Abstract. Because of the extremely short photochemical lifetime of tropospheric OH, comparisons between observations and model calculations should be an effective test of our understanding of the photochemical processes controlling the concentration of OH, the primary oxidant in the atmosphere. However, unambiguous estimates of calculated OH require sufficiently accurate and complete measurements of the key species and physical variables that determine OH concentrations. The Tropospheric OH Photochemistry Experiment (TOHPE) provides an extremely complete set of measurements, sometimes from multiple independent experimental platforms, that allows such a test to be conducted. When the calculations explicitly use observed NO, NO 2, hydrocarbons, and formaldehyde, the photochemical model consistently overpredicts in situ observed OH by -50% for the relatively clean conditions predominantly encountered at Idaho Hill. The model bias is much higher when only CH4-CO chemistry is assumed, or NO is calculated from the steady state assumption. For the most polluted conditions encountered during the campaign, the model results and observations show better agreement. Although the comparison between calculated and observed OH can be considered reasonably good given the +30% uncertainties of the OH instruments and various uncertainties in the model, the consistent bias suggests a fundamental difference between theoretical expectations and the measurements. Several explanations for this discrepancy are possible, including errors in the measurements, unidentified hydrocarbons, losses of HO x to aerosols and the Earth's surface, and unexpected peroxy radical chemistry. Assuming a single unidentified type of hydrocarbon is responsible, the amount of additional hydrocarbon needed to reduce theoretical OH to observed levels is a factor of 2 to 3 greater than the OHreactivity-weighted hydrocarbon content measured at the site. Constraints can be placed on the production and yield of various radicals formed in the oxidation sequence by considering the observed levels of certain key oxidation products such as formaldehyde and acetaldehyde. The model results imply that, under midday clean westerly flow conditions, formaldehyde levels are fairly consistent with the OH and hydrocarbon observations, but observed acetaldehyde levels are a factor of 4 larger than what is expected and also imply a biogenic source. Levels of methacrolein and methylvinylketone are much lower than expected from steady state isoprene chemistry, which implies important removal mechanisms or missing information regarding the kinetics of isoprene oxidation within the model. In a prognostic model application, additional hydrocarbons are added to the model in order to force calculated OH to observed levels. Although the products and oxidation steps related to pinenes and other biogenic hydrocarbons are somewhat uncertain, the addition of a species with an oxidation mechanism similar to that expected from C 10 pinenes would be consistent with the complete set of observa...

No abstract

With the continuous methods used here, no unequivocal interferences were seen when SO2, NO2, 03, and isoprene impurities were added to prepared mixtures or when these were present in ambient air. The measurements with the C-18 DNPH (no 03 scrubber) and silica gel DNPH cartridges (with 03 scrubber) showed a reasonable correlation with the TDLAS measurements, although the results from the silica cartridges were about a factor of two below the standards in the spike experiments and about 35% below in the ambient measurements. Using the NCAR gas-phase spike data to calibrate the response of the silica gel cartridges in the ambient studies, the results are the same within statistical uncertainty. When the same gas phase calibration was used with the C-18 cartridges, the results showed a positive bias of about 35%, presumably reflecting a positive ozone interference in this case (no ozone scrubber used). The silica DNPH cartridge results from the second participant were highly scattered and showed no significant correlation with the TDLAS measurements.

No abstract

No abstract