Nighttime observations of anomalously high levels of hydroxyl radicals above a deciduous forest canopy

Diurnal measurements of hydroxyl and hydroperoxy radicals (OH and HO2) made during the Program for Research on Oxidants: Photochemistry, Emissions, an d Transport (PROPHET) summer intensive of 1998 indicate that these key comp onents of gas phase atmospheric oxidation are sustained in significant amou nts throughout the night in this northern forested region. Typical overnigh t levels of OH observed were 0.04 parts per trillion (pptv) (1.1 x 10(6) mo lecules/cm(3)), while HO2 concentrations ranged from 1 to 4 pptv. Results o f diagnostic testing performed before, after, and during the deployment sug gest little possibility of interferences in the measurements. Collocated me asurements of the reactive biogenic hydrocarbon isoprene corroborate the ob served levels of OH by exhibiting significant decays overnight above the fo rest canopy. The observed isoprene lifetimes ranged from 1.5 to 12 hours in the dark, and they correlate well to those expected from chemical oxidatio n by the measured OH abundances. Possible dark reactions that could generat e such elevated levels of OH include the ozonolysis of extremely reactive b iogenic terpenoids. However, in steady state models, which include this hyp othetical production mechanism, HO2 radicals are generated in greater quant ities than were measured. Nonetheless, if the measurements are representati ve of the nocturnal boundary layer in midlatitude temperate forests, this o bserved nocturnal phenomenon might considerably alter our understanding of the diurnal pattern of atmospheric oxidation in such pristine, low-NOx envi ronments.