Impact of biomass emissions on particle chemistry during the California Regional Particulate Air Quality Study

This paper describes aerosol time-of-flight mass spectrometry (ATOFMS) measurements of the size and chemical composition of individual particles during the California Regional Particulate Air Quality Study (CRPAQS) (December 2000-February 2001). In Fresno, biomass particles display distinct diurnal variations, peaking at night and reaching a minimum during the day. These biomass particles are small (D-a <= 1.0 mu m) and comprise 25% of the total analyzed particles with fractions ranging from 5% during the day to more than 60% at night. In addition, a unique collection of high mass organic carbon (HMOC) particles was identified with similar diurnal variations. The HMOC particles contain characteristic peaks between mass-to-charge (m/z) 100 and 200 in both the positive and negative ion mass spectra. HMOC particles only appear at night and have larger aerodynamic diameters (D-a <= 1.0 mu m). Furthermore, the HMOC particles show fragment ions of organic carbon, aromatic compounds, as well as non-mineral potassium, levoglucosan, and marker ions indicative of fog processing. We hypothesize the observed diurnal variations are due to an increase in direct biomass emissions followed by gas/particle partitioning of semivolatile species which undergo aqueous phase processing at night. (c) 2006 Published by Elsevier B.V.