Production of Nitrous Acid from Nitrogen Dioxide Uptake on Illuminated Aerosol Surfaces

Abstract:

Haze events in megacities in China lead to asthma, respiratory diseases, and increased risk for premature death for exposed populations. The production of major components of the haze, secondary organic aerosol and ozone, is controlled by hydroxyl radicals, previously measured by our group in Beijing to reach high daily maximum values of 2 x 10⁷ molecules cm^-3. The largest source of hydroxyl radicals in Chinese megacities is the photolysis of HONO, thought to contribute as much as 95% of the OH initiation. However, especially in the summer during the daytime, box models equipped with a gas phase chemistry, combustion emission estimates, and dark heterogeneous hydrolysis of on surfaces cannot reproduce observed HONO levels. There is thought to be an unquantified source of HONO from illuminated aerosol surfaces, which is the objective of this study. In our experiment, nitrogen dioxide was flowed over collected ambient aerosol from Beijing in a flow tube in the presence of near-UV light. HONO was detected by photodissociation to OH followed by laser induced fluorescence. Experiments were done under a range of environmental conditions such as light intensity and relative humidity. The metal and inorganic nitrate compositions of the particulate matter samples are also varied to understand the effect of chemical composition. In addition to the measured HONO production rates under different environmental conditions, bulk nitrate measurements using ion chromatography were used to infer the role of nitrate in the mechanism. The uptake of NO₂ to produce HONO was added to a zero-dimensional box model utilizing the Master Chemical Mechanism and constrained to measurements of radical species, particulate matter, and VOCs from a recent field campaign in Beijing, AIRPRO. Using this model we quantify the contribution of HONO production from heterogeneous sources during the daytime in the summer and the winter to the observed HONO.