“…The HCHO from primary emissions was highest between 08:00 and 10:00 (about 2.63, 2.50, and 4.30 ppb in Hefei, Huaibei, and Tai' an, respectively), then gradually decreased, reaching the lowest concentration around 15:00, about 1.78, 1.54, and 1.82 ppb in Hefei, Huaibei, and Tai' an, respectively, and then gradually rising. The high primary HCHO concentrations in the morning and evening may be due to emissions from traffic 10 Lin et al, 2022;Hong et al, 2018). Secondary HCHO concentrations were lowest in the morning, about 0.58, 0.84, and 0.65 ppb in Hefei, Huaibei, and Tai' an, respectively, with the enhancement of photochemical reactions and the resumption of human activities, the secondary HCHO concentrations gradually increased from 08:00 to 12:00, and the first peak usually occurred at 11:00-14:00.…”