Seasonal variation of the particle size distribution of polycyclic aromatic hydrocarbons and of major aerosol species in Claremont, California

摘要

As part of the Southern California Particle Center and Supersite (SCPCS) activities, we measured, during all seasons, particle size distributions of 12 priority pollutant polycyclic aromatic hydrocarbons (PAHs), concurrently with elemental carbon (EC), organic carbon (OC), sulfate (SO_4~(2-)), and nitrate (NO_3~-) size distributions, from October 2001 to July 2002 in Claremont, CA, a receptor site located about 40 km downwind of central Los Angeles. Samples were collected approximately once every week, for 24-h periods, from midnight to midnight. MOUDI impactors collected samples at 30 LPM which were composited for analysis into monthly periods in three aerodynamic diameter size intervals, defined for the purpose of this work, as: 0-0.18 μm (ultrafine mode), 0.18-2.5 μm (accumulation mode), and 2.5-10 μm (coarse mode). For the monthly composites from October to February, the size distributions of the target PAHs are similar. However, from March to July, notable differences are observed: a significant fraction of the PAH mass is found in the coarse mode, as compared with the previous period. During the entire 1-year period, the form and shape of the EC size distributions did not vary much and are distinguished by prominent mass in the ultrafine and accumulation size mode. For the individual modes of the major species, the highest Pearson's correlation coefficients for the variation of temperature with species concentration were found in the ultrafine mode for both SO_4~(2-) (0.92) and EC (0.90), and in the coarse mode for both OC (0.85) and NO_3~- (0.54). High SO_4~(2-) correlations are consistent with increased gas-to-particle formation during the warmer months from (precursor) SO_2 emissions in the Los Angeles and Lon Beach seaport areas and, similarly for EC, increased atmospheric transport to Claremont as the season progresses from winter to summer. Although not statistically significant, the correlations were negative for the less volatile or particle phase group (log [p_L~O] ≤-3.22), consistent with increased partitioning from the vapor phase with decreasing temperature.