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 (NOx) 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.

Factors influencing the outdoor concentration of carbonaceous aerosols at urban schools in Brisbane, Australia: Implications for children's exposure.

This comprehensive study aimed to determine the sources and driving factors of organic carbon (OC) and elemental carbon (EC) concentrations in ambient PM2.5 in urban schools. Sampling was conducted outdoors at 25 schools in the Brisbane Metropolitan Area, Australia. Concentrations of primary and secondary OC were quantified using the EC tracer method, with secondary OC accounting for an average of 60%. Principal component analysis distinguished the contributing sources above the background and identified groups of schools with differing levels of primary and secondary carbonaceous aerosols. Overall, the results showed that vehicle emissions, local weather conditions and secondary organic aerosols (SOA) were the key factors influencing concentrations of carbonaceous component of PM2.5 at these schools. These results provide insights into children's exposure to vehicle emissions and SOA at such urban schools.