ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-13-1011-2013 Effect of aerosols and NO<sub>2</sub> concentration on ultraviolet actinic flux near Mexico City during MILAGRO: measurements and model calculations Palancar G. G. 1 2 Lefer B. L. 3 Hall S. R. 1 Shaw W. J. 4 Corr C. A. 5 Herndon S. C. 6 Slusser J. R. 7 Madronich S. 1 Atmospheric Chemistry Division, National Center for Atmospheric Research, Boulder, CO, USA INFIQC-CONICET, Departamento de Físico Química, Facultad de Ciencias Químicas, Universidad Nacional de Córdoba, Centro Láser de Ciencias Moleculares, 5000, Córdoba, Argentina Department of Earth and Atmospheric Sciences, University of Houston, Houston, TX, USA Pacific Northwest National Laboratory, Department of Energy, Richland, WA, USA Earth Systems Research Center, University of New Hampshire, Durham, NH, USA Aerodyne Research Inc., Billerica, MA, USA UV-B Monitoring and Research Program, USDA, Colorado State University, Ft. Collins, CO, USA 24 01 2013 13 2 1011 1022 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys.net/13/1011/2013/acp-13-1011-2013.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/13/1011/2013/acp-13-1011-2013.pdf

Urban air pollution absorbs and scatters solar ultraviolet (UV) radiation, and thus has a potentially large effect on tropospheric photochemical rates. We present the first detailed comparison between actinic fluxes (AF) in the wavelength range 330–420 nm measured in highly polluted conditions and simulated with the Tropospheric Ultraviolet-Visible (TUV) model. Measurements were made during the MILAGRO campaign near Mexico City in March 2006, at a ground-based station near Mexico City (the T1 supersite) and from the NSF/NCAR C-130 aircraft. At the surface, measured AF values are typically smaller than the model by up to 25% in the morning, 10% at noon, and 40% in the afternoon, for pollution-free and cloud-free conditions. When measurements of PBL height, NO<sub>2</sub> concentration and aerosols optical properties are included in the model, the agreement improves to within ±10% in the morning and afternoon, and ±3% at noon. Based on daily averages, aerosols account for 68% and NO<sub>2</sub> for 25% of AF reductions observed at the surface. Several overpasses from the C-130 aircraft provided the opportunity to examine the AF perturbations aloft, and also show better agreement with the model when aerosol and NO<sub>2</sub> effects are included above and below the flight altitude. TUV model simulations show that the vertical structure of the actinic flux is sensitive to the choice of the aerosol single scattering albedo (SSA) at UV wavelengths. Typically, aerosols enhance AF above the PBL and reduce AF near the surface. However, for highly scattering aerosols (SSA > 0.95), enhancements can penetrate well into the PBL, while for strongly absorbing aerosols (SSA < 0.6) reductions in AF are computed in the free troposphere as well as in the PBL. Additional measurements of the SSA at these wavelengths are needed to better constrain the effect of aerosols on the vertical structure of the AF.

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