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

As an important precursor of secondary inorganic aerosols (SIAs), ammonia (NH3) plays a key role in fine particulate matter (PM2.5) formation. In order to investigate its impacts on haze formation in the North China Plain (NCP) during winter, NH3 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 NH3 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 (SO42−), nitrate (NO3−) and ammonium (NH4+) were 9.54 ± 5.97 μg/m3, 19.09 ± 14.18 μg/m3 and 10.72 ± 6.53 μg/m3, respectively. Under the nitrate-dominated atmospheric environment, aerosol liquid water content (ALWC) was crucial for NH3 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 (NH4NO3) plays a dominant role in the increase of PM2.5 during haze events. Consequently, future efforts to mitigate fine particulate pollution in this region should focus on controlling NH4NO3 levels. In ammonia-rich environments, NO3− formation is more dependent on the concentration of nitric acid (HNO3). The sensitive analysis of TNO3 (HNO3 + NO3−) and NHX (NH3 + NH4+) reduction using the thermodynamic model suggested that the NO3− concentration decreases linearly with the reduction of TNO3. And the concentration of NO3− decreases rapidly only when NHX is reduced by 50–60 %. Reducing NOX emissions is the most effective way to alleviate nitrate pollution in this region. [Display omitted] •The gas-particle partitioning of NH3 to NH4+ could be self-amplified with increasing of aerosol liquid water content.•Large production of ammonium nitrate dominated the increase of PM2.5 concentration during haze.•NOX has better reduction potential than NHX and SO2 for PM2.5 alleviation in ammonia-rich environments.