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

Abstract

Abstract. In March 2006, a multiagency field campaign was undertaken in Mexico City called the Megacities Initiative: Local and Global Research Observations (MILAGRO). Two of the five field components of the MILAGRO study focused a major part of their efforts on atmospheric particulate emissions from the Mexico City basin and their effects on radiative balance as a function of time, location and processing conditions. As part of these two MILAGRO components, measurements of aerosol optical properties were obtained at a site located in the northern part of Mexico City (T0) and also at a site located 29 km northwest (T1) to estimate the regional effects of aerosol emissions from the basin.Measurements of aerosol absorption and scattering for fine mode aerosols were obtained at both sites. Aerosol absorption at 550 nm was similar at both sites, ranging from 7-107 Mm −1 at T0 and from 3-147 Mm −1 at T1. Aerosol scattering measured at 550 nm at T0 ranged from 16-344 Mm −1 while the aerosol scattering values at T1 were much lower than at T0 ranging from 2-136 Mm −1 . Aerosol single scattering albedos (SSAs) were calculated at 550 nm for the fine mode aerosols at both sites using these data. The SSAs at T0 ranged from 0.47-0.92 while SSAs at T1 ranged from 0.35-Correspondence to: J. S. Gaffney (jsgaffney@ualr.edu) 0.86. The presence of these highly absorbing fine aerosols in the lower atmosphere of the Mexico City area will result in a positive climate 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 average of 64 µW/cm 2 at solar noon, than at site T1, which had an average of 54 µW/cm 2 at solar noon. Comparisons of clear-sky modeled UVB intensities with the simultaneous UVB measurements obtained at sites T0 and T1 for cloudless days indicate a larger diffuse radiation field at site T0 than at site T1. The determination of aerosolÅngstrom scattering coefficients at T0 suggests that this is due to the predominance of aerosols in the size range of 0.3 micron, which leads to scattering of UVB radiation peaked in the forward direction and to an enhanced UVB radiation observed at ground level. This enhancement of the UVB diffuse radiation field would explain the enhanced photochemistry observed in the Mexico City area despite the reduction in UVB anticipated from light absorbing species.

An overview of the MILAGRO 2006 Campaign: Mexico City emissions and their transport and transformation

“…Substantial vertical layering of the aerosol and regional differences in the Valley of Mexico for aerosol scattering and extinction was also found. Enhanced UV absorption (300-450 nm) of incoming solar radiation was found to be substantial and was likely due to both SOA formation as well as from biomass burning sources (Corr et al, 2009;Marley et al, 2009b), consistent with a previous study during MCMA-2003. The wavelength dependence of absorption measured at 470, 530 and 660 nm on the C-130 was higher with higher organic fraction of non-refractory mass of submicrometer particles, and this relationship was more pronounced under high single scattering albedo, as expected for the interplay between soot and colored weak absorbers (some organic species and dust) .…”

“…Thus other urban and regional sources of modern carbon are important in Central Mexico, such as biogenic SOA , food cooking (Mugica et al, 2009), and other anthropogenic open burning activities (e.g., trash burning and biofuel use (Christian et al, 2010)). Larger relative amounts of biomass burning aerosols were noted at the T1 regional site than the urban T0 site Marley et al, 2009b), consistent with the megacity having a significant fossil fuel input, but both sites were heavily impacted by recent carbon sourced carbonaceous aerosols from local and regional burning and also other modern carbon-containing sources as discussed above.…”

“…The AAE values were derived from a simple exponential fit of the broadband aerosol absorption spectra obtained from either a seven-channel wavelength Aethalometer (5-min time resolution) or from 12-h integrated submicrometer aerosols samples by using an integrating sphere/spectrometer to obtain complete UV-visible-NIR spectra (Marley et al, 2009b). This result showed the significant impact that grass fires had in the region during the MILAGRO campaign on the lower atmospheric aerosol radiative forcing.…”

Physical and chemical properties of the regional mixed layer of Mexico's Megapolis

“…The scattering coefficient, B sct , is extremely well correlated with the POM for all air flow directions. Marley et al (2009) Marley el al. (2009).…”

Aerosol effects on the photochemistry in Mexico City during MCMA-2006/MILAGRO campaign

“…Marley et al (2009) measured the aerosol absorption and scattering in the MCMA during MILAGRO; they reported that the single scattering albedo (SSA) at 550 nm for the fine mode aerosols ranged from 0.47 to 0.92 at the T0 supersite (located in the northern part of Mexico City), suggesting that Mexico City environment has high levels of fine mode absorbing aerosols. Using direct measurements of the optical and microphysical characteristics of atmospheric soot particles, Moffet and Prather (2009) found that, within 3 h after sunrise, photochemical activity leads to the rapid conversion of fresh non-spherical soot to aged spherical coated soot particles, which become the dominant contributor to aerosol absorption during mid-day.…”