Continuous and filter-based measurements of PM2.5 nitrate and sulfate at the Fresno Supersite

PM2.5 nitrate (NO3-) and sulfate (SO4=) were measured continuously with R&P8400N and R&P8400S instruments, respectively, and compared with filter-based measurements at the Fresno Supersite from October, 2000 through December, 2005. NO3- concentrations were higher in winter than summer with a long-term decreasing trend. Correlations between 24-h average continuous and filter-based NO3- were greater than 0.96 in 4 out of 5 years. Continuous NO3- was generally lower than filter-based NO3- although the difference decreased over time, from -52% in 2001 to +13% in 2005. These differences were similar in winter (-23%) and summer (-19%) while the corresponding differences between ambient and instrument temperature were -12 and 0.7 degrees C, respectively. Neither seasonal nor long-term trends in NO3- can be explained by variations in ambient temperature, the difference between ambient and instrument temperature, or changes in aerosol chemical composition. There were no seasonal or long-term trends in SO4= concentrations, partially due to low concentrations observed in Fresno. Long-term variability in the performance of R&P8400 NO3- and SO4= instruments suggest that collocation with filter measurements is needed for long-term measurements.