Atmospheric Amino Acids under Lockdown: The Glycine-to-Alanine Ratio as a Source Indicator

Abstract

Primary emissions and secondary formation are potential sources of free amino acids (FAAs) in atmospheric aerosols, but their relative importance and governing mechanisms remain unclear. Leveraging COVID-19 lockdown as a natural experiment with reduced primary emissions and variable atmospheric oxidation, we investigated FAA sources and evolution in PM_2.5 in Beijing. Neutral glycine consistently dominated, forming primarily via secondary aqueous-phase processes driven by relative humidity and aerosol liquid water content, especially in winter. In spring, the level of hydrophilic FAAs increased but acid–base chemistry enriched basic amino acids with sharp aerosol pH shifts, unlike pure plant-derived enhancement. Hydrophobic alanine, second to glycine, became dominant under enhanced primary dust/combustion events, matching glycine-to-alanine (Gly/Ala) spatial ratio decreases: city-to-rural with primary anthropogenic emissions and background with primary productivity. Our findings demonstrate that FAA profiles provide a useful indicator of both source shifts and aerosol physicochemical processing, offering critical insights into the atmospheric nitrogen cycle.