Understanding particulate matter formation in the California San Joaquin Valley: conceptual model and data needs

Quantitative information from the 1995 Integrated Monitoring Study (IMS95) is used to develop a conceptual model, which describes the chemical charact eristics and the physical processes responsible for the accumulation of PM in the San Joaquin Valley of California. One significant finding of the con ceptual model is the sensitivity of ammonium nitrate (46% of winter PM2.5) and nitric acid to oxidants, which may be VOC-sensitive rather than NOx-sen sitive. Key gaps in current knowledge are identified using the conceptual m odel, e.g., the relative sensitivity of winter oxidants to VOC and NOx, mec hanistic details of secondary organic aerosol formation, mechanisms of disp ersion under calm conditions, and the importance of dry deposition. Some re commendations are also provided for the formulation of air quality models s uitable to address the accumulation of PM in the San Joaquin Valley. (C) 19 99 Elsevier Science Ltd. All rights reserved.