[Concurrent Collection of Ammonia Gas and Aerosol Ammonium in Urban Beijing During National Celebration Days Utilizing an Acid-Coated Honeycomb Denuder in Combination with a Filter System].

Since 2013, the Chinese government implemented the Air Pollution Prevention and Control Action Plan. As a result, the atmospheric concentrations of sulfate reduced significantly, whereas the nitrate concentrations remain relatively high due to the excess of ammonia (NH ₃ ). To date, there is no official observation network monitoring NH ₃ concentrations in China. Previous studies have focused on NH ₃ or ammonium (NH ₄ ⁺ ) separately. These limitations hinder a complete understanding of their dynamic changes due to the rapid gas-to-particle conversion. In this study, the concentrations of NH ₃ and NH ₄ ⁺ were measured concurrently in urban Beijing during autumn 2019 utilizing an acid-coated denuder-filter combination with a time resolution from 2 h (PM _2.5 >35 μg·m ^-3 ) to 5 h (PM _2.5 <35 μg·m ^-3 ). The mean concentrations of NH ₃ and NH ₄ ⁺ during the study were (4.1±2.9)μg·m ^-3 and (1.7±1.4) μg·m ^-3 , respectively. The temporal patterns of NH ₃ or NH ₄ ⁺ coincided with that of PM _2.5 , CO, and NO ₂ throughout the sampling period. The diurnal distributions of NH ₃ were bimodal, both on polluted (PM _2.5 >75 μg·m ^-3 ) and non-polluted (PM _2.5 <75 μg·m ^-3 ) days, peaking at 21:30-05:30 and 05:30-08:30, respectively. The NH ₃ concentrations on polluted days were relatively lower during 17:30-21:30, which may be related to higher wind speeds. In contrast to NH ₃ , NH ₄ ⁺ had an obvious peak during 17:30-21:30 due to the formation of ammonium nitrate. The meteorological conditions favor the gas-to-particle conversion on polluted days, resulting in a lower NH ₃ /NH ₄ ⁺ ratio of 0.8. However, this value may reach 2.8 on non-polluted days. The concentrations of NH ₃ , CO, NO ₂ , SO ₂ , and PM _2.5 in the emission control period showed a significant increase greater than or comparable to those in the non-control period by 54.2%, 40.4%, 33.3%, 0%, and 49.4%, respectively. This result shows that the stagnant conditions offset the benefit of emission control actions implemented during and before the National Celebration Day.