Spatial and temporal variations of particulate precursor gases and photochemical reaction products during SJVAQS/AUSPEX ozone episodes

SJVAQS/AUSPEX acquired PM2.5, PM10, and gas samples at ten sites in central California during five ozone episodes over a period of 14 intensive sampling days. Four sample sets per day were collected for 5-and 7-hour durations during this period and analyzed for particulate mass, elements, ions, and organic and elemental carbon (OC and EC). Gaseous ammonia, nitric acid, and sulfur dioxide were collected with absorbing filters. Ozone (O3), nitrogen oxides (NOx), and peroxyacetyl nitrate (PAN) were measured continuously. Concentrations varied by sampling site, time of the day, and ozone episode. The highest concentrations of most chemical species were found at sites in the southern San Joaquin Valley. Nitrate occurred mainly in the gas phase during the daytime when temperatures exceeded 25°C, and in the particulate phase during the nighttime when temperatures decreased to less than 10°C. Significant amounts of nitrate volatilized from particles collected on quartz-fiber filters. Diurnal patterns of ozone, nitrogen oxides, PAN, nitric acid, and total particle nitrate were similar at most of the sites, which were separated by hundreds of kilometers, showing evidence of both photochemical conversion and transport of polluted air masses.