Rebecca J. Sheesley

Brown Clouds over South Asia: Biomass or Fossil Fuel Combustion?

Carbonaceous aerosols cause strong atmospheric heating and large surface cooling that is as important to South Asian climate forcing as greenhouse gases, yet the aerosol sources are poorly understood. Emission inventory models suggest that biofuel burning accounts for 50 to 90% of emissions, whereas the elemental composition of ambient aerosols points to fossil fuel combustion. We used radiocarbon measurements of winter monsoon aerosols from western India and the Indian Ocean to determine that biomass combustion produced two-thirds of the bulk carbonaceous aerosols, as well as one-half and two-thirds of two black carbon subfractions, respectively. These constraints show that both biomass combustion (such as residential cooking and agricultural burning) and fossil fuel combustion should be targeted to mitigate climate effects and improve air quality.

Natural Abundance 13C and 14C Analysis of Water-Soluble Organic Carbon in Atmospheric Aerosols†

Water-soluble organic carbon (WSOC) constitutes a large fraction of climate-forcing organic aerosols in the atmosphere, yet the sources of WSOC are poorly constrained. A method was developed to measure the stable carbon isotope (δ(13)C) and radiocarbon (Δ(14)C) composition of WSOC for apportionment between fossil fuel and different biogenic sources. Synthetic WSOC test substances and ambient aerosols were employed to investigate the effect of both modern and fossil carbon contamination and any method-induced isotope fractionation. The method includes extraction of aerosols collected on quartz filters followed by purification and preparation for off-line δ(13)C and Δ(14)C determination. The preparative freeze-drying step for isotope analysis yielded recoveries of only ∼70% for ambient aerosols and WSOC probes. However, the δ(13)C of the WSOC isolates were in agreement with the δ(13)C of the unprocessed starting material, even for the volatile oxalic acid probe (6.59 ± 0.37‰ vs 6.33 ± 0.31‰; 2 sd). A (14)C-fossil phthalic acid WSOC probe returned a fraction modern biomass of <0.008 whereas a (14)C-modern sucrose standard yielded a fraction modern of >0.999, indicating the Δ(14)C-WSOC method to be free of both fossil and contemporary carbon contamination. Application of the δ(13)C/Δ(14)C-WSOC method to source apportion climate-affecting aerosols was illustrated be constraining that WSOC in ambient Stockholm aerosols were 88% of contemporary biogenic C3 plant origin.

Source Contributions to Wintertime Elemental and Organic Carbon in the Western Arctic Based on Radiocarbon and Tracer Apportionment.

To quantify the contributions of fossil and biomass sources to the wintertime Arctic aerosol burden source apportionment is reported for elemental (EC) and organic carbon (OC) fractions of six PM10 samples collected during a wintertime (2012-2013) campaign in Barrow, AK. Radiocarbon apportionment of EC indicates that fossil sources contribute an average of 68 ± 9% (0.01-0.07 μg m(-3)) in midwinter decreasing to 49 ± 6% (0.02 μg m(-3)) in late winter. The mean contribution of fossil sources to OC for the campaign was stable at 38 ± 8% (0.04-0.32 μg m(-3)). Samples were also analyzed for organic tracers, including levoglucosan, for use in a chemical mass balance (CMB) source apportionment model. The CMB model was able to apportion 24-53% and 99% of the OC and EC burdens, respectively, during the campaign, with fossil OC contributions ranging from 25 to 74% (0.02-0.09 μg m(-3)) and fossil EC contributions ranging from 73 to 94% (0.03-0.07 μg m(-3)). Back trajectories identified two major wintertime source regions to Barrow: the Russian and North American Arctic. Atmospheric lifetimes of levoglucosan, ranging from 50 to 320 h, revealed variability in wintertime atmospheric processing of this biomass burning tracer. This study allows for unambiguous apportionment of EC to fossil fuel and biomass combustion sources and intercomparison with CMB modeling.

Radiocarbon-based source apportionment of elemental carbon aerosols at two South Asian receptor observatories over a full annual cycle

Black carbon (BC) aerosols impact climate and air quality. Since BC from fossil versus biomass combustion have different optical properties and different abilities to penetrate the lungs, it is important to better understand their relative contributions in strongly affected regions such as South Asia. This study reports the first year-round 14C-based source apportionment of elemental carbon (EC), the mass-based correspondent to BC, using as regional receptor sites the international Maldives Climate Observatory in Hanimaadhoo (MCOH) and the mountaintop observatory of the Indian Institute of Tropical Meteorology in Sinhagad, India (SINH). For the highly-polluted winter season (December–March), the fractional contribution to EC from biomass burning (fbio) was 53 ± 5% (n = 6) at MCOH and 56 ± 3% at SINH (n = 5). The fbio for the non-winter remainder was 53 ± 11% (n = 6) at MCOH and 48 ± 8% (n = 7) at SINH. This observation-based constraint on near-equal contributions from biomass burning and fossil fuel combustion at both sites compare with predictions from eight technology-based emission inventory (EI) models for India of (fbio)EI spanning 55–88%, suggesting that most current EI for Indian BC systematically under predict the relative contribution of fossil fuel combustion. A continued iterative testing of bottom-up EI with top-down observational source constraints has the potential to lead to reduced uncertainties regarding EC sources and emissions to the benefit of both models of climate and air quality as well as guide efficient policies to mitigate emissions.

Temporal trends in motor vehicle and secondary organic tracers using in situ methylation thermal desorption GCMS.

Organic aerosol measurements with high temporal resolution can differentiate primary organic carbon (POC) from secondary organic carbon (SOC) and can be used to distinguish morning rush hour traffic emissions and subsequent photo-oxidation. In the current study, five hour filter samples were collected during the Summer Study for Organic Aerosols at Riverside (SOAR-1 in CA, USA) for analysis of organic molecular markers. To achieve the low detection limits required for the high temporal resolution data, a laboratory-based in situ methylation thermal desorption gas chromatography-mass spectrometry method was developed. This enabled the measurement of potential markers of SOC, including phthalic acid, along with markers for traffic emissions, including norhopane. The aromatic acids correlated well with unapportioned OC from a molecular marker chemical mass balance model (SOC-cmb; r(2) = 0.46-0.70) and SOC from the elemental carbon tracer method (SOC-ec; r(2) = 0.40-0.56). The aromatic acid/norhopane ratio increased substantially over the course of each day. The average mid-day phthalic acid ratio compared to previously published roadway emissions was a factor of 4 times higher, while the average 1,2,3-benzenetricarboxylic acid ratio was a factor of 40 times higher than roadway emissions. Using correlation plots of SOC-cmb and phthalic acid, it was estimated that 2.9 ± 0.6 μg m(-3) SOC was associated with mid-day aromatic acid production in Riverside.

Pressurized liquid extraction technique for the analysis of pesticides, PCBs, PBDEs, OPEs, PAHs, alkanes, hopanes, and steranes in atmospheric particulate matter

An analytical method has been developed for the pressurized liquid extraction (PLE) of a wide range of semi-volatile organic compounds (SVOCs) from atmospheric particulate matter. Approximately 130 SVOCs from eight compound classes were selected as molecular markers of (1) agricultural activity (30 current and historic-use pesticides), (2) industrial activity (18 PCBs), (3) consumer products and building materials (16 PBDEs, 11 OPEs), and (4) motor vehicle exhaust (22 PAHs, 16 alkanes, 9 hopanes, 8 steranes). Currently, there is no analytical method validated for the extraction of all eight compound classes in a single automated technique. The extraction efficiencies of varying solvents and solvent combinations at high temperatures and pressures were examined. Extracts were concentrated and analyzed by gas chromatography coupled with mass spectrometry. The optimized PLE method utilized methylene chloride:acetone (2:1 v/v) at 100 °C with three (5 min) static cycles, flush volume of 80%, and a 100 s N2 purge. Spike and recovery experiments (n=7) provided average percent recoveries for pesticides, PCBs, PBDEs, OPEs, PAHs, alkanes, hopanes, and steranes of 88.8±4.0%, 86.9±2.6%, 83.8±2.9%, 101±6%, 90.3±6.1%, 74.4±8.8%, 104±8%, and 86.5±8.6%, respectively. The developed method was applied to atmospheric particulate matter samples collected in the greater Houston, TX metropolitan area. Ambient concentrations of eight classes of compounds (92 SVOCs) were reported in pg m(-3).

Evaluation of ground‐based black carbon measurements by filter‐based photometers at two Arctic sites

Long-term measurements of the light absorption coefficient (b(abs)) obtained with a particle soot absorption photometer (PSAP), b(abs) (PSAP), have been previously reported for Barrow, Alaska, and ...

Dual‐carbon‐isotope characterization of total organic carbon in wintertime carbonaceous aerosols from northern India

Large-scale emissions of carbonaceous aerosols (CA) from South Asia impact both regional climate and air quality, yet their sources are not well constrained. Here we use source-diagnostic stable an ...

Spatial and Temporal Distributions of Organophosphate Ester Concentrations from Atmospheric Particulate Matter Samples Collected across Houston, TX

Atmospheric particulate matter (PM) samples were collected from four ground-based sites located in the Houston, TX (September 21-28, 2013) and were analyzed for 12 organophosphate esters (OPEs; current-use plasticizers and flame retardants). Samples analyzed included daytime, nighttime, and 24 h PM of <2.5 μm aerodynamic diameter (PM2.5) and total suspended particulate (TSP) samples. PM2.5 and TSP atmospheric ΣOPE concentrations varied over an order of magnitude and were statistically significantly different between urban and suburban and industrial sites. Additionally, significant temporal variability was also identified; for example, daytime atmospheric concentrations of 2-ethylhexyl diphenyl phosphate (EHDPP; 610 ± 220 pg m-3) measured in TSP samples were significantly higher than nighttime concentrations (280 ± 180 pg m-3; p = 0.03). Detailed discussions of the spatial and temporal distribution are given for Tris-(1-chloro-2-propyl) phosphate (TCiPP), EHDPP, tri-n-butyl phosphate (TnBP), and triphenyl phosphate (TPhP). Correlations to bulk measurements of carbonaceous PM including organic carbon, elemental carbon, and water-soluble organic carbon were used to understand potential sources and urban atmospheric transport. These results highlight the fundamental complexity associated with assessing OPE atmospheric concentrations across a large urban landscape and specific knowledge gaps at the intersection of consumer products and safety with environmental and human health.

Urban impacts on regional carbonaceous aerosols: case study in central Texas.

Rural and background sites provide valuable information on the concentration and optical properties of organic, elemental, and water-soluble organic carbon (OC, EC, and WSOC), which are relevant for understanding the climate forcing potential of regional atmospheric aerosols. To quantify climate- and air quality-relevant characteristics of carbonaceous aerosol in the central United States, a regional background site in central Texas was chosen for long-term measurement. Back trajectory (BT) analysis, ambient OC, EC, and WSOC concentrations and absorption parameters are reported for the first 15 months of a long-term campaign (May 2011–August 2012). BT analysis indicates consistent north–south airflow connecting central Texas to the Central Plains. Central Texas aerosols exhibited seasonal trends with increased fine particulate matter (<2.5 μm aerodynamic diameter, PM2.5) and OC during the summer (PM2.5 = 10.9 μg m−3 and OC = 3.0 μg m−3) and elevated EC during the winter (0.22 μg m−3). When compared to measurements in Dallas and Houston, TX, central Texas OC appears to have mixed urban and rural sources. However, central Texas EC appears to be dominated by transport of urban emissions. WSOC averaged 63% of the annual OC, with little seasonal variability in this ratio. To monitor brown carbon (BrC), absorption was measured for the aqueous WSOC extracts. Light absorption coefficients for EC and BrC were highest during summer (EC MAC = 11 m2 g−1 and BRC MAE365 = 0.15 m2 g−1). Results from optical analysis indicate that regional aerosol absorption is mostly due to EC with summertime peaks in BrC attenuation. This study represents the first reported values of WSOC absorption, MAE365, for the central United States. Implications: Background concentration and absorption measurements are essential in determining regional potential radiative forcing due to atmospheric aerosols. Back trajectory, chemical, and optical analysis of PM2.5 was used to determine climatic and air quality implications of urban outflow to a regional receptor site, representative of the central United States. Results indicate that central Texas organic carbon has mixed urban and rural sources, while elemental carbon is controlled by the transport of urban emissions. Analysis of aerosol absorption showed black carbon as the dominant absorber, with less brown carbon absorption than regional studies in California and the southeastern United States.