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Atmospheric Brown Clouds

Summary and Keywords

Atmospheric brown clouds (ABCs) are widespread pollution clouds that can at times span an entire continent or an ocean basin. ABCs extend vertically from the ground upward to as high as 3 km, and they consist of both aerosols and gases. ABCs consist of anthropogenic aerosols such as sulfates, nitrates, organics, and black carbon and natural dust aerosols. Gaseous pollutants that contribute to the formation of ABCs are NOx (nitrogen oxides), SOx (sulfur oxides), VOCs (volatile organic compounds), CO (carbon monoxide), CH4 (methane), and O3 (ozone). The brownish color of the cloud (which is visible when looking at the horizon) is due to absorption of solar radiation at short wavelengths (green, blue, and UV) by organic and black carbon aerosols as well as by NOx. While the local nature of ABCs around polluted cities has been known since the early 1900s, the widespread transoceanic and transcontinental nature of ABCs as well as their large-scale effects on climate, hydrological cycle, and agriculture were discovered inadvertently by The Indian Ocean Experiment (INDOEX), an international experiment conducted in the 1990s over the Indian Ocean. A major discovery of INDOEX was that ABCs caused drastic dimming at the surface. The magnitude of the dimming was as large as 10–20% (based on a monthly average) over vast areas of land and ocean regions. The dimming was shown to be accompanied by significant atmospheric absorption of solar radiation by black and brown carbon (a form of organic carbon). Black and brown carbon, ozone and methane contribute as much as 40% to anthropogenic radiative forcing. The dimming by sulfates, nitrates, and carbonaceous (black and organic carbon) species has been shown to disrupt and weaken the monsoon circulation over southern Asia. In addition, the ozone in ABCs leads to a significant decrease in agriculture yields (by as much as 20–40%) in the polluted regions. Most significantly, the aerosols (in ABCs) near the ground lead to about 4 million premature mortalities every year. Technological and regulatory measures are available to mitigate most of the pollution resulting from ABCs. The importance of ABCs to global environmental problems led the United Nations Environment Programme (UNEP) to form the international ABC program. This ABC program subsequently led to the identification of short-lived climate pollutants as potent mitigation agents of climate change, and in recognition, UNEP formed the Climate and Clean Air Coalition to deal with these pollutants.

Keywords: air pollution, climate change, brown clouds, China and India air pollution, hydrological cycle, monsoon rainfall, public health

Discovery of ABCs: Serendipitous and Curiosity Driven Science

Atmospheric brown clouds (ABCs) (Figure 1) are plumes of air pollution consisting of aerosol particles and reactive gases and should not be confused with water clouds, which are natural phenomena.

Atmospheric Brown CloudsClick to view larger

Figure 1. Atmospheric Brown Clouds photographed in various locations.

(Veerabhadran Ramanathan, personal photos)

ABCs are typically found downwind of populated regions and large urban areas (Ramanathan & Crutzen, 2003; Ramanathan & Ramana, 2003). They can extend up to 3 km and span areas ranging from thousands to millions of square kilometers over land and ocean. When ABCs were first discovered over southern Asia, they were initially named “Asian Brown clouds” (UNEP-C4, 2002). However, it was later realized that these “brown clouds” are not specific only to Asia but are also observed in other parts of the world. Thereafter, they were renamed “Atmospheric Brown Clouds” (Ramanathan & Crutzen, 2003; Ramanathan & Ramana, 2003).

The discovery of ABCs was made serendipitously. During March to April of 1993, The Central Equatorial Pacific Experiment (CEPEX) was conducted in the Equatorial Pacific Ocean with aircraft, ships, and satellites (Ramanathan et al., 1996). The primary aim was to measure the heat budget of the equatorial Pacific Ocean to understand the relative importance of clouds and evaporation in regulating sea surface temperatures. The data revealed a major discrepancy between theory and observations (see Ramanathan et al., 1996). The solar radiation reaching the sea surface was lower than the predicted values by as much as 10%. The primary reason for this discrepancy was that the measured atmospheric absorption was larger than the theoretical or model absorption by about 15–20 W/m2. It was speculated that the model was missing an important source of solar absorption, i.e., some dark substance in the atmosphere was absorbing sunlight and causing a huge dimming at the surface, and this substance was not included in the models. It was the search for this absorbing species that led to the discovery of the large role of black carbon in atmospheric absorption as well as in surface dimming. While black carbon was largely responsible for the missing (as per the CEPEX experiment) atmospheric solar absorption and more than 50% of the surface dimming, numerous other aerosol species including organic carbon, sulfates, nitrates, ash, and dust contributed to the dimming. Figure 2 shows the contribution of various chemical species to the column-averaged aerosol optical depth (AOD) as observed during the INDOEX study.

Atmospheric Brown CloudsClick to view larger

Figure 2. Relative contributions of each chemical species to the column-averaged aerosol optical depth (AOD) during the INDOEX study.

(Adapted from Ramanathan et al., 2001)

The wide spread nature of ABCs was formally discovered by a series of ground-based, airborne, and satellite measurements as part of the multi-national INDOEX (Ramanathan et al., 1995, 1996) conducted over the northern Indian Ocean from the island of Male in Maldives. The multi-platform observations over the three-year period detailed not only the large-scale structure of ABCs in terms of their chemical and optical properties but also their influences on radiative forcing, a measure to assess the climate change potential of a substance on the atmosphere (see Lelieveld et al., 2001; Ramanathan et al., 2001).

Another major motivation for INDOEX was to directly estimate the surface cooling effect of anthropogenic aerosols (Ramanathan et al., 1996). INDOEX started with a pilot phase in 1996 and 1997, when two small-scale feasibility campaigns were launched (Jayaraman et al., 1998; Rhoads et al., 1997) with ship-based surface measurements. The first campaign employed chemistry measurements (Rhoads et al., 1997), and the second investigated aerosol optical properties using radiometers (Jayaraman et al., 1998). Both confirmed that during winter and spring, the Arabian Sea and the northern Indian Ocean will have strong aerosol loading due to air pollution transported from southern Asia. Note that this area is downwind of where over 1.5 billion people live. ABCs were shown to impact every aspect of human well-being, from public health to crop yields, precipitation, regional climate, and weather (Ramanathan & Crutzen, 2003; Ramanathan & Ramana, 2003).

The climate warming effects of ABCs in terms of their primary aerosol and gaseous pollutant constituents (e.g., black carbon, ozone, and methane) as well as their relatively short life spans (up to a decade) has made them ideal targets for short-term climate change mitigation (Ramanathan & Xu, 2010; Shindell et al., 2012). By mitigating these short-lived climate pollutants (SLCPs), the rate of global warming can be cut down by as much as 50% in the next four decades (Ramanathan & Xu, 2010). Mitigation will have co-benefits of saving millions of lives and millions of tons of crops lost due to ABC pollution (Shindell et al., 2012). With this as an objective, the UNEP formed the Climate and Clean Air Coalition (CCAC) with over 50 member countries to promote the mitigation of SLCPs (CCAC, 2012). Thus, what started as a search for a dark sooty substance in the atmosphere resulted in a major effort at mitigating climate change. Such is the nature of curiosity-driven research!

Global Distribution of Atmospheric Brown Clouds

Hotspots of ABCs were identified in several regional areas (Ramanathan et al., 2008): East Asia, the Indo-Gangetic plains in South Asia, Southeast Asia, southern Africa, and the Amazon Basin. Aerosol optical depth (AOD) is a term used to measure aerosols within a column of air between the earth’s surface and the top of the atmosphere (NASA, 2015). The AOD at these locations was found to be more than 0.3 (typical threshold for polluted conditions), while the percentage of contribution by absorbing aerosols exceeded 10% (absorbing AOD > 0.03).

Globally, the various regional ABC hotspots can have distinct peak seasons (Figure 3).

Atmospheric Brown CloudsClick to view larger

Figure 3. Integrated satellite data show anthropogenic aerosol optical depth (AOD) in the period 2001–2003 for four seasons.

(Adapted from UNEP, 2008)

One of the most prominent hotspots is located in East Asia, with the highest AOD observed during the Northern Hemisphere summer. This seasonal peak is attributed to relatively low precipitation, which aids in aerosol deposition.

Causal Factors

Significant research has been carried out in assessing the major drivers and causal factors for the formation of ABCs in different parts of the world. The major drivers that broadly contribute to formation of ABCs are increased population, rapid urbanization rates, uncontrolled industrialization, and poverty. Figure 4 shows the rapid growth of population, gross domestic product (GDP) per capita, and energy use, globally, from 1970 to 2010 (IPCC, 2014).

Atmospheric Brown CloudsClick to view larger

Figure 4. Growth in global population, gross domestic product (GDP), and energy use, 1970–2010.

(Adapted from IPCC, 2014)

While the world population has grown 1.8-fold, income levels and CO2 emissions (energy use proxy) have more than doubled since 1970.

Global Emissions of Sources of ABCs

Multiple sources are responsible for the formation of ABCs. Densely populated cities emit pollutants from transport and industrial sectors, while rural regions use biomass-based fuels. In fact, a major source of black carbon (BC) and organic carbon (OC) emissions are the residential and commercial sectors, which include biomass combustion and charcoal production for cooking and heating (Figure 5).

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Figure 5. Relative contributions of economic sectors to the global emissions of ABC sources, 2000.

(Adapted from Lamarque et al., 2010)

The power sector is responsible for almost half of the global SO2 emissions (SO2 is a precursor of sulfate aerosols) (Husar, Lodge, & Moore, 1978), while the emissions of NOx contribute to the formation of nitrate aerosols and ozone (Lei & Wang, 2014).

Atmospheric Brown CloudsClick to view larger

Figure 6. Global trends of air pollutant emissions, 1970–2010.

(Adapted from IPCC, 2014)

Figure 6 shows the global trends of emissions of the ABC pollutants during 1970–2010 (IPCC, 2014). Due to controls in some sectors, there is a reduction in emissions of SO2, while NOx, carbon monoxide (CO), and BC have either fluctuated or increased during same period. Methane emissions have grown uncontrolled during the same period. Note that BCs and OCs are the constituents of particulate matter (PM) that influence atmospheric warming and cooling properties, respectively. Also, gases like NOx, SO2, and volatile organic carbon (VOCs) convert to secondary particulates, which have cooling effects on the atmosphere. Aerosols have a dimming effect on the earth’s surface by reflecting the sunlight back to space. Further dimming can occur when aerosols act as cloud condensation nuclei, forming more cloud droplets and causing more reflection of sunlight. Dimming can reduce evaporation rates and lead to reduced rainfall (Ramanathan et al., 2007). Figure 7 shows the magnitude of the warming effects of greenhouse gases (GHGs) and the cooling effects of aerosols such as OCs, sulfates, and nitrates. The only exception to the aerosol cooling effect is the BC aerosols, which are the strongest absorbers of solar radiation.

Atmospheric Brown CloudsClick to view larger

Figure 7. Radiative forcing (W/m²) of different atmospheric constituents.

(Adapted from IPCC, 2014)

Because of their short lifetimes of about few weeks or less, ABCs are more concentrated near the source regions (see Figure 3). The regional areas of various pollution sources are given in Figure 8.

Atmospheric Brown CloudsClick to view larger

Figure 8. Regional shares in emissions of different pollutants, 2010. ALM: Africa, Latin America, and Middle East; REF: Central and Eastern Europe, Russia, and NIS.

(Adapted from Cofala et al., 2007)

Multiple sectors/sources contribute to emissions of different pollutants, which eventually form ABCs. Table 1 lists the pollutant sources and their possible impacts on human society. In what follows, we concentrate on China and India, the region with the large pollution sources currently.

Table 1. List of Pollutants, Their Sources, and Impacts on Human Society




Nitrogen oxides (NOx)

Anthropogenic: high-temperature combustion processes (engines, power generation, etc.)

Bronchitis in asthmatic children, reduced lung function growth

Precursor for ozone formation leading to crop damages

Natural: lightning, volcanoes

Precursor for secondary particulate (nitrate) formation leading to dimming effects

Particulate matter (PM2.5, PM10)

Anthropogenic: vehicles, industrial sources, residential fuel burning, road dust resuspension

Cardiovascular and respiratory diseases, lung cancer, acute lower respiratory infection (ALRI), chronic obstructive pulmonary disease (COPD). Metals like lead affect intellectual development of children and at very high doses may cause poisoning and brain and organ damage

Natural: wind-blown dust, volcanoes, forest fires, sea salt

Agricultural impacts: dry deposition leading to reduced photosynthetic activity

Climate impacts: atmospheric warming due to BC and dimming due to other primary aerosols like OC

Reduced visibility

Carbon monoxide (CO)

Incomplete fuel combustion (as in residential cook stoves, gasoline vehicles)

Reduces the oxygen-carrying capacity of blood; causes headaches, nausea, and dizziness; can lead to death at high levels

Sulfur dioxide (SO2)

Burning of sulfur-containing fuels for heating, power, and vehicles

Affects respiratory system and lung function Coughing, mucus secretion, asthma, and chronic bronchitis

Causes acid rain and damage to ecology and buildings

Precursor for secondary particulate (sulfate) formation leading to dimming effect

VOCs (benzene, butadiene, alcohols, acids, etc.)

Incomplete combustion of fuels in residential cook stoves and vehicles; evaporative emissions form fuel handling and distribution, and use of solvent-based products

Respiratory illness, cancer

Precursor for ozone formation leading to crop damages

Precursor for secondary particulate (secondary organic aerosols) formation leading to dimming effect


Formed primarily by the reaction of NOx and VOCs in sunlight

Breathing problems, asthma, reduced lung function

Ozone is one of the most damaging pollutants for plants

Greenhouse gas

India and China

India and China are among the fastest growing economies in the world. However, air quality has faced the brunt of rapid economic growth, and many cities in both countries violate the prescribed national standards and the more stringent WHO guidelines. About 1.2 million people in China and 0.6 million people in India die annually due to ambient air pollution. Outdoor air pollution is the fourth and fifth highest cause of death in China and India, respectively (Figure 9).

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Figure 9. Premature deaths per year due to ambient air pollution in China and India.

(Adapted from Lim et al., 2012)

Additionally, the impact of pollution on materials has also been observed. The famous Taj Mahal in India has been found to be deteriorating due to emissions of sulfur dioxide in the vicinity.

Energy use patterns are different in the two countries, as are the emissions of air pollutants. Figure 10 shows the comparative trends of BC, OC, and PM2.5 emissions in the two countries during 2001–2008.

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Figure 10. BC, OC, and PM2.5 emissions in India and China, 2001–2008. Each color represents emission in a different year from 2001 to 2008.

(Adapted from Kurokawa et al., 2013)

Emissions in India are less than in China, but in a business-as-usual (BAU) scenario of rapid growth in Indian emissions, this difference may decrease. Specifically, BC emissions are higher in China mainly due to more vehicular traffic. On the other hand, lower BC-to-OC ratios suggest significant contributions from residential biomass-based cooking in India.

Figure 11 shows the relative chemical composition of the mean particulate concentrations in India and China. The values are annual averaged values obtained from the ABC assimilation model (Adhikary et al., 2007).

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Figure 11. Chemical composition of aerosols in India and China.

(Adapted from Adhikary et al., 2007)

With higher emissions of SO2 in China due to high-sulfur coal, the contribution of sulfates is higher. On the other hand, higher biomass usage in the residential sector for cooking leads to higher contributions of OC in India in comparison to China. Dust also contributes significantly to surface aerosols in India.

The capital cities of India and China have attracted international attention for their deteriorated air quality. Cheng et al. (2016) finds significant violation of the ambient air quality standards (PM2.5) in the two cities compared to other megacities in the world. Ramachandran et al. (2012) and Xue et al. (2011) reported the yearly trends of AOD values over two of the most heavily polluted cities in the world (Figure 12).

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Figure 12. Trends of aerosol optical depth (AOD) values in New Delhi and Beijing.

(From Ramachandran et al., 2012; Xue et al., 2011)

AOD is an optical measure of the total number of aerosols in the column extending from the surface to the top of the atmosphere and is determined from satellite data in visible wavelengths. It is influenced by both anthropogenic and natural aerosols and generally varies from 0.05 to about 1 (in heavily polluted regions). The AOD values are higher in New Delhi than Beijing. Other than PM2.5, both cities also report violations of prescribed ozone standards (Liu, Wang, Pang, & He, 2013; Sharma, Sharma, & Khare, 2013).

Impact of ABCs on Surface Dimming, Radiative Forcing

The aerosol particles in ABCs absorb and scatter solar radiation. While aerosol effects on radiative forcing have been studied since the 1960s (Rasool, & Schneider, 1971), it was not until the INDOEX campaign (Jayaraman et al., 1998; Ramanathan et al., 2001; Satheesh & Ramanathan, 2000) that we knew that aerosols can cause significant dimming at the surface. Satheesh and Ramanathan (2000) used satellite and surface observations to suggest that nearly two-thirds of the dimming was due to solar absorption by black carbon. Thus, the INDOEX studies answered the question about an atmospheric “dark substance” posed earlier by Ramanathan et al. (1995). The global estimate of the dimming was made by integrating field observations with surface observations (Figure 13c). Figure 13b shows the atmospheric heating by solar absorption due to ABCs across the world.

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Figure 13. (a) Global black carbon emissions; (b) atmospheric solar heating by aerosols; (c) surface dimming by Atmospheric Brown Clouds.

(Adapted from Ramanathan & Carmichael, 2008)

Stanhill and Cohen (2001) and Wild et al. (2005) showed a reduction of 5–10% in solar radiation observed in extratropical regions across the world in the mid-20th century. INDOEX (Ramanathan et al., 2001; Satheesh & Ramanathan, 2000) confirmed the findings that ABCs can lead to dimming by as much as 7–10% (i.e., a reduction of annual mean absorbed solar radiation by about 15 W/m2). Figure 13 (Ramanathan & Carmichael, 2008) shows that the regions of maximum dimming and maximum absorption of solar radiation in the atmosphere coincide with the regions of high BC emission rates (Bond et al., 2004). These highly polluted regions of the Indo-Gangetic plains, Eastern China, and several other regions across the world show good correlations between BC emissions and dimming.

Masking Effect of Aerosols in ABCs on Global Warming

Sulfates, nitrates, and some organics mainly reflect solar radiation and cool the climate. BC, on the other hand, is the second largest contributor to radiative forcing (Chung, Ramanathan, & Decremer, 2012; Ramanathan & Carmichael, 2008), being the strongest absorber of solar radiation with 0.7–0.9 W/m2 of globally averaged radiative forcing. However, the cooling effects of sulfates, nitrates, and organics are much larger than that of BC warming, such that when the effects of all aerosols are summed, manmade aerosols have a net cooling effect of 1 W/m2 (IPCC, 2013). Without the masking effect of aerosols, the radiative forcing of 3 W/m2 by greenhouse gases (IPCC, 2013) would have warmed the planet by about 2°C currently (Ramanathan & Feng, 2008).

Societal Impacts

Impacts on the Asian Monsoon and Related Aspects

The dimming effect at the earth’s surface accompanied by a simultaneous increase in atmospheric solar heating has a significant impact in reducing the strength of monsoon circulation (Ganguly, Rasch, Wang, & Yoon, 2012; Meehl, Arblaster, & Collins, 2008; Ramanathan et al., 2005). The dimming reduces monsoon rainfall via two different mechanisms. First, since ABCs are concentrated north of the equatorial Indian Ocean, the dimming (i.e., cooling effect) reduced northern Indian Ocean surface temperatures more than the southern Indian Ocean surface temperature; as a result, the dimming reduces the north-to-south sea surface temperature gradients, which is an important driver of the summer monsoon circulation. The net effect is to decrease the strength of the overturning monsoonal circulation and decrease in rainfall (Chung & Ramanathan, 2006). In addition, the dimming over continental surfaces reduces evaporation, which also contributes to a reduction in monsoonal circulation. Observations have revealed that the India-averaged monsoon rainfall has reduced by as much as 7% (Dash, Kulkarni, Mohanty, & Prasas, 2009; Ramanathan et al., 2005), and this decrease is mostly concentrated in the Indo-Gangetic plains. This feature was reproduced well by the coupled ocean-atmosphere modeling studies of Ramanathan et al. (2005) when they introduced ABCs in their model.

Impacts on Agriculture Yields

ABCs affect agricultural productivity in many ways. Ozone formation within ABCs causes serious damage to agricultural productivity. Chatani et al. (2014) and Sharma, Chatani, Mahtta, Goel, & Kumar (2016a) show the projected increase in ozone concentrations in South and East Asia and more specifically in India and China in the next two decades. Two recent studies documented the impact of ozone on agricultural productivity in India (Burney & Ramanathan, 2014; Ghude et al., 2014). Burney and Ramanathan (2014), using a statistical model, showed that up to 36% of wheat in India is lost on account of ozone pollution in India. Ghude et al.’s model study (2014) also concludes that ozone has destroyed crops in India, but the percentage reduction in yield in is about 5% or less compared with the 30% or more estimated by the statistical dynamical study of Burney and Ramanathan (2014). Finally, changes in climatic patterns due to ABCs may also impact the agricultural productivities in a region.

Impacts of ozone on agriculture are well documented for different regions across the world. For example, an economic loss of $5 billion in a year is estimated on account of reduced crop (wheat, rice, corn, and soybean) yields in Japan, South Korea, and China (Wang & Mauzerall, 2004). Avnery, Mauzerall, Liu, and Horowitz (2011) estimated the impact of ozone on global agriculture for the years 2000 and 2010. They found a yield loss of 5.4–26% for wheat, 15–19% for soybean, and 4.4–8.7% for maize in the A2 climate scenario in 2030. This aggregates to an economic damage of $17–35 billion, annually. The analysis reveals that if these losses can be averted, close to 900 million people can be fed and brought above the minimum dietary energy requirement threshold.


ABCs are known to cause a variety of negative impacts on human society. At first, impacts are felt in the indoor/local environment, when emissions of pollutants deteriorate the air quality and expose the residents/nearby population to extremely high concentrations of pollutants. This is evident with very high concentrations of pollutants observed in a rural household that uses biomass fuel for cooking or a neighborhood affected by vehicular fumes. Thereafter, the emissions, based on prevailing meteorological conditions, disperse, transform in secondary forms, and pollute the ambient air quality of a wider region. The particulate pollution near the surface is referred to by the epidemiological community as PM2.5 (particle mass less than 2.5 µm in diameter). This is observed primarily at the municipal level, where ambient PM2.5 levels violate the prescribed standards and WHO guidelines. This sometimes exposes huge population bases residing in cities to unacceptable levels of pollution, resulting in premature mortality and morbidity.

While most of the inhaled PM larger than 2.5 µm are exhaled or trapped in the upper areas of the respiratory system and expelled, those smaller than 2.5 µm penetrate deeper into the lungs and cause a variety of health impacts. Respiratory illnesses like acute lower respiratory infection (ALRI) and asthma are directly linked to exposure to PM2.5 and in some cases can trigger heart failure (Brook, Brook, & Rajagopalan, 2003). Many of the particulates have carcinogenic properties. In 2013, the International Agency for Research on Cancer (IARC) classified “outdoor air pollution” as carcinogenic to humans in Group 1, which means that there is sufficient evidence of its carcinogenicity in humans.

Global burden of disease estimates (Lim et al., 2012) show that outdoor air pollution ranks very high among the top global health risks (Figure 14).

Atmospheric Brown CloudsClick to view larger

Figure 14. Annual global mortalities due to different causes.

(Adapted from Lim et al., 2012)

Outdoor air pollution is estimated to claim 3.2 million lives annually worldwide—over 74 million years of healthy life. Outdoor air pollution was ranked fourth in East Asia (China and North Korea), with 1.2 million mortalities, and 6th in South Asia (including India, Pakistan, Bangladesh, and Sri Lanka) with 0.71 million mortalities in 2010.

Corrective Measures

Many sectors are responsible for formation of ABCs in different regions of the world. Table 2 presents different pollutants and relevant sectors that need to be controlled for reducing the impacts of ABCs.

Table 2. Different Pollutants and Their Corresponding Sources and Controls Measures.

Serial No.



Control mechanisms

1. Primary particulates



Residential biomass cooking

Improved biomass cook stoves, LPG penetration in residential sector

Diesel-driven transport

Advanced (Euro-VI equivalent) vehicle emission norms with tailpipe controls, Inspection & Maintenance systems, enhancement of public transport and electric mobility

Brick manufacturing

Open agricultural burning

Technological shifts to zig-zag advanced kiln technologies for brick manufacturing



Residential biomass cooking

Waste to energy options for agricultural waste residues

Gasoline- and diesel-driven transport

Open agricultural burning




Continuous monitoring and emission trading schemes promoting better tailpipe controls




Adherence to top environment management plans for dust control at construction sites

Road dust resuspension

Quality control during construction, regular maintenance and cleaning of roads, landscaping

2. Secondary particulates


Sulfates formed from SO2

Power plants and industries burning sulfur-based fuels

Flue gas desulfurization units, switch to low-sulfur fuels

Vehicles using high-sulfur diesel/gasoline

Introduction of Euro-V equivalent low-sulfur fuels


Nitrates formed from NOx

High-temperature combustion in vehicles, power plants

Euro-VI norms with selective catalytic reduction (SCR), improved I&M systems

SCR, low-NOx burners

DG sets



Secondary organic aerosols (SOAs) formed from VOCs

Residential biomass cooking

Improved cook stoves, LPG penetration

Promotion of low-VOC products

Solvent use in industries, domestic sector

Vapor recovery systems (Stage-I/II)

Oil- and gas-handling vehicles

Euro-VI norms with diesel oxidation catalysts (DOCs)

Control of particulate matter demands control of both primary and secondary particles. While primary PM reduction strategies will help control BC, OC, and other elements, the control of gaseous pollutants will lead to a reduction in secondary particulate formation. Sector-specific strategies that could be employed for control of ABCs are discussed below.

California has reduced its emissions of black carbon, ozone precursor gases, and other pollutants by more than 80% since the 1960s in spite of its increases in population (100%), fuel consumption (200%), and vehicle ownership (200%) (Ramanathan et al., 2013). In addition, for each dollar spent on air pollution mitigation, the state reaped benefits totaling more than $10 in terms of health benefits and jobs created. Air pollution has also been reduced drastically in Western Europe without negatively impacting its economy. Clearly these examples demonstrate that air pollution is a solvable problem and such actions do not have to ruin a country’s economy. Focusing on just China and India, eliminating residential combustion of solid fuels for cooking and lighting could reduce ambient regional air pollution by about 30% and 50%, respectively (Lelieveld, Evans, Fnais, Giannadaki, & Pozzer, 2015). Based on the source apportionment studies, the share of transport sector is about 20–30% in PM2.5 concentrations in Delhi and Beijing, and hence substantial reductions can be observed in the air pollution levels by reducing transport related emissions.


Diesel combustion in the transport sector in developing countries like India is the main driver of BC and NOx emissions. Among these, diesel-driven heavy-duty vehicles contribute the most to atmospheric pollutant composition. Intervention in the transport sector for control of BC and NOx will require introduction of advanced fuel quality and vehicle emission standards for new vehicles. Due to high sulfur concentrations in fuels, the proper after-treatment technologies, such as diesel particulate filter (DPFs) and lean NOx traps, that could reduce vehicle emissions by more than 90% are not possible. Hence, reduction of sulfur content in fuels to 10 ppm (Euro-V equivalent) is essential to allow the proper functioning of these technologies for advancement to Euro-V/VI equivalent vehicle emission standards. While the developed world has moved to advanced Euro-VI equivalent standards, the developing and underdeveloped nations have yet to catch up (Figure 15).

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Figure 15. State of introduction of advanced emission standards in different parts of the world.

(Adapted from Miller & Façanha, 2014)

In Asia, India and China have taken steps to reduce sulfur content in fuels and advance the vehicle emission standards to Euro-IV levels. The Indian government has now announced to introduce Euro-VI standards by 2020. Other than emission standards, enhancing public transport systems could bring significant emission and climate benefits on regional and global scales. Shao and Façanha (2014) show that bus rapid transit (BRT) and other types of bus service can provide substantial benefits.

Residential Cooking

A huge population in developing countries, about 2.8 billion, currently relies on biomass fuel for cooking; furthermore, by 2030, 2.7 billion will continue to rely on biomass (IEA, 2006). This biomass fuel resource is available at almost no cost, a factor that primarily explains its high usage rates. In 2007, the percentage of households in India using liquefied petroleum gas (LPG) in rural areas was just 9% in comparison to 62% in urban areas. This makes biomass the largest source contributing to the BC and non-methane volatile organic compound (NMVOC) emissions in these countries. Sharma et al. (2015) shows that about 60% of NMVOC emissions in India are released from biomass burning in the residential sector.

Even as the governments across developing nations aim at providing clean cooking energy, such as LPG and natural gas to all households, it is evident that several million households will continue to depend on traditional biomass for cooking due to economic, supply, and delivery constraints. The importance of a more efficient use of biomass as a cooking fuel needs to be recognized and supported by policy. In recent years, stove development and dissemination has received an impetus with the arrival of commercial players in the stove market. However, considering that efficiency levels are still quite low (below 40%), there is a need for further research in developing core biomass-based combustion technology options that can use different (or multiple) feedstocks which can be incorporated into customized stove designs. Equally important is the need for research on the development of more efficient processed fuels that have relatively higher calorific value and produce less smoke.

Residential Lighting

Kerosene used for lighting is an important source of BC emissions. Emissions of BCs due to burning kerosene in lamps are higher than BCs emitted from biomass-based cooking. However, a declining trend in kerosene consumption in India has been evident over the past few years. This is likely to be accelerated by the success of programs such as the Rajiv Gandhi Grameen Vidyutikaran Yojana (RGGVY) and the Village Energy Security Program and Remote Village Electrification Program of the government of India. However, until all households have electricity, households using kerosene for lighting could be benefited by solar lamps, which could continue to be used even after grid-based electricity is provided. A solar lantern is a portable lighting device that uses either CFL (compact fluorescent lamp) or LEDs (light-emitting diode) and has an inbuilt battery rechargeable using a solar panel of adequate size. Solar lanterns have been promoted in India for the past several years, but their dissemination is limited due to their high upfront cost. Inadequate institutional mechanisms and delivery channels for sale and after-sale service has further hampered the penetration of this system in rural areas.

It is estimated that by providing solar lanterns to all rural households that use kerosene (for lighting) while simultaneously phasing out the kerosene subsidy, it would be possible to cover all such households in India without additional expenditure in about 4 years. It is also estimated that if a household spent on solar lighting what it currently spends on kerosene, the cost of a lantern could be recovered in about 4.3 years.

Future Projections

Cofala, Amann, Klimont, Kupiainen, and Höglund-Isaksson (2007) show the future projections of different air pollutants at the global scale. While OC emissions are expected to decrease by 30%, BC emissions are projected to decrease at a lower rate (~20%) during 2010–2030. This means an enhanced ratio of BC-to-OC emissions. This also means an enhanced ratio of warming to cooling particulates in the atmosphere in future. The OC:BC emission ratio at the global level is projected to decline from 2.1 in 2010 to 1.75 in 2030. Increased emissions of methane and NOx also suggest increased ozone formation in the troposphere and corresponding impacts. Chatani et al. (2014) showed scenarios of increased ozone concentrations in Asia as a result of increased emissions of its precursors.

Future growth in emissions is expected to translate into serious health impacts. Hutton (2011) showed that while the percentage of economic damage due to air pollution in the global GDP will decrease, overall economic damage will increase substantially. Reduction in percent share of GDP shows the effects of controls/interventions employed for improvement of air quality.

Selin et al. (2009) projected ozone levels out to 2050 and estimated that it may lead to an additional 817,000 acute mortalities (mortalities due to acute exposure) in 2050 and economic losses of $120 billion. Van Dingenen, Raes, Krol, Emberson, and Cofala (2009) estimated the impact of ozone on agriculture in the future under a BAU scenario. They estimated that by 2030, the crop yields may further decrease for wheat (additional 2–6% loss globally) and rice (additional 1–2% loss globally). India will account for 50% of these increased losses. An economic loss of $14–$26 billion is estimated, 40% of which will occur in China and India.

Next Steps

Black carbon in ABCs positively influences climate radiative forcing and leads to a warming effect in the atmosphere due to the absorption of incoming solar radiation. In addition, the masking of CO2 global warming by cooling aerosols in ABCs has been a Faustian bargain and may have even prevented society from taking drastic actions to curb CO2 emissions. Moreover, the asymmetric cooling (by aerosols) of land compared with the oceans and asymmetric cooling (by aerosols) of the Northern Hemisphere compared with the Southern Hemisphere has led to major disruption of global circulation patterns such as the South Asian monsoon system. In addition, the large-scale dimming of solar radiation has decreased tropical precipitation and contributed to droughts worldwide, especially in Asia and Africa. The aerosols in ABCs lead to the premature mortality of 4 million people every year. Surface ozone in ABCs has decreased crop yields by hundreds of millions of tons every year. Finally, the warming effect of black carbon (and organic carbon), ozone, and methane in ABCs accounts for as much as 40% of the current total anthropogenic warming. In summary, these pollution clouds have major negative impacts on the food and water security of the planet and are a major threat for public health.

Clearly there are compelling reasons to drastically mitigate air pollution. Because of their short residence times, the nations that invest in mitigation will benefit the most. Yet the recent Paris agreement of 2015 to limit global warming does not include air pollution mitigation. Likewise, the recently announced Sustainable Development Goals (SDG) do not mention air pollution or ABCs. This is surprising because the SDG focuses on poverty. About 3 billion poor people still cook with solid biomass and pay a huge price, since 3 million people die every year due to indoor air pollution from cooking smoke (which eventually becomes ABCs).

There are many scalable solutions to drastically reduce air pollution and thin the ABCs. These solutions do not inhibit economic development. For example, California has reduced its black carbon emissions by 90% since the 1960s and reduced emissions of ozone precursors by 90% during the same period. During this same period, its population doubled and its GDP grew to be the largest in the United States (Ramanathan et al., 2013). Using the California model, a 12-point action plan for reducing air pollution and ABCs from the transportation sector in India has been developed (Ramanathan et al., 2014). More, recently, a plan suggesting ten scalable solutions for control of pollution in India has been prepared by a group of scientists across the world (Sharma et al, 2016b). The possibilities to drastically reduce air pollution are enormous and can save billions of people from its ravages. All it would take is willingness on the part of nations to enact proven and scalable policies.


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