Effect of NOx and SO2 on the photooxidation of methylglyoxal: Implications in secondary aerosol formation

Methylglyoxal (CH 3 COCHO, MG), which is one of the most abundant α-dicarbonyl compounds in the atmosphere, has been reported as a major source of secondary organic aerosol (SOA). In this work, the reaction of MG with hydroxyl radicals was studied in a 500 L smog chamber at (293 ± 3) K, atmospheric pressure, (18 ± 2)% relative humidity, and under different NO x and SO 2 . Particle size distribution was measured by using a scanning mobility particle sizer (SMPS) and the results showed that the addition of SO 2 can promote SOA formation , while different NO x concentrations have different influences on SOA production. High NO x suppressed the SOA formation, whereas the particle mass concentration, particle number concentration and particle geometric mean diameter increased with the increasing NO x concentration at low NO x concentration in the presence of SO 2 . In addition, the products of the OH-initiated oxidation of MG and the functional groups of the particle phase in the MG/OH/SO 2 and MG/OH/NO x /SO 2 reaction systems were detected by gas chromatography mass spectrometry (GC-MS) and attenuated total reflection fourier transformed infrared spectroscopy (ATR-FTIR) analysis. Two products, glyoxylic acid and oxalic acid , were detected by GC-MS. The mechanism of the reaction of MG and OH radicals that follows two main pathways, H atom abstraction and hydration, is proposed. Evidence is provided for the formation of organic nitrates and organic sulfate in particle phase from IR spectra. Incorporation of NO x and SO 2 influence suggested that SOA formation from anthropogenic hydrocarbons may be more efficient in polluted environment.