Abstract
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.
Highlights
Urban and regional photochemical smog is reasonably well understood as a byproduct of reactions between volatile organic compounds (VOCs) and nitrogen oxides (NOx) under solar ultraviolet (UV) radiation (Haagen-Smit et al, 1953; Finlayson Pitts and Pitts, 1999)
We report measurements of spectral actinic fluxes made at a surface station (T1) on the northern edge of Mexico City during the March 2006 MILAGRO field campaign (Molina et al, 2010), as well as measurements made from the NSF/NCAR C-130 aircraft during several overpasses of the T1 site
Absorption by NO2 typically accounts for about one quarter of these differences, but can become more significant in the afternoon hours accounting occasionally for up to 77 % of the observed differences
Summary
Urban and regional photochemical smog is reasonably well understood as a byproduct of reactions between volatile organic compounds (VOCs) and nitrogen oxides (NOx) under solar ultraviolet (UV) radiation (Haagen-Smit et al, 1953; Finlayson Pitts and Pitts, 1999). Castro et al (2001) reached the opposite conclusion for Mexico City, where UVabsorbing aerosols reduce PBL actinic fluxes, slowing photochemistry and reducing O3 maxima by 20–50 ppb Such changes in O3 due to aerosol-induced UV perturbations are comparable to or larger than O3 reductions currently practical with VOC and NOx emission regulations. Aerosols usually attenuate UV radiation reaching the surface, resulting in lower actinic fluxes and photolysis frequencies (Leighton, 1961; Demerjian et al, 1980; Lefer et al, 2003; Flynn et al, 2010; Li et al, 2011) Other pollutants, such as NO2, may reduce the actinic flux through direct absorption. Dickerson et al (1997) found that for the Eastern US, sulfate aerosols, which primarily scatter UV wavelengths, cause
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