Atmospheric Brown Clouds: Long Range Transport and Climate Impacts

Atmospheric Brown Clouds: Long Range Transport and Climate Impacts V. Ramanathan and M. V. Ramana Scripps Institution of Oceanography (SIO) University of California at San Diego, La Jolla, CA 92037 Corresponding author: V. Ramanathan Center for Atmospheric Sciences (CAS) Center for Clouds, Chemistry and Climate (C4) Scripps Institute of Oceanography (SIO) University of California at San Diego (UCSD) La Jolla, CA 92037. Tel: (858) 534-8815 Fax: (858)534-7452 E-mail: vram@fiji.ucsd.edu Accepted in Environment Management, September 10, 2003 1

Atmospheric brown cloud, such as the Denver Brown Cloud, is a frequently occurring phenomenon in industrialized regions of the world. It is mistakenly assumed to be restricted to urban regions. Recent field campaigns and satellite data have revealed that due to long range transport, the brown cloud (or haze) covers vast areas of the world with potentially large impacts on regional and global climate, fresh water budget, agriculture and health. INTRODUCTION In February 1999, over 200 scientists from Europe, India and the USA gathered in the islands of the Maldives to conduct the Indian Ocean Experiment (INDOEX; http://wwwindoex.ucsd.edu/publications/). The interdisciplinary and comprehensive data gathered from several aircraft, ships, surface stations and satellites 1, 2 helped forewarn the world of a potentially major environmental problem facing the Asian region and the world. INDOEX revealed the so-called brown cloud phenomenon (see Fig. 1) spreading from the Himalayas all the way over the N. Indian Ocean region, which has a large impact on the radiative heating of the region as reported by Ramanathan and coworkers. 2 The impact on the regional gas phase chemistry of the region, as reported by Lelieveld and coworkers, 3 was also large. The fundamental message from INDOEX is that due to longrange transport, what we normally associate with urban haze can span an entire continent plus an ocean basin (see Fig. 2). Both fossil fuel and biomass burning contribute to the particles in the haze. The persistence of the haze during the long dry season from November to May, its black carbon content, the large perturbation to the radiative energy budget of the region and its simulated impact on the monsoon rainfall distribution have significant implications to the regional and global water budget, agriculture and health. The logical implication is that air pollution and climate changes are intricately linked and should be addressed under one common framework (see Ramanathan and Crutzen in http://www-atmosphericbrowncloud.ucsd.edu/). 2

Recent NASA Terra satellite results (Fig. 3) revealed that pollution aerosols are found in and downwind of all inhabited regions of the planet and each of these hazes spreads over a large region. For example, Figure 3 shows for the month of April 2001 (April-May, 2001/2002 average), anthropogenic aerosols extending from the eastern part of N. America all the way into the mid-atlantic ocean and Europe; it also shows heavy aerosol loading in north and eastern Europe and the Mediterranean sea; dust and anthropogenic haze from Mongolia and China extending all the way across the Pacific; and the biomass burning aerosols from Africa and the Amazon region extending into the Atlantic. Thus we can think of an Asian Brown Cloud, an African Brown Cloud, and a South and North American Brown cloud amongst others. Furthermore, air parcels carrying the particles can travel across an entire ocean basin or a continent within about five days as shown in Figure 4. The particle trajectory in Figure 4 is shown for an altitude of about 3 km since peak concentrations of anthropogenic particles are found even at 3 km, e.g., the Indian Ocean region (See Plate 9 in Ref [2]). In order to come to grips with this complex problem, an international program entitled Atmospheric Brown Clouds (ABC), focusing first on Asia has been created (http://www-asianbrowncloud.ucsd.edu/) under the sponsorship of the United Nations Environmental Program (UNEP) and NOAA. We now return to INDOEX findings to discuss the important environmental and climate influences of these brown clouds. The Indian Ocean Experiment Findings The south Asian brown haze covers most of the Arabian sea, Bay of Bengal and the south Asian region (an area equivalent to that of continental USA), occurs every year, extends from about November to May and possibly longer. Radiation observations from an Observatory in Maldives 4 and from space revealed that the haze absorbed a significant amount of radiation within the atmosphere accompanied by a large decrease in radiation reaching the surface. The brownish haze consists of a three kilometers thick mixture of anthropogenic sulfate, nitrate, organics, black carbon, dust and fly ash particles and natural aerosols such as sea salt and mineral dust. The brownish color is due to the absorption and the scattering of solar radiation by anthropogenic black carbon, fly ash, part of the soil dust and NO 2. 3

In-situ measurements of aerosols chemistry from aircraft, ships and surface stations clearly establish that anthropogenic sources (biomass burning and fossil fuel combustion) contribute as much as 75% to the observed aerosol concentration. The sources of the haze over the ocean are the NE trades over the western Arabian Sea and northern Indian Ocean carrying pollution from SW and central Asia; NW-NE flow along the west coast of India; and NE trades over the Bay of Bengal from the eastern part of S. Asia and SE Asia. The Uniqueness of Air Pollution in the Tropics The brown haze is a particularly severe problem over the tropics due to a variety of reasons. The large increase in emissions of aerosols and their precursors is an important reason. For example emissions of SO 2 in South Asia have increased by a factor of 3 to 4 since 1970. However, SO 2 emissions from S. Asia are only 25% of the US emissions. Hence other factors have to be invoked to account for the thickness and extent of the haze. Organic and black carbon and fly ash contribute more than SO 2 to the haze in Asia. The other important contributor is the unique meteorology of the tropics and the subtropics (including S. Asia) which leads to a long dry season extending from late fall and winter until spring. The dryness is caused by subsidence, which precludes the wet removal of haze particles by rain. In the mid and high latitudes, the absence of a long dry season, and seasonally distributed rainfall (and snow fall) cleans the atmosphere more efficiently. Impact on the Radiation Budget Aerosols, by scattering/absorbing solar and emitting/absorbing long wave (IR) radiation, change the radiation fluxes at the surface and the top of the atmosphere, thereby significantly perturbing the atmospheric absorption of solar radiation. These aerosolinduced changes in radiation budget are referred to as the direct forcing. At the surface, aerosols decrease the direct solar beam and enhance the diffuse solar radiation and both of these effects have been measured and included in INDOEX forcing values. Black carbon, which strongly absorbs solar radiation, plays a major role in the forcing by partially shielding the surface from the intense tropical solar radiation. This shielding 4

effect of BC amplifies the surface radiative forcing due to all other manmade aerosols (sulfates, organics, nitrates, fly ash) by a factor of two or more in cloudy skies. BC over the northern Indian Ocean and the Arabian Sea (during the INDOEX measurement campaign) contributes as much as 10% to 14% to aerosol mass, compared with about 5% in the suburban regions of Europe and N. America. The black carbon and other species in the haze reduce the average radiative heating of the ocean by as much as 10% and enhance the atmospheric solar radiative heating by 50 to 100%. Lastly, by nucleating more cloud drops, aerosols increase the reflection of solar radiation by clouds, which adds to the surface cooling effect. This effect is knows as the indirect forcing. In INDOEX it was shown by direct aircraft measurements 5 that the trade cumulus and strato- cumulus clouds in the polluted Arabian Sea had six times as many cloud drops as the pristine clouds south of the ITCZ. Dimming in other regions of the Planet Large reduction of seasonal averaged solar radiation of the order of 10% or larger (due to anthropogenic aerosols) is not restricted to S. Asia. It has now been observed in many regions of the planet including Atlantic, Western Pacific, Mediterranean, Europe, N. and S. America and Africa. 6, 7, 8, 9, 10, 11 More recently we have observed reduction of 10 to 15% in the Himalayas. The potential consequences of this dimming effect of aerosols are discussed next. Impacts on Climate One of the major and unique accomplishment of INDOEX is that it integrated field measurements with satellite data and aerosol assimilation models to estimate the seasonally and the regionally average direct and the indirect forcing for the anthropogenic fraction of the aerosols. The regional radiative perturbation by the anthropogenic aerosols at the surface and within the atmosphere is an order of magnitude greater than that due to the anthropogenic greenhouse gases. This does not imply that the GHGs are not important since their forcing is distributed globally and cumulative with time while the 5

aerosol forcing is concentrated regionally; what it implies however is that regional climate changes may be strongly influences by absorbing aerosols. The second message is that absorbing aerosols may have a large impact on regional hydrological cycle. This is because roughly 50% to 80% of the solar heating of the ocean is balanced by evaporation. Thus one possible response is that the large reduction in solar radiation reaching the surface (about 20 Wm -2 or 10% of the absorbed solar radiation) can lead to a reduction in evaporation which will in turn lead to a reduction in precipitation. The third is that the 50% to 100% enhancement in solar heating of the lower atmosphere can perturb the monsoonal circulation; particularly since the aerosol heating is distributed non-uniformly with latitude and longitude. Indeed, when the INDOEX haze is inserted in global climate models, 12, 13 it results in a significant perturbation of the rainfall patterns with more rains in some regions and drought in others. The haze also leads to a cooling of the land surface during the dry season which has been confirmed by observations, 14 a strengthening of low level inversion and a cooling of the N. Indian Ocean. It is interesting that the N. Indian Ocean is one of the few oceanic regions with very little increase in heat content as shown by Levitus and coworkers. 15 It is highly likely that the greenhouse warming over the N. Indian Ocean has been nearly balanced by the haze cooling with no net change in the ocean heat content. Recent studies have also demonstrated that the anthropogenic haze produces copious amounts of smaller drops in convective clouds, thus suppressing precipitation over polluted areas and favoring higher aerosol concentrations in the upper troposphere. 16 Link between Aerosols and Tropical and Global Mean Precipitation The global mean precipitation has been decreasing since the 1950s. 17 This decrease is due to a pronounced decrease in the tropics between 25 S to 30 N and a smaller increase in the extra-tropics. We must caution that the precipitation data are collected mostly from land stations. Climate models with just the greenhouse gases do not simulate the negative trend, but when sulfate aerosols are added to the model, some of the models are able to simulate a portion of the observed drying trend in the tropics. 17, 18 However, sulfates are 6

only part of the aerosol forcing problem, and the addition of black carbon can enhance the sulfate surface forcing by a factor of two or more. 2, 4 However, the aerosol pollution (particularly the BC) is generally larger in the tropics, and thus aerosols may have played a role in the observed tropical drying. For the Indian subcontinent, models (about 15 of them) with just the greenhouse gas (GHG) increase, predict an increase in the wet season (JJA) precipitation, while all fifteen of the models with GHG and sulfate aerosols show reduced rainfall increases or even strong rainfall decreases 18 (see P. 602 in Ref [18]). The link between aerosols and reduced rainfall has also been made for other tropical regions including the well known Sahelian drought; 19 north-south shift in the East Asian monsoon during the recent decades; 12, 20 and rainfall changes in the south and the south west Asian region. 13 For example, Xu 20 shows that in China the monsoon has moved southwards leading to drying in the northern part and flooding in the south and this movement started in the late 1970s. He suggests that its main cause may be the negative solar forcing by aerosols. In summary then, several recent studies, including the INDOEX study of Chung et al, 13 have suggested that anthropogenic aerosols could have perturbed the tropical as well as the monsoonal rainfall. Globally the aerosol forcing at present may be comparable to that from greenhouse gas emissions. It is important to note that the role of GHGs will increase in the future because of their long life times in the atmosphere, of the order of centuries. By the mid to late half of this century global warming due to GHGs will likely dominate the aerosol impact. The industrialized world shares the major responsibility for the emission of GHGs and some aerosol precursor gases such as SO 2. Developing nations in Asia, Africa and S. America are beginning to contribute in major ways to the aerosol problem, particularly black carbon from inefficient combustion technology and biomass burning, and the effects are particularly great during the dry season. Over 50% of the world s populations inhabit Asia. The region is experiencing impressive industrial and demographic growth rates, and thus could be very vulnerable to unexpected negative impacts from the haze on health, 21 the hydrological cycle and agriculture. 7

ACKNOWLEDGEMENTS We thank NOAA, NSF and the Vetlesen foundation for funding INDOEX and ABC research and thank UNEP for sponsoring ABC. We acknowledge NOAA for funding the ABC observatories. REFERENCES 1. Ramanathan, V., Crutzen, P. J., Kiehl, J. T. and Rosenfeld, D., Aerosols, climate, and the hydrological cycle, Science, 2001a, 294, 2119-2124. 2. Ramanathan, V., et al., The Indian Ocean Experiment: An integrated analysis of the climate forcing and effects of the Great Indo-Asian Haze, J. Geophys. Res, 2001b, 106, 28371-28398. 3. Lelieveld, J., et al., The Indian Ocean Experiment: Widespread Air Pollution from South and Southeast Asia, Science, 2001, 291, 1031-1036. 4. Satheesh, S. K., and V. Ramanathan, Large differences in tropical aerosol forcing at the top of the atmosphere and Earth s surface, Nature, 2001, 405, 60-63. 5. Heymsfield, A. J., and G. M. McFarquhar, Parameterizations of INDOEX microphysical measurements and calculations of cloud susceptibility: Applications for climate studies, J. Geophys. Res., 2001, 106, 28653-28673. 6. Kinne, S., and R. Pueschel, Aerosol radiative forcing for Asian continental outflow, Atmos. Environ, 2001, 35, 5019-5028. 7. Bush, B. C., and F. P. J. Valero, Surface aerosol radiative forcing at Gosan during ACE-Asia campaign, J. Geophys. Res.,2003, 108, 8660,doi:10.1029/2002JD003233. 8. Markowicz, K. M., P. J. Flatau, M. V. Ramana, P. J. Crutzen and V. Ramanathan, Absorbing Mediterranean aerosols lead to a large reduction in the solarradiation at the surface, Geophys. Res. Lett, 2002, 29(20), 1968, doi:10.1029/2002gl015767. 9. Kaufman, Y. J., D. Tanre, and O. Boucher, A satellite view of aerosols in the climate system, Nature, 2002, 419, 215-223. 10. Hignett, P., J. P. Taylor, P. N. Francis, and M. D. Glew, Comparison of observed and modeled direct aerosol forcing during TARFOX, J. Geophys. Res., 1999, 104, 2279-2287 8

11. Ichoku, C., L. A. Remer, Y. J. Kaufman, R. Levy, D. A. Chu, D. Tanre, and B. N. Holben, MODIS observations of aerosols and estimation of aerosol radiative forcing over southern Africa during SAFARI 2000, J. Geophys. Res., 2003, 108, 8499, doi:10.1029/2002jd002366. 12. Menon, S., J. Hansen, L. Nazarenko, and Y. Luo, Climate Effects of Black Carbon Aerosols in China and India, Science, 2002, 297, 2250-2253. 13. Chung, C. E., V. Ramanathan, and J. T. Kiehl, Effects of the South Asian Absorbing Haze on the Northeast Monsoon and Surface-Air heat exchange, J. Climate, 2002, 15, 2462-2476. 14. Krishnan, R., and V. Ramanathan, Evidence of surface cooling from absorbing aerosols, Geophys. Res. Lett., 2001, 29, 10.1029/2002GL014687. 15. Levitus, S., J. I. Antonov, J. Wang, T.L. Delworth, K. W. Dixson, and A. J. Broccoli, Anthropogenic Warming of Earth s Climate System, Science, 2001, 292, 267-270. 16. Rosenfeld, D., Suppression of Rain and Snow by Urban and Industrial Air Pollution, Science, 2000, 287, 1793-1796. 17. Hulme, M., T. J. Osborn, and T.C. John, Precipitation sensitivity to global warming: Comparison of observations with HadCM2 simulations, Geophys. Res. Lett., 1998, 25, 3379 3382. 18. IPCC, 2001: Climate Change, Synthesis Report, Intergovernmental Panel on Climate Change, Cambridge University Press, 2001. 19. Rotstayn, L., and V. Lohman, Tropical Rainfall Trends and the Indirect Aerosol Effets, J. Climate, 2002, 15, 2103-2116. 20. Xu, Q., Abrupt change of the mid-summer climate in central east China by the influence of atmospheric pollution, Atmos. Environ, 2001, 35, 5029 5040. 21. Stone, R., Counting the Cost of London s Killer Smog, Science, 2002, 298, 2106-2107. 9

Haze over the lower Himalayas, South of Mt. Everest 21 March 1999: Arabian Sea; Thick haze (9.2 N, 73.5 E) 25 March 1999: Clouds under thick haze (3.0 N, 74.5 E) 24 February 1999; Just North of ITCZ; Haze extends up to top of Cu (0.5 N, 73.3 E) 24 March 1999; South of ITCZ; Almost pristine clouds (7.5 S, 73.5 E) Figure 1: Top Photo: Photograph of the South Asian brown haze over the Nepalese town of Phaplu taken on 25 March 2001, approximately 30 km south of Mt. Everest from a flight altitude of about 3 km viewing south. Bottom 4 photos: The dry, north-east monsoonal winds carry the haze thousands of kilometers south and south eastward and spread it over most of the tropical Indian Ocean between 25 N to about 5 S (Source: Satheesh and Ramanathan 4 ). 10

Figure 2. Mean aerosol optical depth (AOD) at visible wavelength from December 2001 to May 2002. The data were obtained from the MODIS instrument onboard NASA s Terra Satellite. 9 11

Figure 3. Global distribution of mean aerosol optical depth (AOD) for April and May during 2001 and 2002. Black area represents no data. Data are from MODIS instrument onboard Terra. 9 12

Figure 4: Forward trajectories at 3 km altitude originating from India, China, Mexico, US east and west coasts, London, Paris and Berlin showing potential transcontinental nature of the Haze (courtesy of T. N. Krishnamurti). 13