JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 101, NO. D9, PAGES 14,729-14,739, JUNE 20, 1996 Comparison of hydroperoxide measurements madeduringthe Mauna Loa Observatory Photochemistry Experiment2 Thomas A. Staffelbach • and Gregory L. Kok NationalCenterfor Atmospheric Research, Boulder, Colorado Brian G. Heikes and Brian McCully 2 Center for Atmospheric Chemistry Studies, Graduate School of Oceanography, University of Rhode Island, Narragansett Gervase I. Mackay, David R. Karecki andHarold I. Schiff Unisearch Associates Incorporated, Concord, Ontario,Canada Abstract. From the fallof 1991 through the summer of 1992 the Mauna Loa Observatory Photochemistry Experiment 2 (MLOPEX 2) tookplace in Hawaii. The experiment consisted of four intensive research periods, each approximately I month in duration. We report a comparison of measurements of atmospheric hydroperoxide andmethylhydroperoxide asmeasured during MLOPEX 2a (September 16 to October 23, 1991)andMLOPEX 2b (January 15to February 15, 1992). The analytical techniques used for measuring hydroperoxides included tunable diode laser infrared absorption (TDLAS) andfluorescence from an enzymatically produced dimer. The TDLAS technique measured hydrogen peroxide (H202) only. Three different procedures wereutilized to distinguish between H202 andorganic hydroperoxides in the fluorescence analytical techniques. These werehighperformance liquid chromatographic (HPLC) separation, enzymatic discrimination, anddifferential aqueous solubility. The measured concentrations ranged from0 to 2 parts per billionby volume (ppbv)for H202and0 to ppbvfor methylhydroperoxide (CH3OOH). Depending onthetimeperiod of measurement, theagreement varied between 20% anda factor of 2 or morefor the measurement of H202. The discrepancy for the CH3OOH data was larger. Introduction The MaunaLoa Photochemistry Experiment (MLOPEX) was designedto examine the oxidizing capacity of the remote troposphere. From May 1 to June 4, 1988, MLOPEX 1 was conducted at the MaunaLoa Observatory on the island of Hawaii (55 36' W, 19 32' N, 3.4 km above sea level). The Mauna Loa Observatory was chosen because of its remoteness and the long- termrecord of atmospheric measurements at the site[Ridley and Robinson,1992]. The experiment helped to characterize the distribution, trends, and behaviorof reactive species in the remote Pacific. Starting in the fall of 1991, the Mauna Loa Observatory Photochemistry Experiment 2 (MLOPEX 2) was conducted at the same site. MLOPEX 2 covered an entire seasonal cycle with four intensive studies,each about a month long. A broad range of atmospheric measurements were lNow atEidgenoessische Forschungsanstalt fuer Agrikulturchemie undUmwelthygiene, Liebefeld-Bem, Switzerland. 2Now atMackie Designs Inc., Woodinville, Washington Copyright 1996by theAmerican Geophysical Union. Paper number 95JD02197. 0148-0227/96/95JD-02197509.00 performedincluding NO, NO2, NOx, HNO3, 03, HCHO and higher aldehydes, hydrocarbons, organic acids, CO, SO2, hydroperoxides, RO2, OH, jO3, andjNO2. Some species were measured by several investigators using different methods. Hydroperoxidesare important indicators of free radical behavior at a site with low NOx concentrations, which is the case for Mauna Loa. Kleinman[1991] showed that the total peroxide concentration increases linearly with increasing radical sources for the low NOx environment. Here we report parallel measurements of hydrogen peroxide (H202) and methylhydroperoxide (CH3OOH or MHP) during MLOPEX 2a (September 15 to October 23, 1991) and MLOPEX 2b (January 15 to February 15, 1992). The National Center for Atmospheric Research operated a continuous dual-enzyme hydroperoxide analytical system (NCAR-DE) [Lazrus et al., 1986] and a cryogenic sampling technique combined with speciated hydroperoxide detection using high performance liquid chromatographic (HPLC) separation (NCAR-HPLC). The NCAR-HPLC system will be described in detail elsewhere. The University of Rhode Island (URI) operated a serial scrubber coil system (URI-SC)[Heikes, 1992]. All of the above techniques used the enzyme catalyzed dimerizationof p-hydroxyphenylacetic acid coupled with fluorescence detection as the analytical technique. Unisearch Associates (UNI) measured H202 with a tunablediode laser absorption spectroscopy (UNI-TDLAS) system [Hastieet al., 14,729