10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
Sources of Brown Carbon in Urban Environments: Importance of Vehicular Emissions
NETHMI KASTHURIARACHCHI, Max Adam, Yue Liang, Dong Zhang, Alex Lee, National University of Singapore
Abstract Number: 260 Working Group: Carbonaceous Aerosol
Abstract Carbonaceous aerosols are known to affect the global climate through direct absorption and scattering of solar radiation. While black carbon (BC) is the most efficient light absorbing aerosol, brown carbon (BrC) has been found to efficiently absorb radiation in the near UV and visible wavelengths. Although biomass burning has been extensively studied as a primary source of BrC, there is increasing laboratory evidence that incomplete fossil fuel combustion, such as vehicular emissions, can be a potential source of BrC in urban environments with minimal influence from biomass burning.
In this study, a seven-wavelength Aethalometer (AE33, Magee Scientific), which gives loading factor-corrected absorbance measurements at 370nm, 440nm, 520nm, 590nm, 660nm, 880nm and 950nm, was deployed to measure ambient BC and BrC in Singapore. A co-located soot-particle aerosol mass spectrometer (SP-AMS, Aerodyne Research) was used to measure non-refractory chemical composition of ambient particles. No biomass burning episodes due to transboundary smoke were reported during the month-long sampling period. The mean absorption Ångström exponent (AAE) of ambient particles was 1.06 ± 0.05, further indicating the absence of biomass burning influence (i.e., AAE > 2 for biomass burning smoke). Comparison of ambient and thermodenuded (at 200oC) particles confirms the presence of BrC in ambient particles. Assuming AAE of ambient pure BC is equal to 1, the light absorption properties of ambient BrC were quantified. The mean AAE of BrC was estimated to be in the range of 3 to 4, which is comparable to the values measured in urban environments with strong influence from fossil fuel combustion.
Positive matrix factorization (PMF) of organic fragments measured by SP-AMS identified four major types of organic aerosols, including hydrocarbon-like organic aerosols (HOA), cooking-related organic aerosols (COA) and less and more oxidised oxygenated organic aerosols (LO-OOA and MO-OOA). Out of the four types of organic aerosols, HOA showed the strongest correlation (r2 = 0.6-0.7) with the BrC absorbance at 370nm. HOA are known to be emitted from vehicles and off-road engines and showed strong correlation with BC absorbance at 880nm (r2 > 0.75). For the total absorption at 370nm, multiple linear regression analysis for BC and the four organic aerosol components gave the mass absorption coefficient of HOA (MACHOA,370nm , m2/g), a measurement proxy of BrC, approximately equal to 1.22 m2/g. This value is comparable to the reported MAC365nm values of biomass burning organic aerosols. Multiplying the MAC for each component by the mean mass concentration showed that HOA contributed up to ~6% and ~70% of total (i.e., BrC + BC) and BrC absorbance at 370 nm, respectively. Overall, our preliminary results provide field evidence to highlight that vehicular emissions can be an important source of ambient BrC in urban environments. Volatility of ambient BrC and organic aerosols based on the thermodenuder measurements will be discussed.