## 1 Introduction  
With the rapid development of urban and industrial modernization, atmosphere pollution has become one of major environmental issues in Shanghai due to the high energy consumption and explosive growth of vehicle traffic. Particulate matter (PM) is the main component of air pollutants, usually composed of viruses, bacteria, heavy metal carriers, catalysts or reaction beds [1, 2]. PM can float into the atmosphere and reach a supersaturated state over a relatively long period, which interferes with various biogeochemical processes and material circulation processes in the surrounding environment [3]. Additionally, the balance of the human respiratory and metabolic systems can be severely disrupted through the food chain. Therefore, air particulate pollution has become a research hotspot in recent years. Many studies have shown that traditional chemical analysis can effectively evaluate air quality, but it is time-consuming and expensive [4]. Therefore, there is an urgent need to establish an efficient monitoring method to track the spread of urban atmospheric particulate matter, which has very important practical significance for research into solutions for heavy metal pollution in the air [5, 6, 7]. As a new type of analysis method, the magnetic approach has been increasingly used in the field of pollution monitoring of environmental carriers, such as urban particulate matter, soil, vegetation, etc., because of its rapid positioning, economic testing, and convenient sampling [8, 9, 10, 11]. A large number of studieshave shown that magnetic parameters, such as magnetic susceptibility (\(\chi_{\rm{Hf}}\)), saturation isothermal remanence (SIRM), and non-hysteresis remanence (\(\chi_{\rm{ARM}}\)), are closely related to the concentration of heavy metals in the air. Especially, \(\chi_{\rm{Hf}}\) and SIRM can be used as effective indicators to evaluate and monitor the level of ferromagnetic particles in the air [8; 12; 13]. At the end of December 2019, a novel coronavirus (COVID-19) with extremely high contagiosity was firstly reported in Wuhan City and spread rapidly across the world. In order to effectively control the spread of the virus, the Chinese government issued the highest-level emergency response to the epidemic, for example, community blockades, traffic restrictions, public transportation system suspension, the implementation of online offices for factory and corporate personnel, etc. These measures have not only effectively reduced the risk of new coronavirus infections, but also reduced air pollutants generated by industry, vehicle traffic and other factors [14]. Numerous studies have highlighted the positive effects of the COVID-19 lockdown on water and air quality, comparing the percentage of pollutants found before and during the lockdown [15; 16]. [PERSON] et al. [17] investigated the impact of the COVID-19 pandemic lockdown on the environmental occurrence of eleven heavy metals in the Campania region (Italy) by analyzing bees and bee products. As one of China's important economic centers and inbound cities, Shanghai is a typical city with rapid urbanization on the east coast and a rapid increase in the number of vehicles, and it was found that there are significant differences in the air PM content with regard to the spatial distribution characteristics of the season [18]. Like other cities, Shanghai has been affected by COVID-19, and human activities have been greatly controlled; however, there are a lack of comparative studies of heavy metal pollution before and after the COVID-19 outbreak. This study aims to better understand the trend of air pollutants before and after the COVID-19 outbreak in Shanghai. For comparison, the sampling cycle was firstly divided into two phases, namely, December 2019 and December 2020, and atmospheric dustfall in various districts was taken as the research medium.