AOD distributions and trends of major aerosol species over a selection of the world's most populated cities based on the 1st version of NASA's MERRA Aerosol Reanalysis
Introduction
Microscopic airborne aerosols have long been a prominent topic of study in the field of environmental science, predominantly in the atmospheric sciences. A considerable amount of literature has emerged in order to better understand the nature of these particles, but more specifically, to assess their impacts on various spheres of life and the environment. Aerosols are found in highly variable space and time distribution, size and chemical composition, and they originate from many sources, both natural and anthropogenic (Pöschl, 2005).
Aerosols considerably affect the environment and its living organisms. It is well documented that aerosols are a serious health hazard to humans, fauna and flora. They are linked to cardiovascular, respiratory and allergic diseases, as well as enhanced mortality (Pöschl, 2005, Tager, 2013). Aerosols also affect weather and climate. Acting as cloud condensation nuclei, aerosols are an essential element of cloud formation. As such, they play an indirect role in increasing the clouds' and the Earth's albedo as a whole (Haywood and Boucher, 2000, Lohmann and Feichter, 2005). They also affect the Earth's radiation budget as absorbers of radiation, contributing to a warming of the atmosphere, and as reflectors of radiation, in which case they act as a cooling agent (Haywood and Boucher, 2000). Finally, in high enough concentrations, they can significantly reduce visibility (Charlson, 1969, Cheng and Tsai, 2000). This is often associated with episodes of haze, smog and dust storms.
The seriousness of the impacts listed in the previous paragraph is dependent on the aerosols' concentration and size, but particularly on their chemical composition. It is therefore relevant to distinguish between different aerosol species commonly found in the air:
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Sulfate (SO4) aerosols originate from sulfur dioxide (SO2) that has been neutralized by ammonium (NH4) to form ammonium sulfate ((SO4)2NH4, Forster et al., 2007, sect. 2.4.4.1). SO2 emissions emerge from fossil fuel and, to a much smaller extent, biomass burning, and therefore are vastly considered as anthropogenic. However, small natural contributions originate from volcanoes and the oceans (Haywood and Boucher, 2000);
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Nitrate (NO3) aerosols originate from nitrogen oxides (NOx) that have been neutralized by NH4 to form ammonium nitrate (NO3NH4, Forster et al., 2007, sect. 2.4.4.5). NO3 emissions emanate from a variety of sources such as fossil fuel and biomass burning, bacteria and lightning. Delmas et al. (1997) estimated that 83% of NO3 emissions are anthropogenic in nature;
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Particulate organic matter (POM), composed largely of organic carbon (OC), is the result of fossil fuel and biomass burning. The former is an anthropogenic source while the latter is either a natural or an anthropogenic source. As a whole, sources of POM are widely considered to be anthropogenic (Haywood and Boucher, 2000, Forster et al., 2007, sect. 2.4.4.3);
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Black carbon (BC) particles are the result of incomplete combustion and originate from the same sources as POM (Haywood and Boucher, 2000, Forster et al., 2007, sect. 2.4.4.2);
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Mineral dust (DS) is the product of wind erosion predominantly in arid environments. Sources are therefore considered natural. However, deforestation, agricultural and industrial practices are responsible for a portion of anthropogenic dust aerosols in the atmosphere (Haywood and Boucher, 2000, Forster et al., 2007, sect. 2.4.4.6; Denman et al., 2007, sect. 7.5.1.1);
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Sea salt (SS) aerosols originate from the oceans. The release of salt particles in the air depends on meteorological factors such as surface wind speed and sea surface temperature (Denman et al., 2007, sect. 7.5.1.2).
There is scientific interest in studying aerosol pollution in cities because associated industries and road traffic are major sources of particulate matter and gaseous pollutants capable of forming aerosols through various chemical reactions and physical processes. Cities offer a wide variety of opportunities such as employment, education, health care, entertainment and other services which stimulate an ongoing and accelerating urbanization movement around the world (Moore et al., 2003). The United Nations (2014) estimated that 30% of the world's population lived in an urban area in 1950. This proportion grew to 47% in 2000 and is expected to reach 66% in 2050 (United Nations, 2014). Particularly in developing countries, where the rate of urbanization is the greatest (Subbotina, 2004, chap. 10), cities are lacking the means to adjust fast enough to fulfill the demand of their rapidly growing population and economic development. In this respect, urbanization comes with its fair share of environmental consequences (Sharma and Joshi, 2016). The phenomenon known has global dimming, which consists of a significant decrease in solar radiation flux around the world since the 1950s, is actually spatially inconsistent and much more pronounced over densely populated urban areas (Alpert et al., 2005, Alpert and Kishcha, 2008). Indeed, atmospheric aerosol concentrations are significantly higher in populated cities as opposed to rural or remote areas (Cheng and Tsai, 2000), and the cities' population growth in developing countries tends to correlate with an increase of aerosol concentration (Kishcha et al., 2011). Rapid urbanization and development in India and China resulted in a sharp increase of air pollutant emissions during the last decade (Lu et al., 2011) and frequently recurring episodes of air pollution and haziness. On the other hand, urbanization in developed countries, albeit occurring at a slower rate, hasn't had such a negative impact. Developed countries did indeed struggle with severe air pollution issues in the past, but their economic and democratic situation provides them with the means to enforce clean air regulations and develop green technologies. As a result, air quality has significantly improved over the last decades in the United States (Hand et al., 2012), Europe (Vestreng et al., 2007, Tørseth et al., 2012) and Japan (Wakamatsu et al., 2013), even though their population and economy kept on growing.
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 community 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, Kishcha et al., 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.
Section snippets
MERRA Aerosol Reanalysis
NASA's Version 1 of MERRAero incorporates the latest version of the Goddard Earth Observing System (GEOS-5). It contains components for atmospheric circulation and composition (including atmospheric data assimilation), ocean circulation and biogeochemistry, and land surface processes. GEOS-5 also includes an atmospheric particulate matter (PM) module (Colarco et al., 2010, and references therein). This module is based on a version of the Goddard Chemistry, Aerosol, Radiation and Transport
North and Central America
The proportions of aerosol species to total AOD for a selection of major cities in North and Central America are shown in Fig. 1. The reader is referred to the supplementary material for AOD values in all cities. The highest urban AOD values are observed in Central and Eastern United States and Canada, ranging from 0.133 in Miami to 0.190 in Houston. Denver is an exception with the lowest mean AOD in the whole region (0.095). The Northeastern United States is highly populated and
Total AOD
The linear trend between 2003 and 2015 for total AOD and for all the cities is mapped in Fig. 12. The color grading is indicative of the level of change with lighter colors indicating an insignificant change. Globally, the AOD is decreasing in a wide majority of cities. The trends are negative in every single city of the Americas, except in Sacramento and Santiago where they are positive but insignificant. The decreases are significant in cities of Eastern Canada and U.S., and the southeastern
Discussion and conclusion
The MERRA Aerosol Reanalysis was used to study urban air pollution issues around the world by using its assimilation of AOD observations and modeled concentrations of particulate matter over a 13-year period (2003–2015). MERRAero's differentiation of particle speciation makes it a unique and innovative tool capable of estimating the AOD of individual aerosol species with a global and constant coverage, unlike remote sensing instruments. This is particularly useful for studying urban air
Acknowledgments
This study was made with support from and in cooperation with the international Virtual Institute DESERVE (Dead Sea Research Venue) funded by the German Helmholtz Association (grant number VH-VI-527).
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