GAW Report No. 205 - IGAC Project

CHAPTER 7 – OVERVIEW OF INTERNATIONAL COLLABORATIVE RESEARCH ACTIVITIESsecondary aerosols at the regional scale and black carbon and SOA are contributing to singlescattering albedos in the MCMA and downwind that are substantially smaller than in other areas(such as the eastern United States). Studies from MILAGRO have reported significant enhancedUV-Visible absorption from biomass burning, SOA, and aged carbonaceous aerosol components.At the T0 and T1 surface sites single scattering albedos (SSA) were frequently in the 0.7-0.8 rangewith some early morning values having even lower SSA. This is consistent with high absorbingaerosol concentrations from both fossil and biomass burning sources during MILAGRO. Aerosolcontributions from biomass burning sources contained both black carbon and oxidized organics thatyielded enhanced UV absorption. This observation indicated biomass burning activities can haveimportant impacts on the absorption or heating by carbonaceous aerosols in megacity (urban) aswell as regional scales. Oxidized organics from primary fires and from secondary aerosol formationwere also found to have strong absorption in the 300-400 nm region that leads to enhanced opticalabsorption by these aerosols over that anticipated from black carbon alone.The results from LIDAR and aircraft operation as well as aerosol mass spectrometers allindicate that there is significant transport of aerosols and that most of this aerosol is in the lowerlayer of the atmosphere, but can be exported aloft into the free troposphere during venting events.Satellite retrievals of aerosols are being improved by comparisons with measurements of radiationand aerosol properties at the surface and from aircraft. Measurements of surface albedo andreflectance in the MCMA showed that many urban surfaces are more reflective than assumed incommon satellite retrieval algorithms, and that use of larger visible surface reflectance in algorithmscan produce more accurate retrieved aerosol optical depth (AOD).As described above, a very large number of instruments were used in the MCMA duringMILAGRO for both ground-based and aircraft measurements; and some innovative instruments andmeasurement techniques were deployed for the first time. The MILAGRO campaign has shown thesynergy of using multiple measuring platforms, instrumentation, and data analysis techniques forobtaining an improved understanding of the physical and chemical characteristics of emissions in amegacity. Furthermore, the deployment of a significant number of advanced instruments, manyoperating with sensitive, fast (~1 s) response times, along with a large number of established airquality monitoring instruments deployed on aircraft and at surface sites, as well as onboard severalmobile laboratories, have provided significant opportunities to intercompare and evaluate a numberof instruments in a highly polluted environment.Despite the use of many advanced PM techniques during MILAGRO, some questionsremain unanswered or strongly debated and should be the focus of further research. For example,the fraction of dust due to road resuspension vs. natural sources is unclear. The impact ofgas-particle reactions is important, for example for nitrate uptake into the coarse dust mode, butneeds to be further investigated to reach a quantitative understanding, including through 3Dmodelling. The identities of industrial sources of metals and organic aerosols and of the urbanchloride sources remain unclear. High time-resolution quantitative analyses of dust and metals mayyield very useful information for source identification.The 2006 MCMA emissions inventory underestimates primary PM 2.5 and needs to beupdated with the information arising from MILAGRO and other studies. Forest fire PM 2.5 appears tobe underestimated by an order of magnitude in the official MCMA inventory, but perhapsoverestimated about two-fold on a custom satellite-based inventory used in 3D modelling. Theimpact of some primary organic aerosol sources such as food cooking, biofuel use, and open trashburning may be important, but remains poorly characterized. Some differences in the apportionmentof biomass burning PM between different approaches were observed and need further research, asthese techniques together represent the state of the art for source apportionment. The differencesin the relative oxidation of organic aerosols in urban vs. background samples between differenttechniques need to be further investigated.The influence of “hot” sources of radiocarbon in aerosols needs to be further investigated asit could bias assessments of fossil vs. modern carbon. SOA from traditional precursors such asaromatics is much smaller than the observed SOA in the Mexico City urban area, but the dominant261