Investigating atmospheric nitrate sources and formation pathways between heating and non-heating seasons in urban North China

In urban North China, nitrate ( ${\text{NO}}_3^ - $ ) is a primary contributor to haze formation. So far, the production processes and source apportionments of atmospheric ${\text{NO}}_3^ - $ during the heating season (i.e. the wintertime) have not yet been well understood. This study determined δ ^15 N– ${\text{NO}}_3^ - $ , δ ^18 O– ${\text{NO}}_3^ - $ , and Δ ^17 O– ${\text{NO}}_3^ - $ of aerosol samples to compare the potential sources and formation pathways of atmospheric ${\text{NO}}_3^ - $ during heating (November to March) and non-heating (April to May) seasons. Combining stable isotope composition with the MixSIAR model based on Δ ^17 O– ${\text{NO}}_3^ - $ showed that NO _3 + DMS/HC (dimethyl sulfate/hydrocarbon) pathway was the dominant process of atmospheric nitrate formation during the heating season (mean = 52.88 ± 16.11%). During the non-heating season, the contributions of NO _3 + DMS/HC (mean = 37.89 ± 13.57%) and N _2 O _5 + H _2 O (mean = 35.24 ± 3.75%) pathways were comparable. We found that Δ ^17 O– ${\text{NO}}_3^ - $ was negatively correlated with wind speed and positively correlated with relative humidity during the heating season, possibly associated with the sources and production of atmospheric ${\text{NO}}_3^ - $ . In specific, in a dust storm event, the very low Δ ^17 O– ${\text{NO}}_3^ - $ is likely associated with particles from land surface. Under the premise of considering ^15 N fractionation, the constraint-based on δ ^15 N– ${\text{NO}}_3^ - $ illustrated that coal combustion was the major source of NO _x emission during the heating season, and the relative contribution of coal combustion decreased rapidly from the heating season (mean = 42.56 ± 15.50%) to the non-heating season (mean = 21.86 ± 4.91%). Conversely, the proportion of NO _x emitted by soil microbes rose significantly from the heating (mean = 9.67 ± 5.99%) to non-heating season (mean = 24.02 ± 11.65%). This study revealed differences in the sources and formation processes of atmospheric ${\text{NO}}_3^ - $ during the heating and non-heating seasons, which are of significance to atmospheric nitrogen oxide/nitrate pollution mitigation.