Photochemical modeling of glyoxal at a rural site: observations and analysis from BEARPEX 2007

Abstract. We present roughly one month of high time-resolution, direct, in situ measurements of gas-phase glyoxal acquired during the BEARPEX 2007 field campaign. The research site, located on a ponderosa pine plantation in the Sierra Nevada mountains, is strongly influenced by biogenic volatile organic compounds (BVOCs); thus this data adds to the few existing measurements of glyoxal in BVOC-dominated areas. The short lifetime of glyoxal of ~1 h, the fact that glyoxal mixing ratios are much higher during high temperature periods, and the results of a photochemical model demonstrate that glyoxal is strongly influenced by BVOC precursors during high temperature periods.

A zero-dimensional box model using near-explicit chemistry from the Leeds Master Chemical Mechanism v3.1 was used to investigate the processes controlling glyoxal chemistry during BEARPEX 2007. The model showed that MBO is the most important glyoxal precursor (~67 %), followed by isoprene (~26 %) and methylchavicol (~6 %), a precursor previously not commonly considered for glyoxal production. The model calculated a noon lifetime for glyoxal of ~0.9 h, making glyoxal well suited as a local tracer of VOC oxidation in a forested rural environment; however, the modeled glyoxal mixing ratios over-predicted measured glyoxal by a factor 2 to 5. Loss of glyoxal to aerosol was not found to be significant, likely as a result of the very dry conditions, and could not explain the over-prediction. Although several parameters, such as an approximation for advection, were found to improve the model measurement discrepancy, reduction in OH was by far the most effective. Reducing model OH concentrations to half the measured values decreased the glyoxal over-prediction from a factor of 2.4 to 1.1, as well as the overprediction of HO₂ from a factor of 1.64 to 1.14. Our analysis has shown that glyoxal is particularly sensitive to OH concentration compared to other BVOC oxidation products. This relationship arises from (i) the predominantly secondary- or higher-generation production of glyoxal from (mainly OH-driven, rather than O₃-driven) BVOC oxidation at this site and (ii) the relative importance of photolysis in glyoxal loss as compared to reaction with OH. We propose that glyoxal is a useful tracer for OH-driven BVOC oxidation chemistry.