Abstract
Abstract. In the present study, the impact of aerosols on the photochemistry in Mexico City is evaluated using the WRF-CHEM model for the period from 24 to 29 March during the MCMA-2006/MILAGRO campaign. An aerosol radiative module has been developed with detailed consideration of aerosol size, composition, and mixing. The module has been coupled into the WRF-CHEM model to calculate the aerosol optical properties, including optical depth, single scattering albedo, and asymmetry factor. Calculated aerosol optical properties are in good agreement with the surface observations and aircraft and satellite measurements during daytime. In general, the photolysis rates are reduced due to the absorption by carbonaceous aerosols, particularly in the early morning and late afternoon hours with a long aerosol optical path. However, with the growth of aerosol particles and the decrease of the solar zenith angle around noontime, aerosols can slightly enhance photolysis rates when ultraviolet (UV) radiation scattering dominates UV absorption by aerosols at the lower-most model layer. The changes in photolysis rates due to aerosols lead to about 2–17 % surface ozone reduction during daytime in the urban area in Mexico City with generally larger reductions during early morning hours near the city center, resulting in a decrease of OH level by about 9 %, as well as a decrease in the daytime concentrations of nitrate and secondary organic aerosols by 5–6 % on average. In addition, the rapid aging of black carbon aerosols and the enhanced absorption of UV radiation by organic aerosols contribute substantially to the reduction of photolysis rates.
Highlights
Atmospheric particulate matter or aerosols, formed from natural and anthropogenic sources, are a chemical mixture of solid and liquid particles suspended in the atmosphere, with diameters ranging from a few nanometers to several micrometers or more
black carbon (BC) and primary organic aerosol (POA) that are directly emitted into the atmosphere in particulate form, the WRF-CHEM model reproduces the measured variation of BC and POA during the second episode (27–29 March), but it frequently overestimates during the first episode (24–26 March), at night when the pollutants are transported from the Tula industrial complex located 70 km northwest of the city
We have developed an aerosol radiative module with detailed consideration of aerosol size, composition and mixing, and coupled it into the WRF-CHEM model to evaluate the aerosol impacts on the photochemistry in Mexico City
Summary
Atmospheric particulate matter or aerosols, formed from natural and anthropogenic sources, are a chemical mixture of solid and liquid particles suspended in the atmosphere, with diameters ranging from a few nanometers to several micrometers or more. Barnard et al (2008) used the data measured during MCMA2003 (Molina et al, 2007) and MILAGRO-2006 to investigate the absorption of solar radiation by the organic component of aerosols. They reported that the organic aerosol enhances the absorption in the near-UV spectral range (250 to 400 nm) and the mass absorption cross section for the organic aerosol decreases from 10.5 m2 g−1 at 300 nm to around zero at about 500 nm. The objective of the present study is to examine the aerosol impact on the photochemistry in Mexico City using the WRF-CHEM model based on the measurements taken during MCMA-2006.
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