•Ambient HCHO was studied in 30 Chinese cities.•A mail-based sampling device was developed.•Peak hours of 12pm-3pm were identified.•Median concentration in summer peak hour was 0.027 mg/m3.•Ambient HCHO pose high carcinogenic risk. Formaldehyde is an important carcinogen commonly found indoors. Its indoor sources have been intensively investigated. But study on outdoor formaldehyde concentration, which is potentially an important source to indoors, remains scarce. This study attempts to characterize temporal and spatial distribution of the atmospheric formaldehyde concentration in Chinese cities. Diurnal variation of ambient formaldehyde was examined in 6 cities and peak hours were identified between 12:00 pm and 3:00 pm. Consequently, outdoor formaldehyde concentrations were measured in the peak hours in 30 cities during the summer months of Jul.-Aug., 2022. The formaldehyde concentrations in the peak hours fell into a range of 0.005–0.087 mg/m3 (median value is 0.027 mg/m3), 87.7% of which have exceeded the chronic reference exposure criteria of 0.009 mg/m3 set by Office of Environmental Health Hazard Assessment. Health risk analysis suggests that exposure to ambient formaldehyde could cause a median carcinogenic risk of 1.9 × 10−5 (3.17×10−6–6.13×10−5), higher than threshold limit of 10−6. Pearson correlation analysis of the 30 cities shows that ambient summertime atmospheric formaldehyde concentrations of the city are positively correlated with its Gross Domestic Product (r = 0.48). We also found that the outdoor formaldehyde concentrations in urban areas (median: 0.017 mg/m3) is slightly higher than those in suburban areas (median: 0.013 mg/m3). Results here prove that outdoor formaldehyde is ubiquitous in Chinese cities and reduces effectiveness of ventilation in dilution indoor concentrations. Neglecting it would underestimate air cleaner capacity needed by a factor of about two. It should be accounted for in health analysis and air quality engineering control design of built environment in the future. Display omitted