Importance of direct anthropogenic emissions of formic acid measured by a chemical ionisation mass spectrometer (CIMS) during the Winter ClearfLo Campaign in London, January 2012

•Formic and nitric acid measurements from the Winter ClearfLo campaign are reported.•Results indicate a significant direct source of anthropogenic HCOOH from vehicles.•Our results suggest that vehicle emissions can be responsible for up to 30% of HCOOH.

Formic acid, an ubiquitous trace gas in the atmosphere, was measured using a chemical ionisation mass spectrometer (CIMS) during the winter ClearfLo campaign in London, 2012. Daily calibrations of formic acid gave sensitivities of 3 ion counts s−1 pptv−1 for the complete campaign and a limit of detection of 2 ppt. No correlation with nitric acid was observed, R2 of 0.137, indicating no significant secondary source of formic acid. However, a strong positive correlation with NOx, CO, and production in line with rush hour periods indicated a direct anthropogenic emission of formic acid from vehicle emissions. Peaks of 6.7 ppb of formic acid were observed with a mean of 610 ppt. Global models indicated that this emission source dominates in the northern hemisphere where global models underestimate formic acid most significantly, thus increasing the accuracy of modelling of global formic acid emissions.