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

Abstract. We present ~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 is used to investigate the processes controlling glyoxal chemistry during BEARPEX 2007. The model shows 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 calculates 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-predict measured glyoxal by a factor 2 to 5. Although several parameters, such as an approximation for advection and increased glyoxal loss to aerosol can improve the model measurement discrepancy, reduction in OH is by far the most effective. Reducing OH to half the measured values, which is suggested by preliminary OH measurements using a different technique, decreases 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.