Applications of lagrangian dispersion modeling to the analysis of changes in the specific absorption of elemental carbon
We use a Lagrangian dispersion model driven by a mesoscale model with\nfour-dimensional data assimilation to simulate the dispersion of\nelemental carbon (EC) over a region encompassing Mexico City and\nits surroundings. The region was the study domain for the 2006 MAX-MEX\nexperiment, which was a component of the MILAGRO campaign. The results\nare used to identify periods when biomass burning was likely to have\nhad a significant impact on the concentrations of elemental carbon\nat two sites, T1 and T2, downwind of the city, and when emissions\nfrom the Mexico City Metropolitan Area (MCMA) were likely to have\nbeen more important. They are also used to estimate the median ages\nof EC affecting the specific absorption of light, alpha(ABS), at\n870 nm as well as to identify periods when the urban plume from the\nMCMA was likely to have been advected over T1 and T2. Median EC ages\nat T1 and T2 are substantially larger during the day than at night.\nValues of alpha(ABS) at T1, the nearer of the two sites to Mexico\nCity, were smaller at night and increased rapidly after mid-morning,\npeaking in the mid-afternoon. The behavior is attributed to the coating\nof aerosols with substances such as sulfate or organic carbon during\ndaylight hours, but such coating appears to be limited or absent\nat night. Evidence for this is provided by scanning electron microscopy\nimages of aerosols collected at the sampling sites. During daylight\nhours the values of alpha(ABS) did not increase with aerosol age\nfor median ages in the range of 1-4 h. There is some evidence for\nabsorption increasing as aerosols were advected from T1 to T2 but\nthe statistical significance of that result is not strong.