Measurements of aerosol absorption and scattering in the Mexico City Metropolitan Area during the MILAGRO field campaign: a comparison of results from the T0 and T1 sites

摘要

In March 2006, a multiagency field campaign was undertaken in Mexico Citycalled the Megacities Initiative: Local and Global Research Observations(MILAGRO). Two of the five field components of the MILAGRO study focused amajor part of their efforts on atmospheric particulate emissions from theMexico City basin and their effects on radiative balance as a function oftime, location and processing conditions. As part of these two MILAGROcomponents, measurements of aerosol optical properties were obtained at asite located in the northern part of Mexico City (T0) and also at a sitelocated 29 km northwest (T1) to estimate the regional effects ofaerosol emissions from the basin.Measurements of aerosol absorption and scattering for fine mode aerosolswere obtained at both sites. Aerosol absorption at 550 nm was similar atboth sites, ranging from 7–107 Mm at T0 and from 3–147 Mm at T1. Aerosol scattering measured at 550 nm at T0 ranged from16–344 Mm while the aerosol scattering values at T1 were much lowerthan at T0 ranging from 2–136 Mm. Aerosol single scattering albedos(SSAs) were calculated at 550 nm for the fine mode aerosols at both sitesusing these data. The SSAs at T0 ranged from 0.47–0.92 while SSAs at T1ranged from 0.35–0.86. The presence of these highly absorbing fine aerosolsin the lower atmosphere of the Mexico City area will result in a positiveclimate forcing and a local warming of the boundary layer in the region.Broadband UVB intensity was found to be higher at site T0, with an averageof 64 μW/cm at solar noon, than at site T1, which had an averageof 54 μW/cm at solar noon. Comparisons of clear-sky modeled UVBintensities with the simultaneous UVB measurements obtained at sites T0 andT1 for cloudless days indicate a larger diffuse radiation field at site T0than at site T1. The determination of aerosol Ångstrom scatteringcoefficients at T0 suggests that this is due to the predominance of aerosolsin the size range of 0.3 micron, which leads to scattering of UVB radiationpeaked in the forward direction and to an enhanced UVB radiation observed atground level. This enhancement of the UVB diffuse radiation field wouldexplain the enhanced photochemistry observed in the Mexico City area despitethe reduction in UVB anticipated from light absorbing species.