Using a combination of satellite and ground-based observations to study air pollution changes in China during COVID-19 lockdowns, researchers report up to 90% reductions of certain emissions, but also an unexpected increase in particulate matter pollution. The results will inform efforts to regulate precursor gases from all possible sectors when developing an emission control strategy. In China, as elsewhere, COVID-19 shutdowns reduced or suspended motor vehicle traffic and some manufacturing. Here, to assess the related atmospheric effects, in a country that has continued to battle particulate haze pollution, Tianhao Le and colleagues used a combination of satellite-retrieved atmospheric compositions, national ground station measurements of major pollutants, meteorology data, and atmospheric chemistry model simulations. Using spaceborne measurements of nitrogen dioxide - an important precursor for both ozone production and secondary aerosol formation - Le and colleagues report that in 2020, Wuhan experienced a 93% fractional reduction in nitrogen dioxide at the peak of the outbreak. "Such a short-term human-induced reduction in nitrogen dioxide is unprecedented," the authors say. Unexpectedly, at the same time, and particularly in northern China, particulate matter levels were extremely high, the authors found, likely due to anomalously high humidity, along with stagnant airflow and uninterrupted emissions from power and petrochemical facilities. These features contributed to severe haze formation driven by a "multi-phase chemistry" approach that requires further study, Le and colleagues write. The authors say their work shows that a protocol of reducing emissions by focusing on the traffic and manufacturing sectors alone achieves only limited effects. "We suggest a more comprehensive regulation of precursor gases from all possible sectors when developing an emission control strategy," they say, including power plants and heavy industry, such as petrochemical facilities.