Continuous measurements of atmospheric monochloroacetic acid: Evidence of photochemical formation

Monochloroacetic acid (MCAA) is a regulated disinfection byproduct in drinking water due to toxicity, but its presence in environmental samples unrelated to disinfection is poorly understood. Here, we apply two online mass spectrometry-based techniques to quantify atmospheric MCAA: chemical ionization mass spectrometry with acetate reagent ion for 1 Hz measurements of gas-phase MCAA and ambient ion monitoring ion chromatography mass spectrometry for hourly measurements of MCAA in the gas and particle phases. Sampling campaigns in Toronto, Canada determined average gas-phase MCAA mixing ratios of 0.57 ± 0.44 pptv from February – April 2022 and 1.4 ± 1.0 pptv from June – August 2022. No MCAA was detected in particulate matter (PM2.5), suggesting particles were not reservoirs of MCAA. Summer MCAA gas-phase data displayed clear diurnal trends and correlated with UV irradiance and known secondary pollutants ozone and formic acid. Thus, we propose MCAA is photochemically formed from volatile precursors. Modelling MCAA from three known precursors accounted for < 0.2% of the measured MCAA in July, suggesting unknown MCAA precursors including the potential for it to be a product of chlorine atom-initiated photochemistry.