Particulate Nitrate Photolysis in HONO Production: Impacts of Aerosol Phase States, Acidity, and Actinometry Calibration
JUN ZHENG, He-shan Ning, Yan Ma, Nanjing University of Information Science and Technology
Abstract Number: 77
Working Group: Aerosol Chemistry
Abstract
Productions of nitrous acid (HONO) from the photolysis of ambient particulate nitrate (PN) samples were investigated using a solar irradiation simulator. The measured apparent HONO production rate (JPN-HONO) ranged from 4.4×10-6 s-1 to 8.3×10-5 s-1, with an average of (2.5±1.7)×10-5 s-1. Release of nitrogen dioxide (NO2) was simultaneously detected with HONO with a production rate (JPN-NO2) ranging from 3.2×10-6 s-1 to 7.9×10-5 s-1 and an average of (2.3±1.6)×10-5s-1. JPN-HONO and JPN-NO2 appeared to vary with the time of sample collection, showing a distinctive daily maximum around early morning and a minimum around noontime. It was found that irradiated particle filter samples ceased to release HONO, while ~90% PN was estimated to remain on the filter. Moreover, the sum of HONO and NO2 produced during the experiment only accounted for about ~2% of the consumed PN. These observations indicated that HONO and NO2 formation from PN photolysis can only occur within a thin layer of PN-containing shell. Limited aerosol liquid water content (ALWC) may prevent most PN from dissolving into the aqueous phase. It was also found that the irradiation intensity within the solar simulator was underestimated by the traditional aqueous-nitrate actinometry due to the solvent cage effects within the bulk nitrate solution, leading to the recombination of free radicals generated by the irradiation. Interestingly, the effects of overestimated reactive PN contents and underestimated irradiation intensities may cancel each other somewhat, leading to similar values of JPN-HONO and JPN-NO2 reported in previous studies.