Chemical Characteristics and Source of PM<sub>2.5</sub> in Hohhot, a Semi-arid City in Northern China: Insight from the COVID-19 Lockdown

Abstract. A knowledge gap exists concerning how chemical composition and sources respond to implemented policy control measures for aerosols, particularly in a semi-arid region. To address this, a single year’s offline measurement was conducted in Hohhot, a semi-arid city in northern China, to reveal the driving factors of severe air pollution in semi-arid region and assess the impact of the COVID-19 lockdown measures on chemical characteristics and sources of PM2.5. Organic matter, mineral dust, sulfate, and nitrate, accounted for 31.5 %, 14.2 %, 13.4 %, and 12.3 % of the total PM2.5 mass, respectively. Coal combustion, vehicular emissions, crustal sources, and secondary inorganic aerosols were the main sources of PM2.5 in Hohhot, at 38.3 %, 35.0 %, 13.5 %, and 11.4 %, respectively. Due to the coupling effect of emission reduction and improved atmospheric conditions, the concentration of secondary inorganic components, organic matter, elemental carbon, and chloride declined from the pre-lockdown period to the lockdown and post-lockdown period. Compared with the pre-lockdown period, the percentage of secondary inorganic components declined during the lockdown and post-lockdown period, while the mineral dust, organic matter, and elemental carbon increased. The rapid generation of secondary inorganic components caused by unfavorable meteorological conditions during lockdown led to serious pollution. This study elucidates the complex relationship between air quality and environmental policy.