Estimation of particulate organic nitrates from thermodenuder-aerosol mass spectrometer measurements in North China Plain

Abstract. Particulate organic nitrates (pON) are an important component of secondary organic aerosol in biogenic emission dominant environments, and play a critical role in NO_x cycles. However, estimation of pON has been a challenge in polluted environments, e.g., North China Plain with high concentrations of inorganic nitrate and NO_x. Here we developed a method for estimation of pON from the measurements of high-resolution aerosol mass spectrometer coupled with a thermodenuder based on the volatility differences between inorganic nitrate and pON. The results generally correlated well with those estimated from positive matrix factorization of combined organic and inorganic mass spectra and from the ratio of NO⁺ to NO₂⁺ (NO_x⁺ ratio), yet had improvements in reducing negative values due to the influences of high concentration of inorganic nitrate and constant NO_x⁺ ratio of organic nitrates (R_ON). By applying this approach to the measurements at an urban (Beijing) and a rural site (Gucheng) in summer and winter in North China Plain, we estimated that the average mass concentrations of NO_3,org (1.8 µg m⁻³ vs.1.0 µg m⁻³) and pON to OA (27.5 % vs. 14.8 %) were higher in summer than in winter in Beijing, indicating more pON formation in biogenically and anthropogenically mixed environments. In addition, the average NO_3,org loading in Gucheng was 1.9 µg m⁻³, and the pON at the rural site also showed higher contribution to OA than that in Beijing during wintertime due to higher primary emissions and gaseous precursors in Gucheng. In addition, R_ON was determined and showed considerable differences between day-night and clean-polluted periods, highlighting the complexity of pON compounds from different chemical pathways (e.g., OH and NO₃ oxidation) and sources.