1Center for Research in Environmental Science, School of Public and Environmental Affairs, Dept. of Chemistry, Indiana Univ., Bloomington, USA
2Massachusetts Institute of Technology, Cambridge MA, USA
3University of California at San Diego, La Jolla CA, USA
4University of Colorado at Boulder and CIRES, Boulder, CO, USA
5Molina Center for Energy and the Environment, La Jolla CA, USA
*now at: Molecular Physics Laboratory, SRI International, Menlo Park, CA, USA
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 3.9×105molec cm−3 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 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 conditions of the MCMA. A comparison with other urban and sub-urban field measurements suggests that OH concentrations are highly buffered under these conditions. In contrast, HO2 concentrations are highly variable between different urban sites.