Ozone enhancement due to the photodissociation of nitrous acid in eastern China

Abstract

PM2.5, particulate matter with a diameter of 2.5 µm or less, is one of the major components of air pollution in eastern China. In the past few years, China's government has made strong efforts to reduce PM2.5 pollution. However, another important pollutant (ozone) is becoming a problem in eastern China. Ozone (O3) is produced by photochemistry, which requires solar radiation for the formation of O3. Under heavy PM2.5 pollution, solar radiation is often depressed, and the photochemical production of O3 is prohibited. This study shows that during late spring and early fall in eastern China, under heavy PM2.5 pollution, there was often strong O3 photochemical production, causing a co-occurrence of high PM2.5 and O3 concentrations. This co-occurrence of high PM2.5 and O3 is unusual and is the main focus of this study. Recent measurements show that there were often high HONO surface concentrations in major Chinese megacities, especially during daytime, with maximum concentrations ranging from 0.5 to 2 ppbv. It is also interesting to note that high HONO concentrations occurred during high aerosol concentration periods, suggesting that there were additional HONO surface sources in eastern China. Under high daytime HONO concentrations, HONO can be photodissociated to OH radicals, which enhance the photochemical production of O3. In order to study the above scientific issues, a radiative transfer model (TUV; tropospheric ultraviolet–visible) is used in this study, and a chemical steady-state model is established to calculate OH radical concentrations. The calculations show that by including the OH production of photodissociated HONO, the calculated OH concentrations are significantly higher than the values without including this production. For example, by including HONO production, the maximum OH concentration under high aerosol conditions (AOD = 2.5) is similar to the value under low aerosol conditions (AOD = 0.25) in the no-HONO case. This result suggests that even under high aerosol conditions, the chemical oxidizing process for O3 production can occur, which explains the co-occurrence of high PM2.5 and high O3 in late spring and early fall in eastern China. However, the O3 concentrations were not significantly affected by the appearance of HONO in winter. This study shows that the seasonal variation of solar radiation plays important roles for controlling the OH production in winter. Because solar radiation is at a very low level in winter, adding the photolysis of HONO has a smaller effect in winter than in other seasons, and OH remains at low values by including the HONO production term. This study provides some important scientific insight to better understand O3 pollution in eastern China.