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 NO_x 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 NO_x 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.