Primary and Secondary Organic Nitrate in Northwest China: A Case Study

Particle-bound organic nitrate is ubiquitous in the atmosphere, but its primary emission and secondary formation processes are poorly understood and are, therefore, generally ignored in models. In this study, based on the ambient measurements and a series of combustion experiments, we found that the nitrate functionality from organic nitrate (primary + secondary) accounted for 20%–53% of the total NO3 signal measured with a long-time-of-flight (LToF) soot particle aerosol mass spectrometer (SP-AMS) in northwestern China in mid-November 2020. In particular, secondary organic nitrate accounted for 17% and 36% of the measured NO3 during the haze and reference periods, respectively. It was further estimated that secondary organic nitrate, on average, accounted for 46% of the total organic matter (or 18% of PM2.5) during the haze period, representing a significant portion of the secondary organic aerosol, while primary organic nitrate, mostly from biomass burning, was estimated to account for 8%–9% of the total organic matter (3%–5% of PM2.5). Moreover, we showed that photochemical (OH-initiated + elevated NO2) formation of organic nitrate was probably a more important pathway than the nighttime chemistry (NO3 initiated), which is in great contrast to that in European areas where nighttime chemistry was found to be the dominant pathway.