SOME CONSIDERATIONS OF THE ORIGIN OF NIGHTTIME PEROXY-RADICALS OBSERVED IN MLOPEX 2C

The ''chemical amplifier'' (CA) was used to measure the concentration of the peroxy radicals present in the nighttime atmosphere at the Maun a Loa Observatory (MLO) during the Mauna Loa Observatory Photochemistr y Experiment (MLOPEX 2) from April 15 to May 15, 1992. Simultaneous me asurements were also made of peroxy radicals using a modified OH instr ument during one period of 56-hour duration. Both instruments observe experimentally significant nighttime signals, but the magnitude oi the larger CA signal appears to result from organic peroxy radicals to wh ich the modified OH instrument is insensitive. The data from about one half of the nights showed a decay of the radical signal from 2000 to 0400 HST the next morning. The decay rates followed roughly those expe cted for primary or secondary alkyl peroxy radicals. However, data fro m the other half of nights showed an increase in radical concentration with time, which reflected significant radical generation rates. Seve ral alternatives are considered in explanation of the results. (1) The possible origin of the signals from the interference of HO2NO2, presu mably transported from reservoirs at higher elevations, was considered . The data are inconclusive, and this hypothesis is tentatively reject ed. (2) The nighttime chemistry of the O-3 and NO3 was also considered . The observed trace gases provide in theory only a small fraction of the peroxy radical signal seen on many nights. However, the presence o f 1-30 parts per trillion by volume (pptv) of CH3SCH3, a compound not measured but expected to be present at MLO, can react with NO3 to prod uce peroxy radical signals of the magnitude observed. It is concluded that this ubiquitous compound over the oceans offers the best current hypothesis to rationalize the nighttime generation of peroxy radicals at MLO.