Characterization And Sources Of Organic Nitrogen In PM_2.5 In Beijing

Atmospheric organic nitrogen（ON） could potentially influence nitrogen cycle in ecosystems, secondary organic aerosol（SOA） formation, cloud nucleation nucleaus and human health, which has raised increasing research interest, however study on fine aerosol organic nitrogen in China is rare and limited. Beijing, as the biggest city in China, is suffering serious PM_2.5, influenced by multi-emission sources including transportation, coal burning,factory processing, dust, biomass burning, which correspond to some of the main sources recognized for atmospheric ON. Study on organic nitrogen in aerosol in Beijing may promote the knowledge of the influence of ON on SOA formation, nitrogen cycle and human health.This study concentrates on bulk characterizations of ON in PM_2.5, providing confinement and reference for further single species identification analysis. Daily aerosol samples of Jan.,Apr.,Jul.,Oct. and the aerosols with frequency of 2 hours under heavy haze conditions in Jan. and Jun. were taken to explore the seasonal variations and day&night difference under heavy haze conditions separately. Chemical analysis data including organic nitrogen, water soluble ions, carbonaceous species, minor elements, combined with meteorological data, went through correlation analysis and positive matrix factor analysis to explore the sources of fine aerosol organic nitrogen qualitatively and quantitatively.According to the results of daily samples of the four seasons, organic nitrogen in PM_2.5 is ubiquitous across the year, with a mean concentration of 1.49μg·m-3, with autumn ranking first with a mean concentration of 2.44μg·m-3. Organic nitrogen contributes to 20% of total nitrogen, with summer contributing most. Organic nitrogen correlate with total nitrogen, ammonium-N, and nitrate-N significantly, reflecting anthropogenic nature of organic nitrogen sources. N/C ratios correlate positively with ON concentration significantly in spring and autumn. Evidence shows that extra organic sources with high N/C ratios and high density may exist. Peak values of organic nitrogen coincide with peak values of sulfate concentration, relative humidity and acidity. Correlation relationship between organic nitrogen and other components shows that sources of organic nitrogen may correlate with dust related source and multi anthropogenic sources（e.g. coal burning, factory processing, biomass burning）. Organic nitrogen correlate more significantly with secondary organic carbon shows secondary aerosol nature of ON.Intensive observations in heavy haze in winter and summer shows that, the 2hrs average concentrations of ON are 2.86 μg m-3, the highest concentrations are both about 10 μg m-3. Organic nitrogen takes up 20-30% of total nitrogen in PM_2.5, with the ratio higher in summer; N/C ratios are higher summer too, consistent with the result of daily samples set. ON sources may get strengthened and be more sensitive to sulfate concentration. Organic nitrogen present secondary nature and may be influenced by acidity catalysis. Dust related source and anthropogenic emission related secondary source are identified as important sources for ON according to the correlation relationship between N/C ratios and ON concentrations under different sulfate loads. ON concentrations are higher at night in the both seasons, however with distinguished day and night difference patterns influenced by RH（relative humidity） conditions. In winter, ON concentrations increase with RH on average through low RH values to high RH values, which variations are far larger than the ones caused by day and night difference. However in summer, day and night difference dominates the variations of ON concentrations at low RH values, and RH conditions promote ON concentrations increase significantly only at high RH values.Positive Matrix Factors analyze the data of the daily sample set with the best fitting result of 7 factors which are nitrate factor, sulfate factor, organic carbon factor, chlorine factor, dust factor, Vanadium factor and Arsenic factor. The main sources of ON are the sources represented by nitrate factor, organic carbon factor and Vanadium factor with the contributions of 57.2%,37.2% and 7.6% respectively. The result provides the quantitative evidence of anthropogenic nature of the sources of ON, except the vanadium factor represents a combustiong source of heavy oil influenced by dust.