Total OH reactivity measurements at Manitou Experimental Forest in summer season during BEACHON-ROCS field campaign

The hydroxyl radical (OH) is well known to play a central role in initiating the oxidation of a number of atmospheric species. We have developed an OH reactivity measurement system using a laser pump probe technique and have deployed this instrument in urban and suburban areas of Tokyo. During these measurements of OH reactivity, we confirmed the existence of unknown reactive species. However, there are only few examples for measurements of OH reactivity in rural areas where the effect of anthropogenic emissions is thought to be small. Therefore measurements of OH reactivity in rural areas are essential to confirm whether our knowledge of trace species in these areas is valid. We participated in the BEACHON-ROCS field campaign and measured the total OH reactivity at Manitou Experimental Forest (MEF) during August 8th-23rd, 2010. MEF is located in the southern part of the Rocky Mountains and about 80 km south from Denver. MEF is dominated by ponderosa pine; observations indicate that the effect of anthropogenic emissions is small. The OH reactivity was about 5-20 s-1, lower than that measured in urban or suburban areas of Tokyo. Diurnal variation of OH reactivity was observed for all measurement days and OH reactivity for daytime was lower than that for nighttime. In addition to the measurement of OH reactivity, trace species such as CO, NOx, O3, SO2, aldehydes and VOCs, including biogenic VOCs, were analyzed. From the calculation of OH reactivity based on the analysis of the observed trace species, about 15-30% of OH reactivity for VOCs came from biogenic VOCs, which were dominated by a monoterpene and 2-methyl-3-butene-2-ol (MBO). A comparison of the observed and calculated OH reactivity shows that the calculated OH reactivity is up to 60% less than the observed value.