Origins of aerosol nitrate in Beijing during late winter through spring.

Recent studies have identified aerosol nitrate (NO3-) as one of the most important inorganic ions; however, quantitative studies of aerosol NO3- sources are rarely undertaken. Total suspended particulate samples were collected in Beijing from 1 February to 31 May 2013, and water-soluble ions and δ15N-NO3- were analysed to examine the potential sources of aerosol NO3-. Using a Bayesian model, the fractional contributions of NOx from different sources to aerosol NO3- were quantified herein. The maximal concentrations of NO3-, Cl-, and K+, as well as values of δ15N-NO3- during the heating period (from 1 February to 15 March) implied that coal combustion was the dominant source of aerosol NO3-. Concentrations of NO3- and K+ in the transition period (from 16 March to 15 April, when heating is gradually reduced in northern China) were similar to those during the non-heating period (from 16 April to 31 May). However, δ15N-NO3- and Cl- were obviously higher in the transition period than those in the non-heating period, suggesting a shift in NO3- sources from the transition period to the non-heating period. The fractional contribution of NO3- from coal combustion was 70.6 ± 5.0% during the heating period, which confirms that coal combustion is the dominant source of NO3- in late winter to early spring 2013 in Beijing. The fractional contribution of biomass burning to aerosol NO3- obviously increased from the heating period to the non-heating period, indicating that biomass burning was an important source of NOx in late spring 2013 in Beijing. This study verified that reduced coal consumption and prohibition of biomass burning can lower aerosol NO3- concentrations in northern China.