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.

Effects of halogens on European air-quality.

Halogens (Cl, Br) have a profound influence on stratospheric ozone (O3). They (Cl, Br and I) have recently also been shown to impact the troposphere, notably by reducing the mixing ratios of O3 and OH. Their potential for impacting regional air-quality is less well understood. We explore the impact of halogens on regional pollutants (focussing on O3) with the European grid of the GEOS-Chem model (0.25° × 0.3125°). It has recently been updated to include a representation of halogen chemistry. We focus on the summer of 2015 during the ICOZA campaign at the Weybourne Atmospheric Observatory on the North Sea coast of the UK. Comparisons between these observations together with those from the UK air-quality network show that the model has some skill in representing the mixing ratios/concentration of pollutants during this period. Although the model has some success in simulating the Weybourne ClNO2 observations, it significantly underestimates ClNO2 observations reported at inland locations. It also underestimates mixing ratios of IO, OIO, I2 and BrO, but this may reflect the coastal nature of these observations. Model simulations, with and without halogens, highlight the processes by which halogens can impact O3. Throughout the domain O3 mixing ratios are reduced by halogens. In northern Europe this is due to a change in the background O3 advected into the region, whereas in southern Europe this is due to local chemistry driven by Mediterranean emissions. The proportion of hourly O3 above 50 nmol mol-1 in Europe is reduced from 46% to 18% by halogens. ClNO2 from N2O5 uptake onto sea-salt leads to increases in O3 mixing ratio, but these are smaller than the decreases caused by the bromine and iodine. 12% of ethane and 16% of acetone within the boundary layer is oxidised by Cl. Aerosol response to halogens is complex with small (∼10%) reductions in PM2.5 in most locations. A lack of observational constraints coupled to large uncertainties in emissions and chemical processing of halogens make these conclusions tentative at best. However, the results here point to the potential for halogen chemistry to influence air quality policy in Europe and other parts of the world.

Theoretical study of the reactions of Criegee intermediates with ozone, alkylhydroperoxides, and carbon monoxide.

The reaction of Criegee intermediates (CI) with ozone, O3, has been re-examined with higher levels of theory, following earlier reports that O3 could be a relevant sink of CI. The updated rate coefficients indicate that the reaction is somewhat slower than originally anticipated, and is not expected to play a role in the troposphere. In experimental (laboratory) conditions, the CI + O3 reaction can be important. The reaction of CI with ROOH intermediates is found to proceed through a pre-reactive complex, and the insertion process allows for the formation of oligomers in agreement with recent experimental observations. The CI + ROOH reaction also allows for the formation of ether oxides, which don't react with H2O but can oxidize SO2. Under tropospheric conditions, the ether oxides are expected to re-dissociate to the CI + ROOH complex, and ultimately follow the insertion reaction forming a longer-chain hydroperoxide. The CI + ROOH reaction is not expected to play a significant role in the atmosphere. The reaction of CI with CO molecules was studied at very high levels of theory, but no energetically viable route was found, leading to very low rate coefficients. These results are compared against an extensive literature overview of experimental data.