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 NOx 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 PM2.5 formation. The mean mass ratio of NH3/NHx was 0.76±0.13. This revealed that NHx (NH3+NH4+) was influenced by local sources during the summer period and that NH3 dry deposition could contribute substantially to NHx deposition. A high temperature, relative humidity, and degree of oxidization could increase photochemical reactions and aqueous processing, having the important effect of SO2 to SO42- 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.