Development and validation of a NO⁺_x ratio method for the quantitative separation of inorganic and organic nitrate aerosol using a unit-mass-resolution time-of-flight aerosol chemical speciation monitor equipped with a capture vaporizer (CV-UMR-ToF-ACSM)

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 monitor 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 NO⁺₂ /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. Data pre-treatment filters concentrations of particulate nitrate below 0.6–2.0 µg m⁻³, depending on the time averaging. The method detection limit, when considering ±10 % absolute uncertainty of organic nitrate fraction, is found to be 2 µg m⁻³ (120 min averaging) to 10 µg m⁻³ (10 min averaging) for total particulate nitrate concentration and 10 % (120 min) to 20 % (10 min) for organic nitrate fraction. 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) and positive matrix factorization (PMF) method shows similar response of increasing particulate organic nitrate fraction with uncertainties mainly from sensitivity to fragmentation table correction when obtaining the NO⁺₂ signal. We propose that researchers use this NO⁺_x ratio method for CV-UMR-ToF-ACSM (adapting the appropriate fragmentation table and data pre-treatment for each specific application) to quantify the particulate organic nitrate fraction at existing monitoring sites in order to improve understanding of nitrate formation and speciation.