Secondary inorganic aerosols formation during haze episodes at an urban site in Beijing, China

Abstract Severe PM 2.5 pollution was observed frequently in Beijing. We conducted highly time-resolved measurements of inorganic ions associated with PM 2.5 at an urban site in Beijing from 10 February to 19 March, 2015. The average PM 2.5 mass concentrations during the six haze episodes ranged from 113.0 μg/m 3 to 182.6 μg/m 3 , which were more than 8 times higher than those observed in clean periods. The secondary inorganic species (NH 4 + , SO 4 2− and NO 3 − ) in PM 2.5 sharply increased during the haze episodes, indicating more extensive formation of SO 4 2− and NO 3 − . The sulfur oxidation ratios (SOR) and the nitrogen oxidation ratios (NOR) in haze episodes were higher than those in clean periods, which indicated that secondary transformation in haze episodes was more significant than those in clean periods. No correlations between SOR and the oxidants (O 3 and HONO) and the temperature were found, whereas a high correlation between SOR and relative humidity (RH) was found in haze episodes, which implied that sulfate was mainly produced by the aqueous-phase oxidation of SO 2 rather than the gas-phase conversion of SO 2 to sulfate. The conversion of SO 2 to SO 4 2− was observed to be sensitive to changes in RH. Furthermore, the SOR sharply increased at RH > 60% with the highest value of 0.88 at RH around 80% during complicated pollution. NO 2 played an important role in the rapid sulfate formation with high RH and NH 3 neutralization conditions in haze episodes in Beijing. The impact of RH was less apparent for nitrate than for sulfate. Nitrate was found to be produced mainly by photochemical and heterogeneous reactions, while heterogeneous reactions had a greater influence on NOR at nighttime. The NO 3 − /SO 4 2− ratio indicated that mobile sources contributed more to the formation of PM 2.5 than stationary sources. The result suggested the need for control of vehicle emissions to reduce the high levels of NO x and nitrate and the severe PM 2.5 pollution in Beijing.