Factors Influencing the Formation of Nitrous Acid from Photolysis of Particulate Nitrate

Enhanced photolysis of particulate nitrate (pNO₃) to form photolabile species, such as gas-phase nitrous acid (HONO), has been proposed as a potential mechanism to recycle nitrogen oxides (NO_x) in the remote boundary layer ("renoxification"). This article presents a series of laboratory experiments aimed at investigating the parameters that control the photolysis of pNO₃ and the efficiency of HONO production. Filters on which artificial or ambient particles had been sampled were exposed to the light of a solar simulator, and the formation of HONO was monitored under controlled laboratory conditions. The results indicate that the photolysis of pNO₃ is enhanced, compared to the photolysis of gas-phase HNO₃, at low pNO₃ levels, with the enhancement factor reducing at higher pNO₃ levels. The presence of cations (Na⁺) and halides (Cl^-) and photosensitive organic compounds (imidazole) also enhance pNO₃ photolysis, but other organic compounds such as oxalate and succinic acid have the opposite effect. The precise role of humidity in pNO₃ photolysis remains unclear. While the efficiency of photolysis is enhanced in deliquescent particles compared to dry particles, some of the experimental results suggest that this may not be the case for supersaturated particles. These experiments suggest that both the composition and the humidity of particles control the enhancement of particulate nitrate photolysis, potentially explaining the variability in results among previous laboratory and field studies. HONO observations in the remote marine boundary layer can be explained by a simple box-model that includes the photolysis of pNO₃, in line with the results presented here, although more experimental work is needed in order to derive a comprehensive parametrization of this process.