Nitrogen-Containing Organic Compounds and Oligomers in Secondary Organic Aerosol Formed by Photooxidation of Isoprene

Abstract

Electrospray ionization high-resolution mass spectrometry (ESI HR-MS) was used to probe molecular structures of oligomers in secondary organic aerosol (SOA) generated in laboratory experiments on isoprene photooxidation at low- and high-NO(x) conditions. Approximately 80-90% of the observed products are oligomers and up to 33% by number are nitrogen-containing organic compounds (NOC). We observe oligomers with maximum 8 monomer units in length. Tandem mass spectrometry (MS(n)) confirms NOC compounds are organic nitrates and elucidates plausible chemical building blocks contributing to oligomer formation. Most organic nitrates are comprised of methylglyceric acid units. Other important multifunctional C(2)-C(5) monomer units are identified including methylglyoxal, hydroxyacetone, hydroxyacetic acid, and glycolaldehyde. Although the molar fraction of NOC in the high-NO(x) SOA is high, the majority of the NOC oligomers contain only one nitrate moiety resulting in a low average N:C ratio of 0.019. Average O:C ratios of the detected SOA compounds are 0.54 under the low-NO(x) conditions and 0.83 under the high-NO(x) conditions. Our results underscore the importance of isoprene photooxidation as a source of NOC in organic particulate matter.