Role of gas-particle conversion of ammonia in haze pollution under ammonia-rich environment in Northern China and prospects of effective emission reduction

Abstract

As an important precursor of secondary inorganic aerosols (SIAs), ammonia (NH 3 ) plays a key role in fine particulate matter (PM 2.5 ) formation. In order to investigate its impacts on haze formation in the North China Plain (NCP) during winter, NH 3 concentrations were observed at a high-temporal resolution of 1 min by using the SP-DOAS in Tai'an from December 2021 to February 2022. During the observation period, the average NH 3 concentration was 11.84 ± 5.9 ppbv, and it was determined as an ammonia-rich environment during different air quality conditions. Furthermore, the average concentrations of sulfate (SO 4 2− ), nitrate (NO 3 − ) and ammonium (NH 4 + ) were 9.54 ± 5.97 μg/m 3 , 19.09 ± 14.18 μg/m 3 and 10.72 ± 6.53 μg/m 3 , respectively. Under the nitrate-dominated atmospheric environment, aerosol liquid water content (ALWC) was crucial for NH 3 particle transformation during haze aggravation, and the gas-particle partitioning of ammonia played an important role in the SIAs formation . The reconstruction of the molecular composition further indicated that ammonium nitrate (NH 4 NO 3 ) plays a dominant role in the increase of PM 2.5 during haze events. Consequently, future efforts to mitigate fine particulate pollution in this region should focus on controlling NH 4 NO 3 levels. In ammonia-rich environments, NO 3 − formation is more dependent on the concentration of nitric acid (HNO 3 ). The sensitive analysis of TNO 3 (HNO 3 + NO 3 − ) and NH X (NH 3 + NH 4 + ) reduction using the thermodynamic model suggested that the NO 3 − concentration decreases linearly with the reduction of TNO 3 . And the concentration of NO 3 − decreases rapidly only when NH X is reduced by 50–60 %. Reducing NO X emissions is the most effective way to alleviate nitrate pollution in this region.

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