Aerosol time-of-flight mass spectrometry (ATOFMS) instruments measure the size and chemical composition of individual particles in real-time. ATOFMS chemical composition measurements are difficult to quantify, largely because the instrument sensitivities to different chemical species in mixed ambient aerosols are unknown. In this paper, we develop a field-based approach for determining ATOFMS instrument sensitivities to ammonium and nitrate in size-segregated atmospheric aerosols, using tandem ATOFMS-impactor sampling. ATOFMS measurements are compared with collocated impactor measurements taken at Riverside, CA, in September 1996, August 1997, and October 1997. This is the first comparison of ion signal intensities from a single-particle instrument with quantitative measurements of atmospheric aerosol chemical composition. The comparison reveals that ATOFMS instrument sensitivities to both (NH{sub}4){sup}+ and (NO{sub}3){sup}- decline with increasing particle aerodynamic diameter over a 0.32-1.8 μm calibration range. The stability of this particle size dependence is tested over the broad range of fine particle concentrations (PM{sub}1.8 = 17.6 ± 2.0-127.8 ± 1.8 μg m{sup}(-3)), ambient temperatures (23-35 ℃), and relative humidity conditions (21- 69), encountered during the field experiments. This paper describes a potentially generalizable methodology for increasing the temporal and size resolution of atmospheric aerosol chemical composition measurements, using tandem ATOFMS-impactor sampling.
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