Different Response Mechanisms of N-Bearing Components to Emission Reduction Across China During COVID-19 Lockdown Period

The response characteristics of secondary inorganic aerosols to COVID-19 lockdown measures have been extensively reported, while spatial disparities of these response characteristics across China and key deriving mechanisms still remained poorly understood. In our study, a chemical transport model (GEOS-Chem) was applied to simulate the national concentrations of four N-bearing components during lockdown periods in 2020 and business-as-usual (BAU) period (the same period in 2019). Three distinct regional response mechanisms were distinguished across China. The increases of (Formula presented.) levels in the northern part of Beijing-Tianjin-Hebei (BTH) might be attributable to increased relative humidity (RH) and abundant ammonia (NH₃) level (13.5 μg/m³). The dramatic decreases of secondary inorganic nitrogen in Southeast China were contributed by substantial emission reduction (−15.3%) and slightly favorable (pollutant removal) meteorological conditions (−4.36%). The consistent increases of (Formula presented.) levels, but decreased NH₃ levels in tropical regions suggested extremely high RH (84%) and enhanced atmospheric oxidation capacity were dominant factors. The response mechanisms of secondary N-bearing components showed the distinct characteristics between urban and rural regions, which might be closely associated with variations of emission reduction and meteorology during the COVID-19 period. The natural experiment demonstrates priority control of NH₃ emission might be the primary target for BTH, while the coordinated controls of VOCs and NO_x emissions should be exerted in Southeast China especially the tropical regions.