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 (Da ≤ 1.0 μm) 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 (Da ≤ 1.0 μm). 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.