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

Abstract

Methylglyoxal (CH₃COCHO, 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 NOx and SO₂. Particle size distribution was measured by using a scanning mobility particle sizer (SMPS) and the results showed that the addition of SO₂ can promote SOA formation, while different NOx concentrations have different influences on SOA production. High NOx suppressed the SOA formation, whereas the particle mass concentration, particle number concentration and particle geometric mean diameter increased with the increasing NOx concentration at low NOx concentration in the presence of SO₂. In addition, the products of the OH-initiated oxidation of MG and the functional groups of the particle phase in the MG/OH/SO₂ and MG/OH/NOx/SO₂ 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 NOx and SO₂ influence suggested that SOA formation from anthropogenic hydrocarbons may be more efficient in polluted environment.

Keywords: Methylglyoxal; NOx; OH-Initiated oxidation; SO(2); Secondary organic aerosol.