Abstract. Diurnal measurements ot hydroxyl and hydroperoxy radicals (OH ---'during the Program for Research on Oxidants: Photochemistry, Emissions. and Transport (PROPHET) summer intensive of 1998 indicate that these key components of gas phase atmospheric oxidation are sustained in significant amounts throughout the night in this northern forested region. Typical overnight levels of OH observed were 0.04 parts per trillion (pptv) (1.1 x 106 molecules/cm3), while HO2 concentrations ranged from 1 to 4 pptv.Results of diagnostic testing performed before, after, and during the deployment suggest little possibility of interferences in the measurements. Collocated measurements of the reactive biogenic hydrocarbon isoprene corroborate the observed levels of OH by exhibiting significant decays overnight above the torest canopy. The observed isoprene lifetimes ranged t¾om 1.5 to 12 hours in the dark, and they correlate well to those expected from chemical oxidation by the measured OH abundances. Possible dark reactions that could generate such elevated levels of OH include the ozonolysis of extremely reactive biogenic terpenoids. However, in steady state models, which include this hypothetical production mechanism, HO: radicals are generated in greater quantities than were measured. Nonetheless, if the measurements are representative of the nocturnal boundary layer in midlatitude temperate forests, this observed nocturnal phenomenon might considerably alter our understanding of the diurnal pattern of atmospheric oxidation in such pristine, low-NOt environments.

Abstract. A rapid nighttime decay of isoprene (2-methyl-1,3-butadiene) has been observed at several forest sites. Data from the Program for Research on Oxidants: PHotochemistry, Emissions, and Transport (PROPHET) have been carefully examined with respect to this phenomenon. Essentially every evening (at PROPHET), isoprene concentrations fall from several ppb to levels below 100 ppt, with an average lifetime of 2.7 hours. Since this decay rate exceeds that expected from established nighttime chemistry, other possible mechanisms are suggested and discussed. Reaction with ozone will not occur at a rate consistent with the observed decay. Calculations of nitrate radical concentrations reveal that this oxidant only becomes an important sink for isoprene after the majority of the isoprene decay has taken place. The isoprene flux data were not consistent with dry deposition playing a significant role in nighttime forest loss. On the basis of ambient measurements of OH radical concentrations at the PROPHET site, calculated isoprene decay rates were compared with observations. For some nights the observed decay can be fit strictly by OH consumption; however, the reported OH data overpredict the isoprene loss rate on most nights. We estimate that vertical mixing with isoprene-depleted air probably contributes to the fast isoprene decay observed; however, the measurements needed to support this suggestion have yet to be made.

[1] A large set of isoprene and isoprene oxidation product concentration data from four North American sites was examined to assess the NO x dependence of the daytime oxidation of isoprene. Sites that represent a wide range of NO x (50 ppt to 30 ppb) were studied and include the Dickson, Tennessee, and Cornelia Fort Air Park sites during the 1999 Southern Oxidants Study, the Pellston, Michigan, site during the 1998 PROPHET summer intensive, and the Kejimkujik National Park site during the Atlantic 1996 study. Knowledge of NO x and HO x concentrations were critical for this evaluation. While NO x data are readily available at all sites, HO x data are limited. We employed a simple 10-reaction HO x model to calculate steady state OH radical concentrations as a function of [NO x ] to enable analysis of the data from all sites. Here, we use methyl vinyl ketone (MVK) concentrations to quantify the extent of isoprene-OH oxidation. Making use of the MVK/isoprene ratio, we show that the rate of production of isoprene oxidation products at various North American sites, although highly variable, exhibits the crossover from NO x -dependent to VOC-dependent conditions at $8 ppb [NO x ], in agreement with what is calculated from HO x measurements and our calculations.

Abstract. The Program for Research on Oxidants: PHotochemistry, Emissions, and Transport (PROPHET) 1998 summer campaign, conducted at the University of Michigan Biological Station, provided a unique opportunity to compare isoprene measurement techniques that were automated, sampled and analyzed on-line, and provided relatively fast time resolution. Assessment of the data quality for fast isoprene measurements is important because isoprene dominates the surface chemistry at many rural sites and even some urban environments. An informal intercompalison was conducted by evaluating ambient isoprene mixing ratio data generated by five different instruments: quadrupole ion trap (QIT) MS, the chemiluminescent-based fast isoprene sensor (FIS), and three gas chromatograph/mass spectrometry (GC/MS) techniques. The GC/MS methods were deployed and maintained by Purdue University (GC/MS-P), the National Center for Atmospheric Research (GC/MS-NCAR), and the Rosenstiel School of Marine and Atmospheric Science (GC/MS-RSMAS).The FIS was deployed and maintained by NCAR, Hills-Scientific.com and Washington State University, while the QIT was implemented by Purdue University. The GC/MS-P was chosen as the reference method to evaluate the agreement of the data set. The data were evaluated for time-matched samples through regression analysis, ratio analysis, and percent difference analysis relative to GC/MS-P. For measurement data in the central 90th percentile relative to the median, the mean percent difference was 21% for GC/MS-NCAR, 41% for QIT, 42% for GC/MS-RSMAS, and 88% for the FIS. Potential sources of disagreement, especially for low-concentration data, such as variations in sampling time, interferences, method precision and accuracy, and limited cross-calibration, are discussed.

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