Heterogeneous uptake of NH3 on ambient PM2.5 in Beijing and Shijiazhuang: Possible influence of aerosol acidity

Ammonium salts (NH₄⁺) is the important component of PM_2.5 and has a significant impact on air quality, climate, human health, and natural ecosystems. The contribution of NH₄⁺ to PM_2.5 is increasing at urban sites. Ammonia (NH₃) with global emissions estimated at greater than 33 Tg(N) Yr^-1 is the only precursor of particulate NH₄⁺ in the atmosphere. Thus, it is important to understand the conversion kinetics from NH₃ to NH₄⁺ in the atmosphere. However, the uptake coefficient of NH₃ (γ_NH3) on aerosol particles are scarce at the present time. In this work, we reported the γ_NH3 on ambient PM_2.5 in Beijing and Shijiazhuang in China. The γ_NH3 values on ambient PM_2.5 are (1.13±12.4)×10^-4 and (6.88±40.7)×10^-4 in Shijiazhuang and Beijing, respectively. They are significantly lower than those on sulfuric acid droplet (0.1-1), aqueous surface (~5×10^-3-0.1) and acidified secondary organic aerosol (~10^-3-~10^-2), while are comparable with that on ice surface (5.3±2.2 ×10^-4) and on sulfuric acid in the presence of organic gases (2×10^-4-4×10^-3). An annual increase of γ_NH3 in the statistic sense is observed and the possible reason related to the aerosol acidity has also been discussed.