Decomposing PM_2.5 air pollution rebounds in Northern China before COVID-19

China's efforts to curb air pollution have drastically reduced its concentrations of fine particulate matter (PM_2.5) from 2013 to 2018 nationwide. However, few studies examined the most recent changes in PM_2.5 concentrations and questioned if the previous PM_2.5 declining trend was sustained or not. This study took a deep dive into the PM_2.5 trend for 136 northern cities of China from 2015 to early 2020 before the coronavirus disease 2019 (the COVID-19 hereafter) crisis, using ground-based PM_2.5 data notably adjusted for a key measurement method change. We find that mean PM_2.5 concentrations in northern China increased by 5.16 µg/m³ in 2019, offsetting 80% of the large reduction achieved in 2018. The rebound was more significant during the heating seasons (HS; Nov to next Mar) over the 2 years: 10.49 µg/m³ from the 2017 HS to the 2019 HS. A multiple linear regression analysis further revealed that anthropogenic factors contributed to around 50% of the PM_2.5 rebound in northern cities of China. Such a significant role of anthropogenic factors in driving the rebound was tightly linked to deep cuts in PM_2.5 concentrations in the previous year, systemic adjustment of policy targets and mitigation measures by the government, and the rising marginal cost of these measures. These findings suggest the need to chart a more sustainable path for future PM_2.5 emission reductions, with an emphasis on key regions during key pollution periods.