Ethylene glycol emissions from on-road vehicles: implications for aqueous phase secondary organic aerosol formation

Ethylene glycol (HOCH2CH2OH), used as an engine coolant for most on-road vehicles, is an intermediate volatility organic compound (IVOC) with a high Henry's Law Coefficient (kH > 10,000 M atm-1) . Oxidation of ethylene glycol, especially in the atmospheric aqueous phase (clouds, fog, wet aerosol), can lead to the formation of glycolaldehyde, oxalic acid, and ultimately secondary organic aerosol. We present measurements of unexpectedly high ethylene glycol emissions in the Caldecott Tunnel near San Francisco (Summer 2010) and the Washburn Tunnel near Houston (Spring 2009). Ethylene glycol was detected using a proton-transfer reaction mass spectrometer (PTR-MS) at m/z = 45, which is usually interpreted as acetaldehyde. Although not necessarily a tailpipe emission, effective fuel-based emission factors are calculated using the carbon balance method and range from 50 to 400 mg ethylene glycol per kg fuel. Total US and global emissions are estimated using these emission factors and fuel consumption rates and are compared to previous model estimates of ethylene glycol emissions (e.g., the Regional Atmospheric Chemistry Model). Compared to biogenically emitted isoprene, ethylene glycol is likely a minor source of glycolaldehyde globally, but may contribute significantly to glycolaldehyde, oxalate and SOA formation in areas dominated by urban emissions.