Primary Organics Drive Poor Wintertime Air Quality in Los Angeles

RYAN X. WARD, Haroula D. Baliaka, Benjamin Schulze, Richard Flagan, John Seinfeld, California Institute of Technology

     Abstract Number: 567
     Working Group: Urban Aerosols

Abstract
The Los Angeles basin is well known for its historically poor air quality, and despite strengthening of regulations, declines in particulate matter (PM) concentrations have stagnated over the last decade. Comprehensive, multi-institutional surveys of air quality in Los Angeles (e.g. CalNex) have been most frequently performed in the summer (nominally when routine air quality measurements of PM are highest) and have revealed that much of the aerosol pollution is driven by oxygenated SOA. In this work, we deploy a suite of aerosol- and gas-phase instruments to characterize the wintertime atmosphere between January and March in Pasadena, CA, including a ToF-ACSM with PM_2.5 inlet for aerosol composition. We observe PM loadings that are often comparable to summertime AMS measurements of organic aerosol, but with a diurnal cycle whose maximum appears in the evening as opposed to the afternoon. Positive matrix factorization of the mass spectral data demonstrates that, in contrast to summertime conditions, aerosol is predominately derived from primary (e.g., combustion and cooking-related emissions) rather than secondary sources. Further, we compare with routine air quality measurements of PM, CO, and NO_x, which largely suggests that boundary layer dynamics play a dominant role in the shifted diurnal profile, and additionally that this profile is a robust feature of the wintertime atmosphere in Los Angeles. Our results show that primary aerosol sources remain particularly relevant to LA air quality in the winter, underscoring the need to consider seasonality in the aerosol source distribution when developing policies to further reduce regional air pollution.