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An improved chemical amplifier for atmospheric RO, (HO + HOz + RO + ROz) radical measurement is reported. The reaction chamber in this version was equipped with a Teflon filter-nitrogen wall to maintain the nature of the wall constant and reduce the free radical wall loss. The results of laboratory studies were consistent with theoretical calculations assuming a radical wall loss coefficient of 1 s~*. Radical calibration gave relatively stable chain lengths, i.e., 26% variability over a period of 3 months. In addition, the zero modulation and the interferences from the ambient PAN and PNA were reduced by using a smaller reactor which yielded a short residence time. The details of the instrumentation and chain length measurement procedure are described. Results of a local field study in the summer of 1993 using this chemical amplifier are presented.Free radicals in the earth's atmosphere play significant roles in tropospheric chemistry. The hydroxyl radical, HO, the most important tropospheric gas-phase oxidant, comes from the ultraviolet photolysis of ozone (O3), nitrous acid (HONO), and various precursors. HO can initiate the reactions of hundreds of trace gases, both natural and anthropogenic, in many instances as the first and ratedetermining step.1 For example, nitrogen and sulfur oxides are converted respectively to nitric and sulfuric acids, and nonmethane hydrocarbons (NMHC) are oxidized to organic acids after reacting with HO.1 2 Reactions of HO with hydrocarbons and CO lead to the production of hydroperoxy, HOz, organic oxy, RO, and organic peroxy, ROz, radicals. HOz is also formed following formaldehyde photolysis, nitrate radical (NO3) reactions, and the reactions of alkenes with ozone.3-9 These RO, radicals (HO + HOz + RO + ROz) are known to be involved in the production and removal of ozone in photochemical cycles through the competition between the oxidation of NO by ROz and O3, which govern the so-called NO-NO2-O3 photostationary-state system in the atmo-

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