Several years ago NASA s Global Modeling and

Several years ago, NASA\’s Global Modeling and Assimilation Office (GMAO) introduced the Modern-Era Retrospective Analysis for Research and Application (MERRA, Rienecker et al., 2011), a reanalysis tool incorporating satellite and model data to reproduce spatially consistent observations of many environmental variables. While the original MERRA included only meteorological parameters (wind, temperature, humidity, etc.), it has recently been extended to include assimilation of biased-corrected aerosol optical depth (AOD) from the Moderate Resolution Imaging Spectroradiometers sensors (MODIS, Remer et al., 2005) on board the Aqua and Terra satellites, which led to its rebranding as MERRAero. Although only total AOD is constrained by MODIS observations, the data assimilation algorithm in MERRAero provides speciated hourly data, with the relative contributions from five of the major aerosol species listed previously. Version 1 of MERRAero doesn\’t assimilate NO3 particles. Nevertheless, MERRAero provides an innovative tool to the scientific Sennoside A to study aerosol pollution issues around the world, especially in regions where reliable surface-based monitoring is scarce or unavailable. Examples of MERRAero\’s applicability can be found in Kessner et al. (2013), Colarco et al. (2014), Kishcha et al. (2014, 2015) and Yi et al. (2015).
In this study, AOD data from MERRAero is used to assess the state of air quality over a large selection of major metropolitan areas around the world (hereafter simply referred to as “cities”) over the last thirteen years (2003–2015). Speciation data is used to determine which aerosol species contribute most to AOD over each city and a trend analysis is performed to evaluate how local and regional factors, as well as natural and anthropogenic factors, affect aerosol pollution in urban environments. Alpert et al. (2012) previously and similarly analyzed AOD trends over a selection of major cities around the world based on MODIS data. The advantage of using MERRAero as opposed to just MODIS data is its ability to distinguish between aerosol species which provides substantially more information for analysis.

Methodology and data

Discussion and conclusion
The mean AOD was high (>0.3) in most cities of China, India, the Middle East, Northern and tropical Africa. In contrast, it was relatively low (< 0.2) in most cities of North America, South America, Europe, Australia and South Africa. The high AOD values observed in Northern African and Western Asian cities are caused mostly by their proximity to large and sandy deserts. Advection of DS also affects cities in India and Bangladesh but the high AOD averages in cities of these two countries is mostly the result of anthropogenic activities. Fossil fuel burning is responsible, for the most part, for the high AOD values observed in Chinese cities. However, advection of dust affects to some extent the AOD in cities of Northern China as well. High AOD averages in cities of tropical Africa are caused by deforestation and biomass burning activities.
Cities in North America, Europe, Japan, Southeastern Asia and Oceania tend to have a relatively low AOD on average, while SO4 and POM aerosols contribute to it most. Even though fossil fuel consumption is a major source of pollution in those parts of the world, effective air quality regulations have been successful at keeping emissions and, as a consequence, AOD values low over the last decade. Cities in South America and on the west coast of the U.S. are affected by fossil fuel burning but carbon emissions from wildfires contribute a significant proportion to their mean AOD during the summer. European cities are also affected by DS transport from the Sahara.
Overall, SO4 aerosols represented at least 10% of the mean AOD in all but two of the 200 cities presented in the various maps of this manuscript, those of Dakar in Senegal and Kano in Nigeria, for the only reason that their AOD is overwhelmed by DS particles due to their location close to the Sahara. POM aerosols represented at least 10% of the average in all but 24 cities, mostly located in Northern Africa or Western Asia. The presence of SS aerosols is significant in coastal cities but usually contributes little to the mean AOD.