Performance evaluation and use of a continuous monitor for measuring size-fractionated PM2.5 nitrate

A field evaluation of a new size-fractionating continuous fine particle nitrate monitor from Aerosol Dynamics Inc. (ADI), Berkeley, CA was conducted via comparison to traditional time-integrated filter (HEADS) and impactor (MOUDI) measurements. The new monitor consists of three cascaded integrated collection and vaporization cells (ICVC) and provides 10-min resolution particulate nitrate measurements in three particle diameter size ranges (0.10-0.45, 0.45-1.0, and 1.0-2.5 mum) corresponding to observed submodes in the particle size distribution in Southern California. Side-by-side sampling was conducted for approximately six months at two sites, both at downwind receptor locations east of downtown Los Angeles. Both size-resolved and total PM2.5 nitrate concentrations were compared among the different sampling techniques. The ADI monitor and HEADS PM2.5 nitrate measurements, for which nitrate sampling artifacts are expected to be low, are well correlated (r(2)=0.79) with a geometric mean ADI: HEADS ratio of 0.90. The ADI size-fractionated nitrate data measured consistently more nitrate than the corresponding MOUDI stages due to volatilization of labile ammonium nitrate from the MOUDI impaction substrates. Less disagreement was observed in the 1.0-2.5 mum size range in which nitrate is more likely to exist as nonlabile sodium nitrate. The observed MOUDI nitrate losses are attributable to existing theories of nitrate sampling efficiencies and losses. The continuous nature of the data generated by the ADI monitor will provide valuable information on the spatial and temporal distribution of particulate nitrate in the atmosphere, and the new dimension of size-fractionation can help to determine the sources and formation mechanisms of atmospheric particulate nitrate as well. Examples of the atmospheric data generated in this study are presented and the potential utility of such data provided by the new monitor are discussed.