Variation of Urban Atmospheric Ammonia Pollution and its Relation with PM_(2.5) Chemical Property in Winter of Beijing, China

To understand the air pollution problem in megacities such as Beijing, field measurement investigating the variation of NH_3 and its association with PM_(2.5) chemical property was conducted from 25 November to 24 December 2013. The results indicated that the daily concentration of wintertime NH_3 tended to be high on the days with relatively high temperatures and low wind speeds. Affected by the synoptic condition, NH3 concentration showed a bimodal diurnal variation pattern, which tended to peak at around 09:00 and 22:00 of the day. As the sole precursor for NH_4^+, NH_3 exerted a significant impact on the ion chemistry of PM_(2.5) through enhancing the nighttime NH_4Cl formation and promoting both homogeneous and heterogeneous formation of NO_3^-. During heavy pollution episodes with PM_(2.5) concentrations over 200 μg m^(-3), the NH_3 levels and NH_4^+/NH_3 ratios grew simultaneously with the increase of PM_(2.5) levels, indicating that NH3 is one of the key reasons for heavy pollution events. Revealed by the features of measured ionic species in PM_(2.5), in conjunction with the acidity analysis using thermodynamic model, our results suggested that NH_3 was frequently sufficient in wintertime atmosphere of urban Beijing and the fine particulates were neutralized nearly fully by NH_3.