Observed decreases in on-road CO 2 concentrations in Beijing during COVID-19 restrictions

Abstract. To prevent the spread of the COVID-19 epidemic, restrictions such as\n“lockdowns” were conducted globally, which led to a significant reduction\nin fossil fuel emissions, especially in urban areas. However, CO 2 \nconcentrations in urban areas are affected by many factors, such as weather,\nbiological sinks and background CO 2 fluctuations. Thus, it is difficult\nto directly observe the CO 2 reductions from sparse ground observations.\nHere, we focus on urban ground transportation emissions, which were\ndramatically affected by the restrictions, to determine the reduction\nsignals. We conducted six series of on-road CO 2 observations in Beijing\nusing mobile platforms before (BC), during (DC) and after (AC) the\nimplementation of COVID-19 restrictions. To reduce the impacts of weather\nconditions and background fluctuations, we analyze vehicle trips with the\nmost similar weather conditions possible and calculated the enhancement\nmetric, which is the difference between the on-road CO 2 concentration\nand the “urban background” CO 2 concentration measured at the tower of\nthe Institute of Atmospheric Physics (IAP), Chinese Academy of Sciences. The\nresults showed that the DC CO 2 enhancement was decreased by 41 ( ±1.3 ) parts per million (ppm) and 26 ( ±6.2 ) ppm compared to those for\nthe BC and AC trips, respectively. Detailed analysis showed that, during\nCOVID-19 restrictions, there was no difference between weekdays and weekends\nduring working hours (09:00–17:00 local standard time; LST). The enhancements\nduring rush hours (07:00–09:00 and 17:00–20:00 LST) were almost twice those\nduring working hours, indicating that emissions during rush hours were much\nhigher. For DC and BC, the enhancement reductions during rush hours were\nmuch larger than those during working hours. Our findings showed a clear\nCO 2 concentration decrease during COVID-19 restrictions, which is\nconsistent with the CO 2 emissions reductions due to the pandemic. The\nenhancement method used in this study is an effective method to reduce the\nimpacts of weather and background fluctuations. Low-cost sensors, which are\ninexpensive and convenient, could play an important role in further on-road\nand other urban observations.