Comparison of two winter air quality episodes during the California Regional Particulate Air Quality Study

The duration, strength, spatial extent, and chemical makeup of particulate matter (PM) are compared for two winter air quality episodes captured during the California Regional Particulate Air Quality Study (CRPAQS). Each episode, from the beginning of the buildup through dissolution, lasted about 3 weeks. The first episode occurred from December 14, 1999, through January 1, 2000, with peak 24-hr average fine particulate matter (PM2.5) concentrations reaching 129 microg/m3. The second episode occurred a year later, from December 18, 2000, through January 8, 2001, with peak 24-hr average PM2.5 concentrations reaching 179 microg/m3. Although similar in duration, each episode exhibited unique characteristics. One significant difference was the episode buildup rate; rapid in 1999, but slow and steady in 2000. The rapid buildup of the first episode resulted in more days with PM2.5 concentrations above the 24-hr federal standard, whereas the slow and steady increase of the second episode produced higher peaks. Spatial extent and progress also differed between the two episodes. The Northern Valley was impacted more during the December 1999 episode, and the Southern Valley during the December 2000 episode. The differences carried over into chemical composition. Ammonium nitrate dominated the PM2.5 mass during the December 1999 episode. The second episode reflected a dichotomy typical to the San Joaquin Valley, with Fresno concentrations dominated by organic and elemental carbon and the rest of the Valley concentrations dominated by ammonium nitrate. Each episode showed a regional as well as a local component. Ammonium nitrate concentrations, which result from more regional-scale secondary formation and mixing of emissions, were fairly uniform among the urban and rural sites. Carbon concentrations were always higher at urban sites than at rural sites, corresponding to the higher emissions density of primary carbon sources in urban areas.