Pergamon Atmospheric Environment Vol. 30, No. 6, pp. 819-830, 1996 Elsevier Science Ltd Printed in Great Britain. 1352-2310/96 $15.00 + 0.00 1352-2310(95) 0034~7 HYDROGEN PEROXIDE AND ORGANIC HYDROPEROXIDE CONCENTRATIONS IN AIR IN A EUCALYPTUS FOREST IN CENTRAL PORTUGAL A. V. JACKSON and C. N. HEWlTT* Institute of Environmental and Biological Sciences, Lancaster University, Lancaster LA1 4YQ, U.K. (First received 25 May 1995 and in final form 25 August 1995) Abstract Ambient gas-phase peroxide concentrations were measured at a forest site in Portugal during FIELDVOC, June 1994. Analysis was by HPLC with specifichydroperoxide detection. Hydrogen peroxide (H202) and methyl hydroperoxide (MHP) were quantified. H202 concentrations ranged from below the limit of detection up to 0.63 ppbv. MHP concentrations reached up to 0.1 ppbv. Both peroxides displayed diurnal cycleswith higher concentrations during the daytime than during the night. Approximate backward air mass trajectories were determined from surface pressure charts for the periods sampled. The site was found to experience both Atlantic air and continental air. H202 concentrations were found not to be significantly different in air masses of different origins. MHP concentrations were found to be consistently higher in air of continental origin. Key word index: Hydrogen peroxide, methyl hydroperoxide, HPLC analysis, gas-phase measurements, forest environment, FIELDVOC. INTRODUCTION Hydrogen peroxide (H202) and certain organic hy- droperoxides play important roles in tropospheric chemistry, for example they are considered as plant phytotoxins (Gaffney and Senum, 1984; Gaffney et al., 1987) and it is understood that H202 is a major oxidant leading to the formation of H2SOa in the aqueous phase (Penkett et al., 1979; Calvert et al., 1985; Stockwell, 1994). Over the last decade the fate of these compounds in both the gas and aqueous phases has become the focus of greater attention. At present a reasonable amount of field data exist on the tem- poral and spatial concentrations and distributions of H202 (for example, Daum et al., 1990; Dollard et al., 1989; Dollard and Davies, 1992; Hewitt and Kok, 1991; Tremmel et al., 1993) but very few data are available on the organic hydroperoxides. A major difficulty has been the lack of a measurement device capable of determining their ambient species-specific concentrations. Many earlier measurements are thought to be unreliable due to sampling and analyti- cal problems (Farmer and Dawson, 1982; Heikes, 1984; Gay et al., 1988; Chameides and Davies, 1982; Staehelin and Hoigne, 1982). Predictions of H202 concentrations from pho- tochemical models range over several orders of magnitude, dependent on atmospheric conditions * Corresponding author. (Schiavone and Graedel, 1981; Hov, 1983; Calvert and Stockwell, 1983), and some studies suggest that or- ganic hydroperoxides such as methyl hydroperoxide, MHP, should be present in both polluted and nonpol- luted air (Logan et al., 1981; Rodhe et al; 1981; Schiavone and Graedel, 1981; Thompson and Cice- rone, 1982; Calvert and Stockwell, 1983; Hov, 1983; Kleinman, 1986; Jungfeng et al., 1987). The formation, occurrence and behaviour of H202 in the atmosphere has been reviewed (Sakugawa et al., 1990; Gunz and Hoffman, 1990). There are no significant direct emissions of H202 and organic hydroperoxides into the atmosphere. It is believed that H202 is formed as a termination reac- tion product by the bimolecular self-reaction of the hydroperoxy radicals, HO2, in photochemical chain reactions in the troposphere (Finlayson-Pitts and Pitts Jr, 1986): HO2 + HO2 ~ H202 + 02. Organic hydroperoxides (ROOH, R = alkyl or acyl group) are formed in the gas-phase as follows (Hanst and Gay, 1983): RO2 + HO2 -~"RO2H + 02. The major source of aqueous-phase H20 2 in rain, cloud and fog water is believed to be dissolved gas- eous H202, due to its high solubility in water. This is largely supported by the fact that a diurnal and sea- sonal trend in aqueous H202 concentrations, similar 819