Megacities, air quality and climate

Atmospheric Environment 126 (2016) 235e249

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Atmospheric Environment journal homepage: www.elsevier.com/locate/atmosenv

Review article

Megacities, air quality and climate Alexander Baklanov a, *, Luisa T. Molina b, Michael Gauss c a

World Meteorological Organization (WMO), Research Department, Geneva, Switzerland Molina Center for Energy and the Environment, CA, USA c Norwegian Meteorological Institute, Bergen, Norway b

h i g h l i g h t s
Climate, air quality and megacities interactions: gaps in knowledge, research needs.
Urban hazards: pollution episodes, storm surge, flooding, heat waves, public health.
Global climate change affects megacities' climate, environment and comfort.
Growing urbanization requires integrated weather, environment and climate monitoring systems.
New generation of multi-scale models and integrated urban services are needed.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 4 June 2015 Received in revised form 23 November 2015 Accepted 26 November 2015 Available online 2 December 2015

The rapid urbanization and growing number of megacities and urban complexes requires new types of research and services that make best use of science and available technology. With an increasing number of humans now living in urban sprawls, there are urgent needs of examining what the rising number of megacities means for air pollution, local climate and the effects these changes have on global climate. Such integrated studies and services should assist cities in facing hazards such as storm surge, flooding, heat waves, and air pollution episodes, especially in changing climates. While important advances have been made, new interdisciplinary research studies are needed to increase our understanding of the interactions between emissions, air quality, and regional and global climates. Studies need to address both basic and applied research and bridge the spatial and temporal scales connecting local emissions and air pollution and local weather, global atmospheric chemistry and climate. This paper reviews the current status of studies of the complex interactions between climate, air quality and megacities, and identifies the main gaps in our current knowledge as well as further research needs in this important field of research. © 2015 Elsevier Ltd. All rights reserved.

Keywords: Megacities Climate Air quality Seamless modeling Monitoring Mitigation

1. Introduction For the past few hundred years, human populations have been clustering in increasingly large settlements. In 2007, the world's urban population exceeded the rural population for the first time. At present, there are 23 cities worldwide with a population of 10 million or greater (UN, 2012). Most of these are situated in poor and developing countries and are characterized by elevated air pollution levels. Megacities are generally defined as urban agglomerations with a population exceeding 10 million and have quickly become a

* Corresponding author. E-mail address: [email protected] (A. Baklanov). http://dx.doi.org/10.1016/j.atmosenv.2015.11.059 1352-2310/© 2015 Elsevier Ltd. All rights reserved.

worldwide phenomenon (Molina and Molina, 2002). However, there is no exact definition of a megacity's boundaries, where it starts and where it ends. In particular, major urban centers usually include people who do not reside within the cities' political boundaries. As a result, the term ‘megacity’ is often used loosely, referring to large agglomerations of people (5 million of more) who share employment, housing, transportation, and security needs (Molina et al., 2004). Only four such conurbations existed in the 1950s, but about 60 have emerged by now. Megacities cover less than 0.2 per cent of the Earth's surface, but account for approximately 10 per cent of the world's population (Demographia, 2014) and have strong and extended effects on environmental conditions. Effects of megacities and urban agglomerations have different aspects, including from one side the ‘urban metabolism’ (Kolb, 2006):

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Cities consume materials and energy (food, fuel, electrical power, water, industrial materials, atmospheric oxygen, etc.);
Cities export/excrete materials (industrial goods, sewage, garbage, industrial wastes, gaseous pollutants, and airborne particulates/aerosols); and from the other side the ‘urban respiration’:
Urban respiration (oxygen in/primary and secondary gaseous pollutants and airborne particulates/aerosols/out) represents the direct impacts of urban metabolism on the atmosphere;
Emitted urban air pollutants have significant impacts on both regional viability (human health, agricultural/ecosystem productivity, visibility), and global change issues (climate, ozone depletion, oxidative capacity). These densely populated regions emit significant amounts of pollution into the atmosphere, and the local effects are especially evident within the boundaries of well-known megacities, such as Beijing, Delhi, Mexico City and Los Angeles. However, this pollution is not confined within the boundaries of the megacities themselves, but can be transported over large distances (up to thousands of kilometers) and contribute to the overall hemispheric background pollution. The impacts of megacities are quite variable and can also be noted in directions opposing the average prevailing winds. The average transport distance for black carbon and other primary fine particulate matter (PM) components is up to 200 km for most megacities considered in Butler and Lawrence (2009). Maximum transport distances are significantly higher, with 25 per cent transported more than 2000 km away. The production of ozone (O3) and aerosol particles was found to continue for hundreds of kilometers downwind of Mexico City (Molina et al., 2010). Airborne emissions from major urban and industrial areas from the developed world, and progressively from megacities in the developing world, influence both air quality and climate change on scales ranging from local to regional and up to global (Molina and Molina, 2004). Megacities have strong urban heat islands (UHI) due to differences in surface properties and waste heat from anthropogenic activity, so megacities can be warmer than their rural surroundings by up to several degrees Celsius (Oke, 1982; Flanner, 2009; Grimmond et al., 2010a; Allen et al., 2011). This heating impacts the local environment directly, but also affects the regional air circulation. The contribution of megacities to global warming through greenhouse gas emissions is unclear, but probably substantial and mostly due to CO2. The global effect of non-CO2 emissions is nearly neutral, with a very small net cooling calculated from climatechemistry models (caused by ozone, methane and aerosols) (Folberth et al., 2012). However, the effect of megacities on global climate is increasing in the future and already can be significant on regional scales. There are two main mechanisms by which growing megacities could further affect local, regional and global climates. Firstly, changing urban features such as increasingly large urban heating will continue to increase local temperatures, influence local air circulation and alter the formation of precipitation and the frequency and intensity of thunderstorms. Secondly, changing emissions and feedbacks with atmospheric pollutants will alter the subsequent effects on climate, both locally and further afield. Global climate change can substantially affect megacities' climate, environment and comfort for citizens and require measures for urban adaptations to the global change. Climate projections, as well as forecasting systems associated with weather

and climate extremes, require further development to provide the evidence base to ensure that policy makers, businesses and citizens can continue to draw on a sound understanding of the state of the climate and the wider environment, the possible response options and their consequences in social, economic and environmental terms. Studies of the complex interactions between climate, air quality and megacities represent a relatively new and important field of research. With increasing numbers of humans now living in urban sprawls, there are needs of examining what the rising number of megacities means for air pollution, local climate and the effects these are having on global climate. Many international studies have been initiated during the last decade. In particular several major projects have been realized recently, including the MILAGRO project (Molina et al., 2010; http://www.mce2.org/) in Mexico City, the European research projects MEGAPOLI (Baklanov et al., 2010; http://www.megapoli.info), CityZen (http://www.cityzen-project. eu/), and ClearfLo (http://www.clearflo.ac.uk), as well as urban studies and measurement networks in Asia, such as SAFAR in Delhi (SAFAR, 2015), WISE in Seoul (http://wise2020.org/eng/) and the Urban Integrated Meteorological Observation Network of Shanghai (Tan et al., 2015). A more comprehensive worldwide overview of projects dealing with the impacts of megacities on air pollution and climate is given in the WMO/IGAC report on megacities (WMO/ IGAC, 2012). These projects assessed, in particular, the impacts of megacities and large air pollution hot spots on local, regional and global air quality; they quantified feedback mechanisms linking megacity air quality, the local and regional climates, and global climate change; and developed improved tools for predicting high impact weather events and air pollution levels in megacities. While important advances have been made, new interdisciplinary research studies are needed to increase our current understanding of the interactions between emissions, air quality and regional and global climates taking place in megacities. These should include studies to address both basic and applied research, to bridge the spatial and temporal scales connecting local emissions, air quality and weather with climate and global atmospheric chemistry, and finally be oriented towards realization of integrated urban services. To help enhance the capabilities of national meteorological services to handle meteorological and related aspects of urban environmental issues, WMO established the Global Atmosphere Watch (GAW) Urban Research Meteorology and Environment (GURME) project (http://mce2.org/wmogurme/). 2. Fast growing urbanization and its effects Urbanization was one of the most striking phenomena of the 20th century. Levels of urbanization correlate with national income, and within a country, wealth is concentrated in urban areas. This higher income is a major cause of urban growth, as people from the countryside tend to move to cities to find jobs, education, and services that an urbanized center provides. Conflicts, land degradation, and the depletion of natural resources also motivate migration, and international migration is another factor. But the largest contributor to urbanization is the general population growth, especially in the developing world (Molina and Molina, 2004; Montgomery, 2008). More than half of the world's population now lives in urban areas (UN, 2012). The number and size of megacities increased dramatically during the second half of the 20th century as population growth became increasingly urban centered. In 1800, London was the only city in the world with a population exceeding 1 million. Cities with a population of at least 1 million increased to three by the beginning of the 20th century, while today there are more than 450 (Demographia, 2014). In 1970, the world had only

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two megacities of at least 10 million inhabitants: Tokyo and New York. Since then, their number has increased markedly and most new megacities have arisen in developing countries. Today, there are 23 megacities (see Table 1), with 13 being located in Asia, four in Latin America, and two each in Africa, Europe and North America. By 2025, when the number of megacities is projected to reach 37, Asia would have gained another nine, Latin America two, and Africa, Europe and Northern America one each. This indicates a clear trend of accelerated urban concentration in Asia (UN, 2012). Such dramatic demographic shifts have wide ranging implications, and few are more fiercely felt by residents of megacities than air quality. Most of the world's megacities are located in poor, developing countries, and do not have adequate financial resources and air quality management plans that cities like London and Los Angeles have implemented to curb air pollution. Until recently, changes in air quality resulting from increasingly dense urban centers had not been quantified in detail and their effects on regional climate and global warming remain poorly understood. Fast growing urbanization has different positive and negative €bel, 2004), e.g.: faces and consequences (Go
Driving forcers in economic growth (about 80%)
Growing emissions, environmental problems and climate change
Rapid and unbalanced growth

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Problems of fast growth: cities are increasingly subject to dramatic crises
In many cities new urban population is equal to poor urban population
Problems aggravated in developing countries by climate change, economic and financial crises. Regional aspects of the urbanization are very important. Comparison of three major global emissions inventories, alongside two city level inventories, examined in Denier van der Gon et al. (2011), showed that there is huge variation in the sources and degree of emissions between megacities, in particular, by geographical region. For example, much of the megacity emissions in Europe and the Americas are associated with road use, whereas Asia and Africa's output largely stems from residential natural/biofuel consumption. The requirements of each city need to be informed by holistic impact and hazard identification in order to map the city's specific vulnerabilities and to identify the services that would be most beneficial. Coastal cities have different concerns to land-locked cities (Pelling and Blackburn, 2014); similarly, requirements of an urban area in the Tropics differ to one often impacted by severe winter weather. Data sharing arrangements between city institutions is a fundamental building block for authorities to identify the priority services and also to design and establish urban

Table 1 Population of urban agglomeration with 10 million inhabitants or more in 2011 and 2025 (millions). Rank

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

2011

Rank

Urban agglomeration

Population

Tokyo, Japan Delhi, India Mexico City, Mexico New YorkeNewark, USA Shanghai, China ~o Paulo, Brazil Sa Mumbai (Bombay), India Beijing, China Dhaka, Bangladesh Kolkata (Calcutta), India Karachi, Pakistan Buenos Aires, Argentina Los Angeles-Long Beach-Santa Ana, USA Rio de Janeiro, Brazil Manila, Philippines Moscow, Russian Federation Osaka-Kobe, Japan Istanbul, Turkey Lagos, Nigeria Al-Qahirah (Cairo), Egypt Guangzhou, Guangdong, China Shenzhen, China Paris, France

37.2 22.7 20.4 20.4 20.2 19.9 19.7 15.6 15.4 14.4 13.9 13.5 13.4 12.0 11.9 11.6 11.5 11.3 11.2 11.2 10.8 10.6 10.6

Source: UN (2012).

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37

2025 Urban agglomeration

Population

Tokyo, Japan Delhi, India Shanghai, China Mumbai (Bombay), India Mexico City, Mexico New YorkeNewark, USA ~o Paulo, Brazil Sa Dhaka, Bangladesh Beijing, China Karachi, Pakistan Lagos, Nigeria Kolkata (Calcutta), India Manila, Philippines Los Angeles-Long Beach-Santa Ana, USA Shenzhen, China Buenos Aires, Argentina Guangzhou, Guangdong, China Istanbul, Turkey Al-Qahirah (Cairo), Egypt Kinshasa, Democratic Rep. of the Congo Chongqing, China Rio de Janeiro, Brazil Bangalore, India Jakarta, Indonesia Chennai (Madras), India Wuhan, China Moskva (Moscow), Russian Federation Paris, France Osaka-Kobe, Japan Tianjin, China Hyderabad, India Lima, Peru Chicago, USA , Colombia Bogota Krung Thep (Bangkok), Thailand Lahore, Pakistan London, United Kingdom

38.7 32.9 28.4 26.6 24.6 23.6 23.2 22.9 22.6 20.2 18.9 18.7 16.3 15.7 15.5 15.5 15.5 14.9 14.7 14.5 13.6 13.6 13.2 12.8 12.8 12.7 12.6 12.2 12.0 11.9 11.6 11.5 11.4 11.4 11.2 11.2 10.3

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observational networks that capture the phenomena of interest at the spatial and temporal resolution required. Connections between Megacities, Air Quality and Climate include the links illustrated in Fig. 1 and have the following specifics (Baklanov et al., 2010):
Nonlinear interactions and feedbacks between urban land cover, emissions, chemistry, meteorology and climate;
Multiple spatial (from urban to global) and temporal (from minutes to centuries) scales;
Complex mixture of pollutants from large sources;
Interacting effects of urban features and emissions. Ultimately, as an ever-increasing number of megacities loom on the horizon, a new generation of multi-scale integrated models and integrated urban services are needed for helping to ensure that we can adapt to the responsibilities associated with fast growing megacities in changing climate and global change (Baklanov, 2012). The numerical models most suitable for an integrated urban weather, air quality and climate assessment and forecasting system are the new-generation limited area models with coupled dynamics and chemistry modules (so-called Coupled Chemistry-Meteorology Models (CCMM)). CCMMs have been developed mostly over the past 15 years following rapid advances in computing resources plus extensive basic scientific research (see e.g., the extensive overviews of CCMMs in America by Zhang, 2008, and in Europe by Baklanov et al., 2014). Current state-of-the-art CCMMs encompass interactive chemical and physical processes, such as aerosols-clouds-

radiation, coupled to a non-hydrostatic and fully compressible dynamic core including monotonic transport for scalars, allowing feedbacks between the chemical composition and physical properties of the atmosphere. However, simulations using fine resolutions, large domains and detailed chemistry over long time duration for the aerosol and gas/aqueous phase are still too computationally demanding due to the huge model complexity. Therefore, CCMMs weather and climate applications still must make compromises between the spatial resolution, the domain size, the simulation length and the degree of complexity for the chemical and aerosol mechanisms. A typical model run on the weather scale for an urban domain use a reduced number of chemical species and reactions because of its fine horizontal and vertical resolutions, while climate runs generally use coarse horizontal and vertical resolutions with reasonably detailed chemical mechanisms (Barth et al., 2007). There are initiatives to expand the related services of large forecast centers. For example the Copernicus Atmosphere Monitoring Service for the global and European scales (http://atmosphere. copernicus.eu/) is providing air quality relevant data which can be downscaled to megacities and urban agglomerations. 3. Megacities air quality and their effects on larger scale atmospheric pollution 3.1. Sources of urban air pollution Air pollution is not a new phenomenon. Gases such as sulfur dioxide (SO2), hydrogen sulfide (H2S), and carbon monoxide (CO)

Fig. 1. MEGAPOLI schematic showing the main linkages between megacities, air quality and climate (Baklanov et al., 2010). In addition to the overall connections between megacities, air quality and climate, the figure shows the main feedbacks, ecosystem, health and weather impact pathways, and mitigation routes which are investigated. The relevant temporal and spatial scales are also included.

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are continually released into the atmosphere as by-products of natural events such as volcanic activity, vegetation decay, and forest fires. In addition, fine particles are distributed throughout the atmosphere by winds, forest fires, volcanic eruptions, and other similar natural events. Added to these natural pollutants are substances resulting from human activities. Early humans created the first anthropogenic air pollution with their heating and cooking fires. During the past 100 years, air pollution of human origin has become a major, persistent problem in many urban areas around the world. Furthermore, the transport of pollutants across national boundaries and between continents are influencing air quality and climate change on a global scale (Molina and Molina, 2002, 2004 and the references therein). Much progress has been made in understanding urban air pollution since Haagen-Smit discovered the nature and causes of the Los Angeles smog in 1952 (Haagen-Smit, 1952). He and his coworkers determined that a major component of smog is ozone formed by the interaction of and nitrogen oxides (NOx, produced by combustion sources, cars, heaters, etc.) and volatile organic compounds (VOCs, from evaporation of gasoline and solvents used in products such as paints) in the presence of sunlight (ultraviolet radiation). Since then, high levels of ozone have been measured in many urban areas throughout the world. Photochemical smog is now recognized as a worldwide problem in areas where VOCs and NOx emissions from major mobile and stationary sources are trapped by thermal inversions and irradiated by sunlight during transport to downwind regions, leading to the formation of a host of secondary pollutants, including ground-level O3. On a larger scale, these same emissions drive the production of ozone (a powerful greenhouse gas) in the free troposphere, contributing significantly to global warming. Urban and industrial areas are also major sources of the potent greenhouse gases, including carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O) and halocarbons. NOx and SO2 emissions are also processed to strong acids by atmospheric photochemistry on regional to continental scales, driving acid deposition to sensitive ecosystems. The polluted atmospheres in large cities often contain high concentrations of particulate matter (PM), formed from a wide variety of anthropogenic and natural sources, and has a wide range of impacts, from damaging human health (Pope and Dockery, 2006) and visibility (Watson, 2002) to altering the earth's climate (Forster et al., 2007; IPCC, 2014a,b). However, despite its abundance and important role, the sources and transformation of fine PM and its climate impacts remain one of the least understood aspects in atmospheric science, especially the organic aerosols (OA) (see e.g., Hallquist et al., 2009). Probably the most uncertain is the formation and evolution of secondary organic aerosol (SOA), particulate matter formed by the chemical transformation of atmospheric organic compounds, which accounts for a large fraction of the OA burden (see Jimenez et al., 2009 and the references therein). In Mexico City, a much larger amount of SOA was observed from reactive anthropogenic VOCs than estimated from the traditional SOA model during MCMA-2003 (Volkamer et al., 2006) and during MILAGRO-2006 (Kleinman et al., 2008). Although much progress has been made recently in our understanding of the detailed processes and how they affect SOA formation, properties and possible environmental impacts, there are still significant gaps in our scientific knowledge that limit our ability to quantify and predict SOA in the ambient atmosphere. Understanding the role of organic aerosol, particularly SOA, in air quality and climate change, represents a major challenge in atmospheric science (see e.g., Docherty et al., 2008; Fast et al., 2009; Hodzic et al., 2009; Li et al., 2011; Im et al., 2012). Different primary pollutants emitted and secondary pollutants formed in the atmosphere have different lifetimes ranging from

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hours to days to centuries. The type of influence or impact an air pollutant can exert e whether local, regional or global e depends on how long the pollutant remains in the atmosphere and, hence, how far it can travel from its source. Substances emitted into the atmosphere can be removed by physical (wet deposition) or chemical processes (loss by chemical reaction and conversion to another species). The efficiency of their removal processes is affected by direct dispersion and transport, as well as meteorological factors such as temperature, intensity of sunlight, and the presence of clouds and fog. 3.2. Impacts of urban air pollution Emissions and ambient concentrations of pollutants in megacities can have widespread effects on the population health, urban and regional haze, visibility impairment and ecosystem degradation. The following sections describe the impacts of megacities air pollution on health and global pollutant transport. 3.2.1. Adverse health impacts According to the 2014 report from the World Health Organization (WHO, 2014a), globally, 4.3 million deaths were attributable to household air pollution (HAP) in 2012, almost all in low and middle income countries, while 3.7 million deaths were attributable to ambient air pollution (AAP) in 2012, about 88% occur in low- and middle-income countries, which represent 82% of the world population. Globally, 7 million deaths were attributable to the joint effects of HAP and AAP in 2012; the Western Pacific and South East Asian regions bear most of the burden with 2.8 and 2.3 million deaths, respectively. There are some limitations in estimating the joint effects due to limited knowledge on the distribution of the population exposed to both household and ambient air pollution, correlation of exposures at individual level as household air pollution is also a contributor to ambient air pollution, and nonlinear interactions. However, in several regions, household air pollution, such as burning biomass, kerosene or coal for cooking and heating, remains mainly a rural issue. Ambient air pollution is predominantly an urban problem, affecting populations living in or around cities, the most harmful pollutants are fine particles (PM2.5), typically emitted by sources such as diesel vehicles and coal-fired power plants (WMO/IGAC, 2012). The new finding from WHO (2014a) more than doubles its previous estimates and confirms that air pollution is now the world's largest single environmental health risk. In particular, the new data reveal a stronger link between both indoor and outdoor air pollution exposure and cardiovascular diseases, as well as between air pollution and cancer, in addition to air pollution's role in the development of respiratory diseases. The new estimates were based on more knowledge about the diseases caused by air pollution and better assessment of human exposure to air pollutants through the use of improved measurements and technology, which enabled scientists to make a more detailed analysis of health risks from a wider demographic spread that includes rural as well as urban areas. Recently, the extremely fast pace of industrialization and economic transformation in China has been accompanied by severe and well-publicized air pollution in Beijing and other major cities. It has been estimated that outdoor air pollution leads to between 350,000 and 500,000 premature deaths in China each year (LANCET, 2014). A new report from WHO (2014b) on ambient air pollution covers 1600 cities across 91 countries e 500 more cities than the previous database in 2011 e reveals that more cities worldwide are monitoring outdoor air quality, reflecting growing recognition of air pollution's health risks. However, the study found that in the cities that had provided data in 2011, air quality had

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mostly deteriorated in the intervening years. It is important to note that an accurate comparison between any two cities is virtually impossible because it would require data from consistently calibrated ground stations. According to the 2014 edition of the Environmental Performance Index (EPI, 2014) e a biennial ranking of countries produced by Yale and Columbia universities e overall, improvements have been made in many of the categories of the environmental health objective, including access to drinking water, child mortality, and access to sanitation; however, air quality has declined. Air quality category measures population-weighted exposure to fine particulate matter and percentage of the population burning solid fuel for cooking. With the expansion of industry and fossil fuels-based transportation sectors in the developing world, the number of people breathing unsafe air has risen by 606 million since 2000. It now totals 1.78 billion. Given high urbanization, industrialization, and population growth, populations in China and India have the highest average exposure to PM2.5 in the world. Developed countries are not immune from pollution, however. While the United States meets air quality standards at the national level, some sites reveal discrepancies. Recently, severe air pollution in Paris prompted officials to temporarily impose a partial driving ban and provide free public transportation. Nonetheless it has to be stressed that there are large differences in exposure levels between continents. For example, PM levels in Beijing have reportedly exceeded 1150 mg/m3 (Zheng et al., 2015), while significantly lower concentrations are considered as ‘severe’ in European and North American cities. Beekmann et al. (2015) have made a comparison of black carbon and elemental carbon concentrations observed in megacities across the world (see their Figure 10 and Table 1). According to these measurements, concentrations in megacities of Asia and South America vastly exceed those in Europe and North America. 3.2.2. Long-range transport of air pollutants Air pollution used to be considered a local concern rather than a long-term global change issue. However, the atmosphere is a shared resource that respects no boundaries. Once released into the atmosphere, air pollutants can be carried by winds, mix with other pollutants, undergo chemical transformations and are eventually deposited on various surfaces. Thus, their impacts can be felt far from their sources (Molina and Molina, 2002; Butler and Lawrence, 2009; Freney et al., 2014). The regional and global dispersion of pollutants generated locally has been well established in the case of acid deposition and stratospheric ozone depletion. Recently, longrange transport of tropospheric ozone has been seen to be increasing throughout the northern hemisphere (AMAP, 2015; HTAP, 2010; NAS, 2009). With the growth of multi-city “megalopolis” regions in North America, Europe, East Asia, and South Asia, the export of air pollutants from urbanized regions to sensitive environments has become a major concern because of wide-ranging consequences for human health and ecosystems, visibility degradation, weather modification, changes in radiative forcing, and tropospheric oxidation capacity (Gurjar et al., 2010). Observations from the ground, aircraft and satellites throughout the global atmosphere have shown that air pollution can be transported over inter-continental distances (see e.g., Quinn et al., 2014; Logan et al., 2012; Parrish et al., 2012; Reidmiller et al., 2009; NAS, 2009). Our ability to quantify the magnitude of transport has improved recently from an increasing volume of observational evidence, including intensive field campaigns and satellite-borne instruments, improved emissions inventories and global and regional chemical transport models (Freney et al., 2014). MILAGRO (Megacity Initiative: Local And Global Research

Observations), an international collaborative project to examine the behavior and the export of atmospheric pollutants generated in megacities using the Mexico City Metropolitan Area (MCMA), took place in March 2006 (Molina et al., 2010). The campaign, together with an earlier intensive field study in 2003 (Molina et al., 2007), has provided a wealth of information on the emissions, dispersion and transformation processes of the pollutants emitted to the MCMA atmosphere and their urban, regional and hemispheric impacts. The MCMA motor vehicles produce abundant amounts of primary PM, elemental carbon, particle-bound polycyclic aromatic hydrocarbons (PAHs), CO and a wide range of air toxics, including formaldehyde, acetaldehyde, benzene, toluene, and xylene. High aerosol concentrations were observed and were composed in large part of organics, as well as black carbon, crustal matter, sulfate and nitrate. Biomass burning (agricultural, forest, wood cooking and trash burning) also contributes to the urban and regional pollution in the Mexico Basin (Christian et al., 2010; Li et al., 2011; Lei et al., 2013). The pollution plume from Mexico City can be observed several hundreds of kilometers downwind; aircraft-based measurements show ongoing production of secondary organic aerosols and ozone for several days downwind (Molina et al., 2010; Singh et al., 2009). Analysis from the MEGAPOLI (Megacities: Emissions, urban, regional and Global Atmospheric POLlution and climate effects, and Integrated tools for assessment and mitigation) Project showed an unexpectedly strong regional control on PM levels (not only for sulfates, but also for nitrate and organic aerosols) in the agglomeration of several major European urban centers (Beekmann et al., 2015). Wood burning during winter time and cooking activities during summer and winter made strong contributors to primary organic aerosol, in addition to traffic-related emissions. During summer, secondary organic aerosol, mostly of biogenic origin, was the major organic aerosol fraction. Aircraft measurements within the plume (up to 200 km downwind of the agglomeration) showed significant additional secondary aerosol build-up due to anthropogenic emissions. The impacts of megacities on O3 levels are usually much smaller than on PM and primary pollutants like NOx (Stock et al., 2013). Indeed, megacities tend to cause a decrease of O3 mixing ratios directly in the cities studied in MEGAPOLI (due to the reaction of ozone with NO), and an increase downwind of the cities (by up to 10 ppb). Meanwhile, ozone pollution in some Asian cities, during summertime, remains large (e.g. Zhang et al., 2014; Xu et al., 2011; Chen et al., 2015). The rapid increase in motor vehicles in Beijing and other developing cities may lead to greater photochemical O3 production and higher future O3 concentrations (Parrish and Stockwell, 2015), especially during afternoon hours. Yim and Barrett (2012) have applied a multiscale air quality modeling system to investigate the impacts of combustion emissions on the UK air quality and using epidemiological evidence to quantify PM2.5 exposure to risk of early death. The authors found that UK combustion emissions cause about 13,000 premature deaths per year, and an additional 6000 deaths in the UK are caused by non-UK combustion emissions from EU. In greater London, nonUK EU emissions account for 30% of the ~3200 air quality related deaths per year. This study indicates that policy measures should be coordinated at an EU-level because of the strength of the transboundary component of PM pollution. The Arctic is often perceived as a pristine place, yet its atmosphere has served as a receptor for air pollution from the industrial regions of northern mid-latitudes continents, as evidenced by the Arctic Haze phenomenon and the accumulation of persistent pollutants such as mercury. The haze affects the highly reflective Arctic ice sheet in ways that can increase temperatures both in the atmosphere and on Earth's surface. A major field study ARCTAS

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(Arctic Research on the Composition of the Troposphere from Aircraft and Satellites) was launched in 2008 to study the role of air pollution in this climate-sensitive region (NASA, 2015). Results from MEGAPOLI (Cassiani et al., 2013) also show that megacities in Europe are significant contributors to deposition of aerosols in the Arctic (especially absorbing aerosols like soot), regardless of whether considering the annual or only the wintertime deposition. Among the cities the largest contributors are Saint-Petersburg, Moscow and the Rhine-Ruhr region (western Germany). This suggests that in order to most effectively reduce soot deposition in the Arctic, the focus needs to be placed also on northern Eurasian sources (Cassiani et al., 2013; Quinn et al., 2014). There are also geo-political and socio-economic dimensions associated with the shift in pollution sources from the developed countries to the developing ones. For example, during the last few decades, important changes have occurred in the nature of industrial production around the world with most of the low skill level manufacturing moving from the developed countries to developing economies where lower cost labor and less stringent emissions standards allow for reduced production costs. On the other hand, a new study by Lin et al. (2014) suggests that pollutant emissions from China's production of goods for export can be carried across the Pacific Ocean and contribute to air pollution in the Western United States. International trade affects global air pollution and transport by redistributing emissions related to production of goods and services and by potentially altering the total amount of global emissions. Lin et al. (2014) analyze the trade influences by combining an economic-emission analysis on China's bilateral trade and atmospheric chemical transport modeling. Atmospheric modeling shows that as the United States outsourced manufacturing to China, sulfate pollution in 2006 increased in the western United States but decreased in the eastern United States, reflecting the competing effect between enhanced transport of Chinese pollution and reduced US emissions in the American East and highlighting the importance of international efforts to reduce transboundary air pollution. 4. Urban climate of megacities The scope of this section encompasses the impact of urbanization at the urban and meso/regional scales. A wide range of urban features in megacities can influence atmospheric flow, its turbulence regime, and the microclimate, and can accordingly modify the transport, dispersion, and deposition of atmospheric pollutants both within and downstream of urban areas (e.g., acid rain). Key examples include the following:
The heterogeneity of building distribution, and more generally of all rough elements of the earth's surface, affects the turbulence regime of the flow.
The massive use of impervious materials and the scarcity of vegetation in urban areas affect the hydroemeteorological regime and pollutant deposition.
The release of anthropogenic heat fluxes by human activity affects the thermal regime.
The release of pollutants (including aerosols) affects the transfer of radiation, cloud formation, and precipitation.
The street geometry (‘street canyons’) affects the flow regime and the exchange of heat between different surfaces (e.g., roads and walls).

4.1. The urban features and processes forming urban climate The most striking characteristic of the urban microclimate is the

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Urban Heat Island (UHI). The UHI effect causes the temperature to be warmer in the city center than in the surrounding area. The difference in temperature can reach several degrees for large conurbations under certain weather conditions. Even though the effects of the UHI phenomenon are usually not catastrophic for megacities they can nevertheless intensify heat-related stress, especially at night during heat waves, and can lead to tragic consequences for public health. The UHI phenomenon has been extensively investigated during the last decades and is quite well reproduced in models (Oke, 1982; Grimmond et al., 2010a; Linli Cui and Jun Shi, 2012; Cui and de Foy, 2012; von Glasow et al., 2013). Anthropogenic heat fluxes for megacities can be very high: up to 50e500 W m2 (e.g. Flanner, 2009, Allen et al., 2011, globally; Iamarino et al., 2011, London; Quah and Roth, 2012, Singapore), locally reaching 1500 W m2 (e.g., Ichinose et al., 1999 in a small area of Tokyo). Under clear skies and light wind conditions, megacities can be more than 10  C warmer than their surrounding rural environments (the UHI intensity) (Oke, 1982). The representation of the urban land surface and urban sublayer has undergone extensive developments but no scheme is capable of dealing with all the surface exchanges (Grimmond et al., 2010b, 2011). To complicate this further, as the resolution of models becomes higher, combined with the large size of urban buildings in many cities, the limits of current understanding are being challenged. Key questions include (Grimmond et al., 2014a,b): should buildings be directly resolved? What can be simplified to make the computations tractable in realistic modeling time? At what scale can the current land surface schemes and model physics be applied? The fine-scale atmospheric flow field within the urban boundary layer (UBL), and its subsequent interaction and effect on transport and dispersion of air pollutants and aerosols are not well understood and are highly dependent on local terrain/orography. It is difficult to accurately assess the extent of the footprint (or “spatial influence region”) of a megacity that often occurs downwind and the intensity of the interactions of the UBL with the surrounding rural and coastal environment. These atmospheric flow changes can make important contributions to regional climate change through their impact on cloud microphysical processes through modified low-level atmospheric convergence, their impact on large aerosol radiative forcing of the atmosphere and surface (solar radiation dimming), and their impact on surface energy balances via modifications to coastal vertical circulations (e.g., landsea breeze). High-resolution numerical weather prediction simulations of the several coastal megacities, e.g., for New York City, Tokyo and Hong Kong (Thompson et al., 2007; Holt et al., 2009; Ng et al., 2011), illustrate the potential impact a megacity could have on regional weather through modifications to the fine-scale atmospheric flow within the UBL. Numerical simulations for the densely urbanized area of Manhattan indicate that the building height and areal plan fraction (or urban footprint) of the urban landscape are equally important in modifying the UBL wind speed. Over the last three decades, numerous field studies (e.g., ChuanYao Lin et al., 2011; Efe and Eyefia, 2014; Keuser, 2014; and the comprehensive review of previous studies in Shepherd, 2005) have shown that rainfall patterns in and downwind of urban areas have been modified. The most consistent finding has been an increase in the frequency of cloud cover and lightning, with associated enhanced precipitation, e.g. for Chicago (Changnon, 2001). During the 1970s, the Metropolitan Meteorological Experiment investigated a possible increase in convective activity in the surroundings of St. Louis, Missouri (Changnon et al., 1991), and confirmed that deep and moist convection was increased up to about 40 km downwind of the city. Since this study, other research has

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confirmed a similar increase in convective activity in other megacities, such as Atlanta (Georgia) (Bornstein and Lin, 2000), Mexico City (Jauregui and Romales, 1996), Taiwan (Chuan-Yao Lin et al., 2011). Recently, the use of lightning detection networks (Orville et al., 2001) to analyze lightning flash data and satellites to derive both cloud frequency and rainfall rates (e.g., Shepherd et al., 2002) have revealed significant increases in cloud-to-ground lightning, low level cloud frequency, and rainfall rates in and within 30e60 km downwind of large cities. There are three possible causes of urban induced convective phenomena: (1) changes in circulation that are induced by the UHI, (2) an increase in urban surface roughness, and (3) an increase in the concentration of condensation nuclei (CCN; e.g., particles to which water can be adsorbed to form cloud droplets) from urban air pollution. The suggested influence of the UHI is a downwind updraft cell. This is consistent with the analysis of surface meteorological data that show a convergence zone induced by the UHI that favors the development of convective thunderstorms (e.g., Bornstein and Lin, 2000; Changnon, 2001). The effect of the increased urban surface roughness is a result of convergence induced on the upwind side of urban areas. Over heavily polluted cities, a large quantity of aerosols (small liquid or solid particles) can also have physically the same effect as GHGs via a direct feedback mechanism that is caused by a change in radiation properties (Doran et al., 2007). Their presence absorbs or reflects the solar radiation, limiting the amount of sunlight received at the surface, sometimes by more than 10% (e.g., over Mexico City, Oke et al., 1999). However, aerosols also actively influence processes of cloud formation (indirect feedback mechanisms). Larger quantities of aerosol particles usually mean a larger quantity of CCN. The effect of CCN could be different for rain than for ice precipitation. Extreme weather events have become more frequent and are likely linked to increases in GHGs and aerosols, which alter the Earth's radiative balance and cloud processes. For example, Fan et al. (2015) showed that high anthropogenic air pollution contributed to catastrophic floods in Southwest China on 8e9 July 2013. However, the statement by Hidalgo et al. (2008) that further research is needed to better understand the mechanisms of interactions and to parameterize them in the models is still valid (Grimmond et al., 2015). 4.2. Urban climate and weather modeling Challenges for modeling sub-grid features in megacities include: spatial and temporal distribution of heat and chemical emission source activities; flow modification by the urban canopy structure; flow modification by the urban surface heat balance; enhancement/damping of turbulent fluxes in the urban boundary layer due to surface and emission heterogeneity; chemical modification of pollutants in the dispersion process. Because megacities represent rather localized, heterogeneous and variable sources of anthropogenic impact on air quality (and ultimately on climate), the major difficulty in megacity forcing in simulations arises from the sub-grid scale features. They are typically unresolved in climate models and barely resolved in regional scale meteorological models. Thus, models rely on parameterizations of megacity features aggregated within the model grid cell. Aggregation is not straightforward given surface heterogeneity and strong non-linearity of the turbulent transport in the urban boundary layer (UBL). The latter prohibits the application of direct averaging to obtain the large-scale forcing. The aggregation problems are still largely ignored in existing urban parameterizations. A more sophisticated approach which accounts for emission at different levels and for the surface thermal and drag heterogeneity

is needed. Recent progress in street- and urban-scale turbulence-resolving simulations has opened the way for the development of a new generation of effective urban parameterizations (Ching, 2013; Barlow, 2014). The models require databases of emissions and surface characteristics as initial and boundary conditions. Feature analysis helps assessment of the megacity climate. It also relaxes the stability constraints on the megacity forcing in large-scale models. Depending on the scientific objectives addressed, several types of urban canopy schemes (and associated atmospheric models) should be used. They can be separated into 3 categories. Two of them are described in Masson (2006) and Martilli (2014) as single or multi-layer canopy schemes. Those two are sufficiently simple (in their geometry) to be coupled into classical numerical atmospheric models. The third one corresponds to explicitly resolved buildings into the Computer Fluid Dynamics (CFD) type of models, including or not thermal effects (Baklanov and Nuterman, 2009; Santiago and Martilli, 2010; Schlunzen et al., 2011). This real geometry has a huge impact on the CFD code, in particular for the construction of the grid (usually triangular grid) and need, if thermal effects are simulated, a 3D radiative code (allowing exact computation of shadows for example). On the contrary, single and multilayer models can be coupled to atmospheric models that use in general horizontal or pseudohorizontal surfaces (e.g. surfaces parallel to the orography). Single-layer urban schemes allow interaction with the atmosphere through only one atmospheric level: the urban scheme is below the lowest atmospheric model layer, and the coupling is done by exchanging atmospheric variables (from the atmosphere) and radiative and turbulent fluxes (from the surface scheme towards the atmosphere) as described, e.g., in Best et al. (2006) and Baklanov et al. (2008). The drawback of single layer schemes is that some details are lost in the canopy description, both for air canopy characteristics or turbulent and radiative exchanges between, e.g. canopy air and wall layers. However, their major advantage is the relative simplicity of coupling them with atmospheric models, even in Numerical Weather Prediction (NWP) models. Multilayer models interact inside the atmospheric model, not just below. This means (i) that several atmospheric levels (a few of the lowest ones) intersect the urban scheme canopy (ii) that atmospheric variables of all of these intersecting levels influences the turbulent fluxes at the corresponding heights in the canopy, (iii) that as a result, there is a direct impact of the surface scheme on some atmospheric levels, needing additional terms in the prognostic equations of the atmospheric models (e.g., drag term in momentum equation, heating term in temperature equation, or production term in turbulent kinetic energy equation). Such models allow to simulate explicitly, and with a greater precision than single layer models, the profiles of air inside the canopy (thanks to the discretization of the atmospheric model) and the interactions between air and the surfaces (walls, roofs) at several heights. The major drawback is twofold: firstly, the coupling with the atmospheric model is complex and needs to interfere with the prognostic equations of the atmospheric model. Secondly, several atmospheric levels must intersect the canopy, leading to a very high vertical resolution of the atmospheric model near the surface. This is an advantage from a scientific point of view, because processes are better represented, but it leads to numerical constraints (especially on the integration time step of the atmospheric model), which are not compatible with NWP model. This limits the use of multi-layer canopy models to operational purposes (see an extensive overview of the current state and perspectives of urban canopy models for weather, climate and air quality applications in

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Ching, 2013). Proceeding from that the MEGAPOLI project aggregated different urban canopy properties to identify a hierarchy of approaches and variety of single and multi-layer canopy parameterisations relevant to different scales (street, urban, regional and global) and purposes of the models (Mahura and Baklanov, 2010). 5. Megacities and global climate change Urban areas emit large amounts of CO2, reactive gases and aerosols and thus affect climate also beyond their boundaries. Megacities in particular are very large, concentrated sources of reactive gases and aerosols, with the potential to alter ozone and aerosol levels on local, regional, and global scales (e.g., Molina and Molina, 2004; Ramanathan et al., 2007). In this chapter we review the effects of megacities on global climate through their emissions of greenhouse gases and particles, but also discuss how global climate change may affect air pollution in densely populated areas. 5.1. The effect of megacities on global climate Given the growing number of megacities and large urban complexes worldwide, the question about their impact on global climate has received increased attention both in the scientific community and among policy makers, resulting in large research programs such as the EU projects MEGAPOLI and CityZen. Earlier, Mills (2007) had provided a review of climatology literature that examined the relationship between cities and atmospheric changes at all scales, noting little overall coherence as partly being a result of the varying operational definitions of the city area and activities that are related to it. What is clear, however, is that megacities, being hotspots of human activity, emit large quantities of greenhouse gases and particles into the atmosphere, with distinct climate effects. Furthermore, their emissions of traditional air pollutants such as NOx, CO, VOC, and SOx, affect climate indirectly through interactions with ozone, methane and aerosols. The question about the effect of megacities on global climate, and more generally on their global environment, has often been interpreted by the atmospheric science community in two different ways, briefly discussed in the following paragraphs. Annihilation approach. The first interpretation considers the effect of emissions from megacities as they occur now, including an assessment of future development. For this interpretation, the effects can be quantified in so-called ‘annihilation’ experiments, where megacity emissions are removed completely in model simulations (e.g. Butler and Lawrence, 2009; Folberth et al., 2012). The effect of megacities is assessed simply by comparing modeled atmospheric composition and climate with the results from a reference run where all emissions are included. The main challenges in this type of experiments are to define megacity emissions accurately and to consider nonlinearities of chemical processes. Firstly, the geographical boundaries of cities are difficult to define (Mills, 2007), especially in the case of megacities. Secondly, not all emissions related to a megacity are released within the geographical boundaries of the megacity itself. For example, energy production for a megacity may in many cases occur outside the megacity's boundaries, and in many countries there is an increasing number of commuters traveling daily between the megacity and its hinterland. Emission maps based on population density only are therefore inaccurate. Finally, the resolution of the model grid (typically tens to hundreds of kilometers) puts limitations on our ability to mask out megacity emissions accurately in model simulations, although various techniques of improving emission masking are available (e.g. Butler et al., 2008). A prominent example of an annihilation experiment is the study of Butler and Lawrence (2009) who calculated the effect of 32

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megacities and agglomeration as chosen in the earlier Butler et al. (2008) publication. The population in these cities combined makes up about 10% of the world population and about 10% of the anthropogenic emissions of NOx, CO and non-methane hydrocarbons. Megacities contribute only a very small amount to global total (natural and anthropogenic) non-methane hydrocarbon emissions (Butler et al., 2008). In the annihilation experiment the anthropogenic emissions from these 32 megacities and agglomerations were removed completely from the emissions datasets at their native 1  1 degree resolution before being interpolated to the grid of the global chemical transport model. They find that global effects of megacities on the oxidizing capacity of the atmosphere and on radiative forcing (through ozone) are generally quite small, and disproportionately smaller than the proportion of anthropogenic emissions from megacities. Another study, including also long-lived greenhouse gases and more focused on the climate response, was conducted by Folberth et al. (2012). They used the Met Office Hadley Centre Earth System Model HadGEM2 to conduct an ‘annihilation’ experiment in which the emissions at megacities were entirely removed. According to the megacity mask applied in that study, megacities account for approximately 12% of the anthropogenic CO2 emissions, while the contribution to air pollutant emissions is on the order of 2e5% of the global anthropogenic emissions. These emissions result in a positive annual-mean top-of-atmosphere direct radiative forcing (AMTOA-DRF) of 120.0, 28.4 and 3.3 mWm2, respectively, from the long-lived components CO2, CH4 and N2O under present-day conditions. Short-lived pollutants lead to both positive and negative forcing depending on the species considered, for example 5.7 ± 0.02 mWm2 due to the increase in tropospheric ozone, and 6.1 ± 0.21 mWm2 from the aerosol AMTOA-DRF in the shortwave spectrum. The combined AMTOA-DRF from all climate-active air pollutants from megacities is slightly negative at 0.8 ± 0.24 mWm2 (i.e., a slight cooling). Nevertheless, the total AMTOA-DRF, from greenhouse gases and air pollutants combined, induces a warming of þ150.9 ± 0.24 mWm2. Even-distribution approach. The second interpretation with regard to the effect of megacities on climate concerns the difference between the current situation and a fictive situation, where megacities do not exist, but are replaced by a number of smaller cities, which together have the same number of inhabitants. An extreme case would be a population distributed evenly in a country, or even over the globe (e.g. Stock et al., 2013). This interpretation addresses aspects of urbanization itself, and in the case of megacities looks at the development of very large agglomerations in particular. Stock et al. (2013) used the global chemistryeclimate model UM-UKCA (UK Met Office Unified Model coupled to the UK Chemistry and Aerosols model) and found that the total redistribution of megacity emissions worldwide would shift the chemical environment locally towards more NOx-limited conditions in the megacities, leading to higher surface ozone levels in megacities (by up to 30% monthly-averaged, depending on latitude and season). However, globally the effects would be small e for example 0.12% change in global annual ozone burden e even in the 100% redistribution scenario carried out in their study. A detailed assessment of the environmental benefits or drawbacks of a more evenly distribution of population would go far beyond the realm of atmospheric chemistry and climate science, as it requires detailed emission inventories based on different population scenarios including different policies, implementation pathways of technologies, and human behavior. However, in the context of impact of different population density distributions it has been stated that megacities, being concentrations of economic activity, may have the ability to ameliorate environmental impact (Parrish and Zhu, 2009), as they are better able to generate the

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wealth needed to address air quality and climate change issues, and to build the infrastructure required for more efficient energy use. In particular, energy efficiency may increase with urban population size, thereby reducing pollutant emissions per capita (Lamsal et al., 2013). And vice versa, as Parrish and Zhu (2009) point out, air quality control strategies such as the implementation of fast and convenient public transport (which can be particularly efficient in megacities with high population densities), have the co-benefit of reducing per capita CO2 emissions. 5.2. The effect of global climate change on air pollution in megacities Various impacts of global climate change on cities were summarized in the latest assessment reports of IPCC (Wilbanks et al., 2007; IPCC, 2014a,b). Interactions exist most notably in developing countries, where urbanization is often concentrated in coastal areas (vulnerable to sea level rise), especially when megacities and rapidly growing medium-sized cities approach possible thresholds of sustainability infrastructures susceptible to damage from extreme weather events or sea-level rise and/or infrastructures already close to being inadequate, where an additional source of stress could push the system over a threshold of failure. A large number of publications exist in the literature about the importance of global change to urban communities (Stone, 2012; Simon and Leck, 2014). However, much of this research is done through case studies which do not usually result in general relationships with widespread applicability. This section is about the effect of global climate on air pollution in megacities. Although not conceptually different from climate effect studies on atmospheric composition in general, the question about the effects on megacities in particular concerns an increasing number of people (see section 1) and involves some specific aspects as pollution levels within megacities tend to be particularly high. Climate change can also be further aggravated in megacities due to the heat island effect, given the relatively large area of megacities. The study of the impacts of climate change on air pollution involve effects of meteorological parameters and CO2 concentrations on emissions, the effect of meteorology on chemical conversions in the atmosphere, and chemico-physical processes such as aerosol scavenging in clouds or deposition of pollutants on the ground. Im et al. (2011, 2012) have investigated the impact of climate change on ozone and particulate matter in the East Mediterranean, respectively. The East Mediterranean is characterized by dense population and is influenced by the megacities of Istanbul, Cairo and other large agglomerations such as Athens. It is further affected by pollutant flow from Western Europe as well as dust events from the Saharan desert. The studies found that increased temperature lead to increases in ozone and PM. More specifically, temperature increases lead to increases in biogenic emissions by 9 ± 3%K1. Ozone mixing ratios increase almost linearly with the increases in ambient temperatures by 1 ± 0.1 ppb O3 K1 for all studied urban and receptor stations except for Istanbul, where a 0.4 ± 0.1 ppb O3 K1 increase is calculated, which is about half of the domainaveraged increase of 0.9 ± 0.1 ppb O3 K1. Im et al. (2012) showed that changes in temperature modify not only the aerosol mass but also its chemical composition. Hodnebrog et al. (2012) looked at a real case, namely the Greek fire episode during the hot summer or 2007, applying a regional meteorology-chemistry model and a regional chemical transport model. They found that during that summer, ozone levels increased substantially due to a combination of forest fire emissions and the effect of high temperatures. They also found that the most relevant

effect of increased temperature occurs through biogenic emissions, closely followed by the effect of reduced dry deposition, which is related to stomata closure in plants at very high temperatures. Their results suggest that forest fire emissions, and the temperature effect on biogenic emissions and dry deposition, will potentially lead to substantial ozone increases in a warmer climate. In addition to emissions from megacities, also agricultural emissions in their surroundings play a role, especially for particulate matter concentrations within cities (nitrates). NH3 emissions are expected to increase in a warming climate (Simpson et al., 2015). Model calculations in that study suggest that the area of ecosystems which exceed critical loads is reduced from 64% for year 2005 emissions levels to 50% for currently estimated 2050 levels. A possible climate-induced increase in NH3 emissions could worsen the situation, with areas exceeded increasing again to 57% (for a 30% NH3 emission increase). This constitutes an example of how climate warming can compensate (and even overcompensate) for emission reductions. As stated above, case studies like this do not necessarily allow for an assessment of effects that would apply to all megacities worldwide, but they point to different processes that couple climate change to air quality, the overall effect of which need to be assessed on a case-by-case basis. The effect of global climate change on megacities will need to receive more attention as our improved understanding is crucial to the design of emission reduction policies but also to the planning of adaptation. According to Birkmann et al. (2010), adaptation of cities to the impacts of climate change is an important issue in particular as urban areas are hotspots of high risk, given their concentrations of population and infrastructure; their key roles for larger economic, political and social processes; and their inherent instabilities and vulnerabilities. Birkmann et al. (2010) call for new forms of adaptive urban governance that go beyond the conventional notions of urban adaptation planning. Linking of different temporal and spatial scales in adaptation strategies are seen as important. Within the United Nations system, urban issues are likely to feature prominently in the post-2015 development agenda. In particular the Intergovernmental Board on Climate Services agreed that urban activities related to climate be included as a priority area. The United Nations Conference on Housing and Sustainable Development, HABITAT-III, will take place in 2016, and in this connection a new United Nations Urban Agenda is being developed. 6. Future trends, scenarios and possible mitigations 6.1. Future trends The emissions of air pollutants per capita in cities and megacities across the world vary widely. For example, the average annual emission of black carbon (a major component of soot emissions from diesel vehicles, biomass cookstoves, biomass burning) varies from 0.4 kg per person in European megacities to 1.2 kg per person in Asian megacities (MEGAPOLI, 2011). The elevated emission levels per capita in the megacities of Asia and Africa are related to the local materials and fuels used for residential heating and cooking. In European and American megacities the dominant source of pollution is transport emissions. The per capita emissions in the European megacities (Paris and London) studied in MEGAPOLI (Denier van der Gon et al., 2011) are much lower compared to the per capita emissions in the rest of their respective countries, particularly due to difference in population densities, as well as due to policy changes towards the use of cleaner fuels in the cities. This example illustrates how emissions per capita can be highly specific and, hence, local inventories for

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cities and megacities in other parts of the world are needed to better understand and quantify the exposure of their population to pollutants. Even if emissions per capita are lower in modern megacities than elsewhere, the high population densities result in correspondingly higher emission densities, pollutant concentrations and population exposure. According to the APHEKOM project, compliance with the WHO's air quality guidelines in 25 large European cities could add up to 22 months of life expectancy for their residents aged 30 or more, which could produce V31.5 billion in savings through health benefits every year (APHEKOM, 2011). There would be more pronounced benefits for children, elderly people and low-income families, as well as for those with respiratory and heart problems. For example, living close to busy roads has been cited as causing 15 per cent of asthma cases in children. But even in instances where emissions are relatively low, the population density of the metropolitan areas means pollutants accumulate and have repercussions for the health of inhabitants (WHO, 2015). Moreover, plume studies (Beekmann et al., 2015) showed that black carbon and aerosols in megacity emissions are often detectable up to 200 km downwind from their sources. Several physical and chemical changes and transformations take place in the plumes, so that nearby suburbs and rural regions are exposed to ozone, sulfates and secondary particulate matter that would not have accumulated locally. Many unknowns remain about the dynamics of megacity emissions and their various impacts, such as the number of underlying reasons for residents of various megacities having so different emissions per capita. 6.2. Mitigation strategies As mentioned above, a dominant source of pollution in European and American megacities is emissions from the transport sector, while the elevated emission levels in the megacities of Asia and Africa are related to the local materials and fuels used for residential heating and cooking. However, with the rising affluence in these cities there has been high growth in private car ownership resulting in increased congestion and pollution. In principle, substantial reduction of harmful emissions to the atmosphere is feasible through the use of clean technologies and policy measures. In practice, there are large socio-economic and political barriers to the transition to new technologies. Effective policies and strategies need to be implemented at sufficient scale. Cities such as Los Angeles and Mexico City have improved air quality by promoting comprehensive emissions reduction measures (Molina and Molina, 2002; Parrish et al., 2011). As mentioned in Section 3.1, since the discovery of photochemical smog in the Los Angeles megacity by Haagen-Smit in 1952, it has been the subject of extensive air pollution control efforts. Peak ozone levels exceeding 600 ppbv in the 1960s have not reach 200 ppbv since 1998. It is an environmental success story that can be followed in many parts of the developing world. In fact, one such megacity is the Mexico City Metropolitan area (Molina and Molina, 2002). The Mexico City Metropolitan Area (MCMA) e North America's most populous and rapidly developing megacity e has been working on improving air quality for a number of years. The city experienced a huge increase in population and urbanized area as it attracted migrants from other parts of the country and industrialization stimulated economic growth during the twentieth century. Population growth, increasing motorization and industrial activities, a constrained basin and intense solar radiation combined to cause intense air quality problems of both primary and secondary pollutants. The ambient air quality monitoring network,

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established in late 1980s, revealed high concentrations of all criteria pollutants. Ozone exceeded the air quality standards more than 90% of the days, and peaked above 300 ppb 40e50 days a year, among the worst in the world (Molina and Molina, 2002). Both the Mexican government and the citizens of Mexico City have recognized air pollution as a major environmental and social concern since the mid-1980s. In the 1990s, comprehensive air quality management programs were developed and implemented, including removal of lead from gasoline and the implementation of catalytic converters in automobile, reduction of sulfur content in diesel vehicle fuel, substitution of fuel oil in industry and power plants with natural gas; reformulation of liquefied petroleum gas used for heating and cooking, strengthening the vehicle inspection and maintenance program, and modernizing the driving restriction (‘no driving day’) program (Molina et al., 2009). As a result of these regulatory actions combined with technology change, the concentrations of criteria pollutants in the MCMA have decreased substantially over the past decade despite the continuing increase in population and economic activity. The MCMA represents around 20% of Mexico's population, but only 9% of its greenhouse gas emissions. Policies focused on greenhouse gas emission reductions include biogas capture and waste management projects, improved public transportation and mobility (bus rapid transit, school bus, bicycle lanes), fleet renewal projects for taxis and medium-capacity buses, sustainable housing development projects (Molina et al., 2009). The experiences in Los Angeles and Mexico City demonstrate that urban and industrial development can proceed simultaneously with air quality improvement, and can be replicated in other parts of the world especially in rapidly developing cities. However, there is no single strategy for addressing air pollution problems in megacities. A mix of policy measures best suited for each city's challenges and customs will be needed to improve air quality. It also has to be noted that the degree of emission control that has been required to improve air quality in developed megacities is quite substantial. For example, vehicle emissions of VOC per distance traveled have been reduced by a factor of about hundred in Los Angeles from 1960 to the present (Parrish and Stockwell, 2015, and references therein). For the European situation, Crippa et al. (2015) recently constructed a fictive emission scenario for 2010, assuming stagnation of technology at 1970 levels, and compared this with the real-world situation. According to their results, EU air quality in 2010 would have suffered from 129% higher SO2, 71% higher NOx and 69% higher PM2.5 emissions than in reality, if technology and European EOP reduction measures had stagnated at 1970 levels. This demonstrates the role of technology that has efficiently reduced emission factors during the 1970 to 2010 period. Advances like this have been motivated by public awareness and policy measures, such as the increasingly stringent European emission standards for exhaust emissions of new vehicles sold in EU member states (EURO 6 for light duty and EURO VI for heavyduty vehicles). 7. Main problems and recommendations As a consequence of rapid urbanization, the world now has more urban than rural residents, leading to the emergence of megacities. Megacities are dense centers of population and economic activity and generate atmospheric pollution that affects public health and ecosystems, causes urban and regional haze, and alters the earth's climate. However, megacities also offer opportunities for effective pollution control strategies that could lead to significant environmental benefit. Despite the large volume of research that has been conducted in recent years, including improved emission inventories, ambient air

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quality measurements and special field studies, the sources and processes leading to high concentrations of main pollutants such as ozone, nitrogen dioxide, and particulate matter in complex urban areas are still not fully understood, limiting our ability to forecast air quality accurately. Substantial progress has been made over the past few decades to prevent and control air pollution in many parts of the world through a combination of technology improvements and policy measures. Many countries have clean air laws that set emission ceilings and ambient air quality standards to protect public health and the environment, with some successes in both developed and developing world megacities. However, increasing human activities are offsetting some of the gains, including continued reliance on fossil fuels such as coal-fired power plants, the dependence on private transport motor vehicles, inefficient use of energy in buildings, and the use of biomass for cooking and heating. Similarly, according to the current Environmental Performance Index (EPI, 2014), air quality measurement capabilities are weak and poorly coordinated with management, international policy targets are largely absent, and the world has observed policy stagnation and alarming air pollution crises in a growing number of cities. Air pollution is especially a problem in many developing world cities that are producing goods for the global economy. Nevertheless, the general improvement in cities, such as Mexico City, which once had significant air pollution problems, is proof that the present situation in other polluted megacities in the developing countries can indeed be improved as well. The new report from WHO (2014a) underscores the perception that, although air quality in developed countries has been generally improved over the last decades, the adverse health effects of air pollution remain a global public health concern, especially in low and middle income countries. WHO estimates that about 13% of premature deaths could be prevented by improving air quality worldwide. Furthermore, actions that reduce urban air pollution will also cut emissions of short-lived climate pollutants, particularly black carbon, a major component of soot emissions from diesel vehicles, biomass cook stoves, flaring from oil and gas production, etc., as well as greenhouse gases, including CO2, that contribute to longer-term climate change impacts (UNEP-WMO, 2011). In the developing world, PM10, PM2.5 and O3 measurements are conducted regularly only in a small number of cities. A high priority action should be to establish comprehensive monitoring in other cities of the developing world. Air pollution problems of megacities differ greatly and are influenced by a number of factors, including topography, demography, meteorology, mobility and transportation patterns, fuel quality and usage, and the level and rate of industrialization and socio-economic development. More extensive atmospheric measurements and modeling are needed to define optimal emission control strategies for the particular urban center under consideration, considering the unique local economic, social and political circumstances. Policy makers should use this information to balance the economic and social benefits of health improvements against the costs of emission control to achieve various air quality improvement goals as well as climate protection. Additionally to air quality, megacities have strong urban heat island effects due to differences in surface properties and waste heat from anthropogenic activity, so megacities can be warmer than the surrounding rural environments by up to several degrees Celsius. This heating impacts the local environment directly, as well as affecting the regional air circulation. One of the most important problems for growing urbanization under global change conditions is the reduction of the risks of natural disasters and high impact weather events. The risks in the urban environment include, but are not limited to, 1) flooding; 2) poor air quality; 3) sea-level rise; 4) extreme heat and human

thermal stress; 5) energy and water sustainability; and 6) public health problems caused by the previous. These urban risks are largely related to weather and climate extremes and a key question would be to better understand how these will change in a changing climate. The understanding of how and to which extent cities modify regional weather and climate, for instance through additional heat and pollution fluxes, is also needed. A common understanding on how best the weather and climate prediction capability can be advanced on all temporal and spatial scales for megacities and large urban complexes needs to be developed. The development of the Integrated Urban Weather, Environment and Climate Systems and Services (IUWECS) is thus an important practical and research-oriented task. The IUWECS concepts relate to the conditions faced by urban populations and the impacts of environmental conditions on the megacity and urban society, the need for a legal framework and clearly defined government agency interactions to enable creation and maintenance of such a system, and the advances of science and technology required to develop and implement such a system. The recent World Meteorological Congress (WMO, Geneva, 25 May e 12 June 2015) adopted the Resolution on ‘Establishing WMO Cross-cutting Urban Focus’ (CG-17, 2015) and recognized the need to elaborate the integrated approach providing weather, climate, water and related environmental services tailored to the urban needs. The development of an IUWECS requires scientific and technological development in many areas, including (Grimmond et al., 2014b): (a) Development of understanding and knowledge regarding enhanced observational needs to meet the requirements of integrated services in megacity and other urban environments, and identification of observational source locations in complex environment; (b) Development of concepts, scientific capabilities and technology for seamless services; (c) Development of the science and technology required for provision of service applications to society; (d) Development of smart delivery approaches, including the application of new technology to create an “intelligent and wise” city; (e) Development of methods for efficiently making use of large, complex databases (i.e., “Big Data”); and (f) Development and implementation of user-relevant approaches for evaluating the quality and benefits of products and services. Accomplishing these activities will require an acceleration of the transition of research capabilities and knowledge to operational systems. The scientific effort is also heavily reliant on extensive sharing of capabilities and knowledge among participating organizations in an IUWECS. In particular the emission inventories used in the models should be reviewed. The uncertainties in this respect are especially large in the developing countries. Research on basic physical and chemical processes and development of numerical models and tools are an integral and central component of a reliable and accurate forecast products and services. For example, the representation of the urban land surface and urban sublayer has undergone extensive developments but no scheme is capable of dealing with all the surface exchanges. To complicate this further, as the resolution of models becomes greater, combined with the large size of urban buildings in many megacities, the limits of current understanding are being challenged. Research needs also relate to secondary organic aerosols and their interaction with clouds and radiation, data assimilation including chemical and aerosol species, dynamic cores with multi-

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tracer transport efficiency capability and the general effects of urban aerosols on weather and climate evolution. All of these areas require an efficient use of models on massively parallel computer systems. In conclusion, as an ever-increasing number of megacities loom on the horizon, a new generation of multi-scale integrated models are needed for helping to ensure that we can adapt to the responsibilities associated with megacities.

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