Aerosol sampling during photochemically active times
across the Los Angeles Basin has provided evidence of
secondary formation of organic aerosol from gas-phase
precursors at midday. Ambient organic carbon/elemental
carbon ratios exceeded the estimated ratio of organic
carbon/elemental carbon in primary source emissions on
most sampling days at all sites. The concentration of
secondary organic aerosol was calculated by using ambient
data and estimates of the organic carbon/elemental carbon
ratio in primary source emissions. Nonparametric sign
correlations comparing calculated secondary organic carbon
concentrations with tracers of both primary and secondary
aerosols supported the method used to quantify
secondary organic carbon. Secondary organic aerosol appears
to have contributed roughly half of the organic
aerosol in Pasadena during midday summer conditions
