Applications of lagrangian dispersion modeling to the analysis of changes in the specific absorption of elemental carbon

Abstract

Abstract. We use a Lagrangian dispersion model driven by a mesoscale model with four-dimensional data assimilation to simulate the dispersion of elemental carbon (EC) over a region encompassing Mexico City and its surroundings. The region was the study domain for the 2006 MAX-MEX experiment, which was a component of the MILAGRO campaign. The results are used to identify periods when biomass burning was likely to have had a significant impact on the concentrations of elemental carbon at two sites, T1 and T2, downwind of the city, and when emissions from the Mexico City Metropolitan Area (MCMA) were likely to have been more important. They are also used to estimate the median ages of EC affecting the specific absorption of light, α ABS , at 870 nm as well as to identify periods when the urban plume from the MCMA was likely to have been advected over T1 and T2. Median EC ages at T1 and T2 are substantially larger during the day than at night. Values of α ABS at T1, the nearer of the two sites to Mexico City, were smaller at night and increased rapidly after mid-morning, peaking in the mid-afternoon. The behavior is attributed to the coating of aerosols with substances such as sulfate or organic carbon during daylight hours, but such coating appears to be limited or absent at night. Evidence for this is provided by scanning electron microscopy images of aerosols collected at the sampling sites. During daylight hours the values of α ABS did not increase with aerosol age for median ages in the range of 1-4 h. There is some evidence for absorption increasing as aerosols were advected from T1 to T2 but the statistical significance of that result is not strong.

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

“…Single scattering albedos in the MCMA and downwind are substantially lower than in other areas (such as the eastern United States) (Doran et al, 2007(Doran et al, , 2008Kleinman et al, 2008;Marley et al, 2009a;ParedesMiranda et al, 2009;Thornhill et al, 2008). These results are consistent with the significant amounts of black carbon and light-absorbing PAH coatings observed during these studies as well as during MCMA-2003Stone et al, 2008;Querol et al, 2008).…”

Evaluating simulated primary anthropogenic and biomass burning organic aerosols during MILAGRO: implications for assessing treatments of secondary organic aerosols

“…While it is likely that T2 is impacted by Mexico City emissions when the regional winds are southwesterly (e.g. higher EC concentrations between March 18 and 22), transport from Mexico City to T2 does not occur every day (Doran et al, 2008). Instead, EC observed at T2 is from dilute plumes originating from many urban and biomass burning sources.…”

“…Initial ocean temperatures, soil temperatures, and soil moisture were also obtained from the GFS model. In addition to constraining the boundary conditions to the large-scale analyzed meteorology, four-dimensional data assimilation was used to nudge (Liu et al, 2006;Doran et al, 2008) the predicted wind, temperature, and specific humidity to the observations obtained from the radar wind profilers and the radiosondes at the T0, T1, T2 sites ( Fig. 1c) and the operational radiosondes in Mexico.…”

Estimation of the mass absorption cross section of the organic carbon component of aerosols in the Mexico City Metropolitan Area

“…For example, using a photoacoustic absorption spectrometer and an OC/EC instrument Doran et al (2007) determined α BC,λ at the T1 MILAGRO site to be 8.9 m 2 /g at 550 nm. At this site, the BC is expected to be at least partially coated, depending on the time of day (Doran et al, 2008). estimated α BC,λ to be 8.2 m 2 /g at 550 nm over MC.…”