Continuous and filter-based measurements of PM 2.5 nitrate and sulfate at the Fresno Supersite

PM(2.5) nitrate (NO-(3)) and sulfate (SO=(4)) 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. NO-(3) concentrations were higher in winter than summer with a long-term decreasing trend. Correlations between 24-h average continuous and filter-based NO-(3) were greater than 0.96 in 4 out of 5 years. Continuous NO-(3) was generally lower than filter-based NO-(3) 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 NO-(3) 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 SO=(4) concentrations, partially due to low concentrations observed in Fresno. Long-term variability in the performance of R&P8400 NO-(3) and SO=(4) instruments suggest that collocation with filter measurements is needed for long-term measurements.