HOx Chemistry during Summer 2005 at a Rural Site in Central Pennsylvania: Observations and Modeling Study

Atmospheric hydroxyl (OH), hydroperoxy (HO2), total peroxy (HO2 and organic peroxy radicals, RO2) mixing ratios and OH reactivity were measured at a rural site in central Pennsylvania during June 2005. OH and HO2 were measured with laser induced fluorescence (LIF); OH and HO2+RO2 were measured with chemical ionization mass spectrometry (CIMS). OH measurements with the two completely different techniques were compared. The daytime maximum mixing ratios were up to 0.5 parts per trillion by volume (pptv) for OH, 40 pptv for HO2, and about 50 pptv for HO2+RO2. A parameterized RACM (Regional Atmospheric Chemistry Mechanism) box model was used to predict steady state OH, HO2 and RO2 concentrations by constraining the model to the measured OH reactivity and measured volatile organic compound (VOC) distributions. The observed radical levels were compared with the model calculations and the agreement is generally good. Possible reasons for some discrepancies between the observations and model calculations will be discussed. Higher-than-expected HO2 and HO2+RO2 at higher NO levels were obtained in this study, which indicates higher-than-expected O3 production at higher NO levels and is consistent with the results in the spring 2002 study at the same site.