Articles | Volume 13, issue 2
Atmos. Chem. Phys., 13, 1011–1022, 2013
Atmos. Chem. Phys., 13, 1011–1022, 2013

Research article 24 Jan 2013

Research article | 24 Jan 2013

Effect of aerosols and NO2 concentration on ultraviolet actinic flux near Mexico City during MILAGRO: measurements and model calculations

G. G. Palancar1,2, B. L. Lefer3, S. R. Hall1, W. J. Shaw4, C. A. Corr5, S. C. Herndon6, J. R. Slusser7, and S. Madronich1 G. G. Palancar et al.
  • 1Atmospheric Chemistry Division, National Center for Atmospheric Research, Boulder, CO, USA
  • 2INFIQC-CONICET, Departamento de Físico Química, Facultad de Ciencias Químicas, Universidad Nacional de Córdoba, Centro Láser de Ciencias Moleculares, 5000, Córdoba, Argentina
  • 3Department of Earth and Atmospheric Sciences, University of Houston, Houston, TX, USA
  • 4Pacific Northwest National Laboratory, Department of Energy, Richland, WA, USA
  • 5Earth Systems Research Center, University of New Hampshire, Durham, NH, USA
  • 6Aerodyne Research Inc., Billerica, MA, USA
  • 7UV-B Monitoring and Research Program, USDA, Colorado State University, Ft. Collins, CO, USA

Abstract. 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, NO2 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 NO2 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 NO2 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.

Final-revised paper