Measurements and modeling of glyoxal: Insights into rural photochemistry and secondary organic aerosol production

Glyoxal is important to atmospheric science as an indicator for oxidation chemistry and secondary organic aerosol formation. Global models indicate that the majority of glyoxal is biogenically derived, but few measurements of glyoxal are available in biogenically dominated regions. The Madison Laser-Induced Phosphorescence Instrument enables rapid measurements of glyoxal in such rural areas and facilitates new science with sensitive, selective, fast measurements of gas-phase glyoxal. We describe the development of this instrument and results from a deployment to a collaborative field campaign near Blodgett Forrest, CA.;A photochemical box model was developed to evaluate the sources and sinks of glyoxal at the site and validated using laboratory data leading to two findings: (1) prompt formation of glyoxal from the reaction of isoprene + OH in the presence of NO; (2) model over-prediction of higher–generation glyoxal from isoprene by a factor of at least eight. Inclusion of these processes will alter mode] predictions of glyoxal and secondary aerosol due to the dominance of isoprene among biogenic emissions.;A model incorporating these changes over-predicted glyoxal at the field site by a factor of 2 to 5. Vertical dilution, deposition, loss to aerosol, and transport were examined; no single process was able to reduce the over-prediction without introducing another problem such as incorrect diurnal profile. The ability of the box model to reproduce chamber but not ambient conditions suggests that the mechanistic representation of isoprene and 2–methyl–3–buten–2–ol (the dominant precursors of glyoxal at the site) should be examined in future studies with attention to the influence of the ratio of alkyl peroxy radical (RO2) to hydrogen dioxide radical (HO2).;Comparison of measurements to results from the Community Multiscale Air Quality model show that the model correctly predicts ozone at the site but fails to match crucial aspects of radical and higher–generation chemistry, leading to excess glyoxal. Current generation global and regional models may place undue emphasis on the formation of aerosol from glyoxal from biogenic precursors as a result of similar over–predictions. Improvements to model photochemistry are suggested, likely leading to changes in predicted ozone.