The optical properties, composition and sources of the wintertime aerosols in the San Joaquin Valley (SJV) were characterized through measurements made in Fresno, CA during the 2013 DISCOVER-AQ campaign. PM2.5 extinction and absorption coefficients were measured at 405, 532, and 870 nm along with refractory black carbon (rBC) size distributions and concentrations. BC absorption enhancements (Eabs) were measured using two methods, a thermodenuder and mass absorption coefficient method, which agreed well. Relatively large diurnal variations in the Eabs at 405 nm were observed, likely reflecting substantial nighttime emissions of wood burning organic aerosols (OA) from local residential heating. Comparably small diurnal variations and absolute nighttime values of Eabs were observed at the other wavelengths, suggesting limited mixing-driven enhancement. Positive matrix factorization analysis of OA mass spectra from an aerosol mass spectrometer resolved two types of biomass burning OA, which appeared to have different chemical composition and absorptivity. Brown carbon (BrC) absorption was estimated to contribute up to 30% to the total absorption at 405 nm at night but was negligible (<10%) during the day. Quantitative understanding of retrieved BrC optical properties could be improved with more explicit knowledge of the BC mixing state and the distribution of coating thicknesses.
Bibliographical noteFunding Information:
We thank the CARB Fresno-Garland monitoring site for providing space and logistical help during the course of the study, and NASA for travel support and coordination of the larger DISCOVER-AQ study. Additional support was provided by the California Air Resources Board, Research Division, under Contract No. 13-330. C.D.C. and X.Z. thank Gavin McMeeking at DMT for the PAX instrument and Haf Jonsson for facilitating the deployment of the SP2. Q.Z., H.K., and C.P. thank NASA and the California Agricultural Experiment Station (Project CA-D-ETX-2102-H) for supporting the acquisition and analysis of the AMS data. This work has not been formally reviewed by the funding agencies and represents the views of the authors.