Article pubs.acs.org/ac
Atmospheric Peroxy Radical Measurements Using Dual-Channel Chemical Amplification Cavity Ringdown Spectroscopy Yingdi Liu Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91109, United States
Jingsong Zhang* Department of Chemistry and Air Pollution Research Center, University of California, Riverside, California 92521, United States ABSTRACT: Peroxy (HO2 and RO2) radicals are important intermediates in tropospheric oxidation of hydrocarbons, and their accurate atmospheric measurements remain challenging. In this work, the peroxy radical chemical amplification (PERCA) method was combined with cavity ringdown spectroscopy (CRDS) to develop a dual-channel instrument for measurements of atmospheric peroxy radicals. In the amplification channel, the peroxy radicals were converted in an excess amount of NO and CO into a higher level of NO2 and measured along with the background NO2, while in the reference channel, only the background NO2 (ambient NO2 and NO2 converted from O3 reaction with NO) was monitored. The NO2 levels from both channels were measured simultaneously at a high time resolution (∼1 s) using two identical CRDS systems with one 408.5-nm diode laser, and their difference gave the amplified NO2 from PERCA. The peroxy radical concentration was obtained from the amplified NO2 and the calibrated amplification factor or chain length (CL). The optimized CL was 190 ± 20 (1σ) using laboratory-generated HO2 and CH3O2 radical sources. The detection sensitivity was 4 ppt/10 s (3σ). Ambient measurements in Riverside, CA were carried out. This dual-channel diode-laser PERCA-CRDS instrument was compact and capable of providing real-time, in situ, and sensitive measurements of atmospheric peroxy radicals with fast time response. (CIMS),17−20 laser-induced fluorescence (LIF),21,22 etc. The MI-ESR method enriches peroxy radicals by cryogenic matrix isolation and directly measures peroxy radicals using off-line electron spin resonance (ESR).3,4 This technique requires a long sampling time (∼30 min) to achieve a sensitivity of ∼5 pptv.23 LIF has been used for direct measurement of the OH radical; by converting HO2 to OH with NO reaction, LIF can be used to indirectly detect the HO2 radical.21,22,24−27 Although LIF has a high sensitivity for HO2 (106−107 molecules/cm3), it is complex and requires careful calibrations24−27 and low pressure that could result in decomposition of some species.26−28 PERCA is an indirect method for in situ measurements of peroxy radicals.5−16 It was initially developed by Cantrell et al.5,6 and later by Hastie et al.9 and others.10−16 Since then, many PERCA studies have been carried out.9−11,15,16,29,30 In PERCA, the peroxy radicals are converted to an amplified level of NO2 in chain reactions with excess amount of NO and CO, followed by subsequent detection of NO 2 using chemiluminescence reaction of NO 2 with luminol.5,9,11,16 The sensitivity varies from 2 pptv in 30 min
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eroxy radicals, both hydroperoxy (HO2) and organic peroxy (RO2), play significant roles in tropospheric oxidation and photochemistry.1,2 The peroxy radicals are produced mainly in atmospheric reactions of OH radical with CO and hydrocarbons. They are also formed in the photolysis of organic carbonyl compounds, atmospheric reactions of alkenes with ozone, and some reactions of NO3 radical.1 In the polluted troposphere, in the presence of NOx (NO + NO2), the peroxy radicals are involved in the photochemical formation of O3, because they can efficiently convert NO to NO2, which is subsequently photolyzed to produce O3. In the absence of NOx, the peroxy radicals undergo self-reactions to form hydroperoxides, organic peroxides, etc. The radical budget of HO2 and RO2 are closely related to that of the OH radical. Consequently, accurate measurement of the peroxy radicals is critical for assessing atmospheric oxidizing capacity and understanding tropospheric photochemistry. Since the peroxy radicals are highly reactive, their concentrations in the troposphere are very low, typically
