated on the event days, whereas steranes, cholesterol and major polycyclic aromatic hydrocarbons (PAHs) did not show obvious increases. It is interesting to note that ambient hopanes increased significantly during wildfire smoke events, implying that hopanes, which are thought as unique tracers of mobile sources, can also be produced by thermal alteration of biogenic hopanoid precursors in the atmosphere. Strong odd over even carbon-number predominance was found for n-alkanes versus even over odd predominance for n-alkanoic acids. Observations suggest that resin acids altered during transport from burning sites to monitors. Our study also indicates that large quantities of biogenic VOCs and semivolatile organic compounds (SVOCs) were released both as products of combustion and unburned vegetation heated by the fire. Higher leaf temperature can stimulate biogenic VOC and SVOC emissions, which enhance formation of SOA in the atmosphere. This is supported by elevated ambient concentrations of secondary organic tracers (dicarboxylic acids, 2-methyltetrols, cis-pinonic acid, and pinic acid). An approximate source profile was built for the aged fire plume to help better understand the evolution of wood smoke emissions and for use in source impact assessment.;CMB-Regular and CMB-MM approaches were used and compared in this study to obtain source apportionment of PM2.5 data from the Southeastern Aerosol Research and Characterization Study (SEARCH) project. Temporal (winter and summer) and spatial impacts (urban and rural) on source contributions were analyzed. Results indicate a few similarities in source contributions between the two approaches. Secondary sources including secondary sulfate, ammonium, and nitrate contribute the majority of PM2.5 mass in the Southeast in both summer (>50%) and winter (>40%). Motor vehicle exhaust and wood burning are the major primary sources of PM2.5 in this area. Motor vehicle exhaust, paved road dust and wood burning impacts were calculated using both CMB-Regular and CMBMM. However, the differences in source apportionments between the two approaches are sometimes rather great. This disagreement can be traced to differences in: (1) fitting species selected; (2) source category identified; (3) source profile applied; and (4) model uncertainty generated. |
