Summertime ambient ammonia and its effects on ammonium aerosol in urban Beijing, China

To improve the understanding of NH₃ variation in urban Beijing, high temporal resolution (1-min averaged) NH₃ data derived during the summer of 2009 were analyzed. The results indicated that after rain there was generally an increase in NH₃ concentration. The analysis of the 1-min data revealed that a weak but significant linear correlation existed between NH₃ and NO_x in August, indicating a connection between NH₃ concentrations and traffic levels during summer. This was further supported by peak NH₃ levels approximately coinciding with morning rush hours. The daily NH₃ concentrations were weakly correlated with NH₄⁺ concentrations, suggesting that NH₃ played an important precursor role in NH₄⁺ in PM_2.5 formation. The mean mass ratio of NH₃/NH_x was 0.76±0.13. This revealed that NH_x (NH₃+NH₄⁺) was influenced by local sources during the summer period and that NH₃ dry deposition could contribute substantially to NH_x deposition. A high temperature, relative humidity, and degree of oxidization could increase photochemical reactions and aqueous processing, having the important effect of SO₂ to SO₄^2- and NH₃ to NH₄⁺ conversion in summer. The back trajectory analysis indicates that the transport of air masses from the North China Plain region contributed to the atmospheric NH₃ and NH₄⁺ aerosol variations in Beijing.