Development and validation of a NO_x⁺ ratio method for the quantitative separation of inorganic and organic nitrate aerosol using CV-UMR-ToF-ACSM

Abstract. Particulate nitrate is a major component of ambient aerosol around the world, present in inorganic form mainly as ammonium nitrate, and also as organic nitrate. It is of increasing importance to monitor ambient particulate nitrate, a reservoir of urban nitrogen oxides that can be transported downwind and harm ecosystems. The unit-mass-resolution time-of-flight aerosol chemical speciation monitoring equipped with capture vaporizer (CV-UMR-ToF-ACSM) is designed to quantitatively monitor ambient PM_2.5 composition. In this paper, we describe a method for separating the organic and ammonium nitrate components measured by CV-UMR-ToF-ACSM based on evaluating the NO₂⁺/NO⁺ ratio (NO_x⁺ ratio). This method includes modifying the ACSM fragmentation table, time averaging, and data filtering. By using the measured NO_x⁺ ratio of NH₄NO₃ and a plausible range of NO_x⁺ ratio for organic nitrate aerosol, the measured particulate nitrate can be split into inorganic and organic fractions. Time averaging and data filtering results in a concentration limit of 0.6 μg m^-3 total particulate nitrate, above which this method could be used. We show that this method is able to distinguish periods with inorganic or organic nitrate as major components at a rural site in the Netherlands. A comparison to a high-resolution time-of-flight aerosol mass spectrometer equipped with a standard vaporizer (SV-HR-ToF-AMS) shows a good correlation of particulate organic nitrate fraction between the instruments (CV/SV = 1.59; r² = 0.92). We propose that researchers use this NO_x⁺ ratio method for CV-UMR-ToF-ACSM to quantify the particulate organic nitrate fraction at existing monitoring sites in order to improve understanding of nitrate formation and speciation.