Refined source apportionment of nitrate aerosols based on isotopes and emission inventories in coastal city of northern China.

The increasing mass percentage of nitrate (NO₃^-) in PM_2.5 in North China Plain (NCP) from 2013 (20.5 %) to 2019 (28.7 %) indicates that NO₃^- became the most prominent composition of atmospheric aerosols. However, accurately quantifying the sources of NO₃^- in aerosols remained questionable. In this study, we coupled dual isotopic composition of NO₃^- with multiple emission inventories during winter 2018 and summer 2019 to accurately identify the sources of NO₃^-. Source apportionment revealed that mobile sources (including road traffic and shipping) contributed 36.7 % to NO₃^-, followed by coal combustion (18.6 %), lightning (10.1 %), biomass burning (9.8 %), industry oil (8.8 %), natural gas (8.6 %), and soil (7.4 %) during summer. In winter, the contributions to NO₃^- shifted to mobile sources (39.6 %), coal combustion (32.3 %), biomass burning (12.0 %), natural gas (8.1 %), and industry oil (8.0 %). The contribution of major sources was consistent with regional emissions inventories, supporting us in further analyzing the contribution of regional emission. Marine air-mass contributed 33.7 ± 19.6 % of NO₃^- during summer. In winter, in addition to local emissions, regional transport from the Shandong area (outside Qingdao) and Beijing-Tianjin-Hebei (BTH) regions was particularly significant (62.2 ± 12.5 %). This study for the first time established a refined methodology for quantifying the contribution of emission sources and regional transport, providing basis for precise and effective control of the sustained increase of proportion of atmospheric NO₃^-.