Abstract
Abstract. Measurements of tropospheric hydroxyl (OH) and hydroperoxy (HO2) radicals were made during the MCMA (Mexico City Metropolitan Area) field campaign as part of the MILAGRO (Megacity Initiative: Local and Global Research Observations) project during March 2006. These radicals were measured using a laser-induced fluorescence instrument developed at Indiana University. This new instrument takes advantage of the Fluorescence Assay by Gas Expansion technique (FAGE) together with direct excitation and detection of OH at 308 nm. HO2 is indirectly measured as OH by titration with NO inside the fluorescence cell. At this stage of development, IU-FAGE is capable of detecting 3.9×105 molecule/cm3 of both OH and HO2, with a signal to noise ratio of 1, an averaged laser power of 10-mW and an averaging time of 5-min. The calibration accuracies (1σ) are ±17% for OH and ±18% for HO2 using the water-vapor photolysis/O2 actinometry calibration technique. OH and HO2 concentrations were successfully measured at an urban site in Mexico City, with observed concentrations comparable to those measured in other polluted environments. Enhanced levels of OH and HO2 radicals were observed on several days between 09:30–11:00 a.m. and suggest an intense photochemistry during morning hours that may be due to elevated sources of HOx (OH+HO2) and a fast cycling between the radicals under the high NOx (NO+NO2) conditions of the MCMA.
Highlights
Hydroxyl (OH) and hydroperoxy (HO2) radicals have been a subject of considerable interest since it was suggested that OH was produced at sufficient concentrations in the troposphere to initiate the oxidation of volatile organic compounds (VOCs) and other trace gases (Levy, 1971, 1972)
In this paper we report the description of a HOx instrument developed at Indiana University (IU-FAGE) based on the Fluorescence Assay by Gas Expansion technique and the resonant detection scheme at 308-nm
This parameter can be characterized under the working conditions of the FAGE apparatus for various concentrations of reagent using an interference-free source of OH radicals such as the photolysis of water-vapor
Summary
Hydroxyl (OH) and hydroperoxy (HO2) radicals have been a subject of considerable interest since it was suggested that OH was produced at sufficient concentrations in the troposphere to initiate the oxidation of volatile organic compounds (VOCs) and other trace gases (Levy, 1971, 1972). Ambient air is expanded inside a detection cell and OH is probed by lowenergy laser pulses generated at a high repetition-rate This sampling technique, known as Fluorescence Assay by Gas Expansion (FAGE), exhibits several advantages: (i) Sampling at low pressure reduces the concentration of both O3 and H2O in the sampled air mass, as well as other trace gases, reducing the production of laser-generated OH as well as unwanted secondary chemistry (Stevens et al, 1994). In this paper we report the description of a HOx instrument developed at Indiana University (IU-FAGE) based on the Fluorescence Assay by Gas Expansion technique and the resonant detection scheme at 308-nm This instrument was first deployed as part of the Mexico City Metropolitan Area (MCMA) field campaign during March of 2006, and successfully measured daytime OH and HO2 concentrations. A more detailed analysis of the data, including model comparisons and a discussion of the radical budget will be presented in a companion paper (Dusanter et al, 2009)
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