Atmospheric chemistry of nitrogenous aerosols in northeastern Asia: biological sources and secondary formation

Abstract. To better understand the sources of nitrogenous aerosols, particularly water-soluble organic nitrogen (WSON) and water-insoluble organic nitrogen (WION), in northeastern Asia, we measured total nitrogen (TN) and water-soluble total nitrogen (WSTN) as well as nitrogen isotope ratios (δ¹⁵N) of TN (δ¹⁵N_TN) and WSTN (δ¹⁵N_WSTN) in the total suspended particulate (TSP) samples collected from Sapporo, northern Japan, for a 1-year period. In general, WION was more abundant (126 ± 117 ng m⁻³), whereas WSON was 89.7 ± 80.6 ng m⁻³, accounting for 14 ± 11 % and 9.2 ± 7.3 % of TN, respectively. WSON peaked in late autumn to winter (maximum 288 ng m⁻³) and WION peaked in mid-spring to early summer (454 ng m⁻³). δ¹⁵N_TN (21.9 ± 4.1 ‰) and δ¹⁵N_WSTN (25.8 ± 8.2 ‰) showed peaks in summer with relatively high ratios in late autumn. Based on the seasonal variations in WSON and WION together with organic tracers, fossil fuel combustion and biomass burning are found to be two major sources of WSON, whereas emissions of biological particles and secondary formation by reactions of biogenic secondary organic species (carbonyls) with NH₃ are suggested as an important source of WION. The seasonality of δ¹⁵N_TN and δ¹⁵N_WSTN, together with the comparisons to literature values, implies that chemical aging (including gas–particle partitioning) and biomass burning are the causes of the enhanced values in summer and autumn, respectively. This study demonstrates that contributions of aerosol N from fossil fuel combustion and biomass burning dominate in autumn and/or winter, whereas emission of terrestrial biological particles and secondary formation from biogenic hydrocarbons and subsequent chemical aging in the atmosphere are important in spring and/or summer in northeastern Asia.