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2019
Chen, SJ, Russell LM, Cappa CD, Zhang XL, Kleeman MJ, Kumar A, Liu D, Ramanathana V.  2019.  Comparing black and brown carbon absorption from AERONET and surface measurements at wintertime Fresno. Atmospheric Environment. 199:164-176.   10.1016/j.atmosenv.2018.11.032   AbstractWebsite

The radiative impacts of black carbon (BC) and brown carbon (BrC) are widely recognized but remain highly uncertain. The Aerosol Robotic Network (AERONET) provides measurements of aerosol optical depth (AOD), aerosol absorption optical depth (AAOD), and other parameters. AERONET AAOD measurements have been used to estimate the relative contributions of BC and BrC to the total absorption at select sites and have the potential to be used across the global network, but the accuracy of the partitioning method has not been established and the uncertainties not characterized. We made surface-level measurements of aerosol optical properties from January 13 to February 10, 2013, and from December 25, 2014, to January 13, 2015, at Fresno, California. The contribution of BrC and BC to the absorption at 405 nm was estimated from the surface-level measurements using a combined mass absorption coefficient and thermodenuder method. The surface-level measurements were compared with BC and BrC absorption at 440 nm estimated from AERONET measurements of the absolute AAOD and the absorption angstrom ngstrom exponent (AERONET-AAE method). In 2013, AERONET results showed that BC and BrC contributed 67% and 33%, respectively, of absorption at 440 nm while the surface-level measurements showed that BC and BrC contributed 89% and 11%, respectively, of absorption at 405 nm. In 2014, AERONET results showed BC and BrC absorption were 72% and 28%, respectively, and the BC and BrC surface measurements were 68% and 32%, respectively. The boundary layer conditions showed that the comparison between AERONET measurements and surface-based estimates was more appropriate in 2014 than in 2013. As a result, AERONET measurements and surface-based estimates had strong or moderate correlations and slopes near unity in 2014. Thus, surface measurements were more representative of column BC and BrC absorption in 2014.

2016
Pistone, K, Praveen PS, Thomas RM, Ramanathan V, Wilcox EM, Bender FAM.  2016.  Observed correlations between aerosol and cloud properties in an Indian Ocean trade cumulus regime. Atmospheric Chemistry and Physics. 16:5203-5227.   10.5194/acp-16-5203-2016   AbstractWebsite

There are many contributing factors which determine the micro- and macrophysical properties of clouds, including atmospheric vertical structure, dominant meteorological conditions, and aerosol concentration, all of which may be coupled to one another. In the quest to determine aerosol effects on clouds, these potential relationships must be understood. Here we describe several observed correlations between aerosol conditions and cloud and atmospheric properties in the Indian Ocean winter monsoon season. In the CARDEX (Cloud, Aerosol, Radiative forcing, Dynamics EXperiment) field campaign conducted in February and March 2012 in the northern Indian Ocean, continuous measurements were made of atmospheric precipitable water vapor (PWV) and the liquid water path (LWP) of trade cumulus clouds, concurrent with measurements of water vapor flux, cloud and aerosol vertical profiles, meteorological data, and surface and total-column aerosol from instrumentation at a ground observatory and on small unmanned aircraft. We present observations which indicate a positive correlation between aerosol and cloud LWP only when considering cases with low atmospheric water vapor (PWV < 40aEuro-kg m(-2)), a criterion which acts to filter the data to control for the natural meteorological variability in the region. We then use the aircraft and ground-based measurements to explore possible mechanisms behind this observed aerosol-LWP correlation. The increase in cloud liquid water is found to coincide with a lowering of the cloud base, which is itself attributable to increased boundary layer humidity in polluted conditions. High pollution is found to correlate with both higher temperatures and higher humidity measured throughout the boundary layer. A large-scale analysis, using satellite observations and meteorological reanalysis, corroborates these covariations: high-pollution cases are shown to originate as a highly polluted boundary layer air mass approaching the observatory from a northwesterly direction. The source air mass exhibits both higher temperatures and higher humidity in the polluted cases. While the warmer temperatures may be attributable to aerosol absorption of solar radiation over the subcontinent, the factors responsible for the coincident high humidity are less evident: the high-aerosol conditions are observed to disperse with air mass evolution, along with a weakening of the high-temperature anomaly, while the high-humidity condition is observed to strengthen in magnitude as the polluted air mass moves over the ocean toward the site of the CARDEX observations. Potential causal mechanisms of the observed correlations, including meteorological or aerosol-induced factors, are explored, though future research will be needed for a more complete and quantitative understanding of the aerosol-humidity relationship.

Hopner, F, Bender FAM, Ekman AML, Praveen PS, Bosch C, Ogren JA, Andersson A, Gustafsson O, Ramanathan V.  2016.  Vertical profiles of optical and microphysical particle properties above the northern Indian Ocean during CARDEX 2012. Atmospheric Chemistry and Physics. 16:1045-1064.   10.5194/acp-16-1045-2016   AbstractWebsite

A detailed analysis of optical and microphysical properties of aerosol particles during the dry winter monsoon season above the northern Indian Ocean is presented. The Cloud Aerosol Radiative Forcing Experiment (CARDEX), conducted from 16 February to 30 March 2012 at the Maldives Climate Observatory on Hanimaadhoo island (MCOH) in the Republic of the Maldives, used autonomous unmanned aerial vehicles (AUAV) to perform vertical in situ measurements of particle number concentration, particle number size distribution as well as particle absorption coefficients. These measurements were used together with surface-based Mini Micro Pulse Lidar (MiniMPL) observations and aerosol in situ and off-line measurements to investigate the vertical distribution of aerosol particles. Air masses were mainly advected over the Indian subcontinent and the Arabian Peninsula. The mean surface aerosol number concentration was 1717 +/- 604cm(-3) and the highest values were found in air masses from the Bay of Bengal and Indo-Gangetic Plain (2247 +/- 370cm(-3)). Investigations of the free tropospheric air showed that elevated aerosol layers with up to 3 times higher aerosol number concentrations than at the surface occurred mainly during periods with air masses originating from the Bay of Bengal and the Indo-Gangetic Plain. This feature is different compared to what was observed during the Indian Ocean Experiment (INDOEX) conducted in winter 1999, where aerosol number concentrations generally decreased with height. In contrast, lower particle absorption at the surface (sigma(abs)(520nm) = 8.5 + 4.2Wm(-1)) was found during CARDEX compared to INDOEX 1999. Layers with source region specific single-scattering albedo (SSA) values were derived by combining vertical in situ particle absorption coefficients and scattering coefficients calculated with Mie theory. These SSA layers were utilized to calculate vertical particle absorption profiles from MiniMPL profiles. SSA surface values for 550 nm for dry conditions were found to be 0 : 94 +/- 0 : 02 and 0 : 91 +/- 0 : 02 for air masses from the Arabian Sea (and Middle East countries) and India (and Bay of Bengal), respectively. Lidar-derived particle absorption coefficient profiles showed both a similar magnitude and structure as the in situ profiles measured with the AUAV. However, primarily due to insufficient accuracy in the SSA estimates, the lidar-derived absorption coefficient profiles have large uncertainties and are generally weakly correlated to vertically in situ measured particle absorption coefficients. Furthermore, the mass absorption efficiency (MAE) for the northern Indian Ocean during the dry monsoon season was calculated to determine equivalent black carbon (EBC) concentrations from particle absorption coefficient measurements. A mean MAE of 11.6 and 6.9m(2) g(-1) for 520 and 880 nm, respectively, was found, likely representing internally mixed BC containing particles. Lower MAE values for 880 and 520 nm were found for air masses originating from dust regions such as the Arabian Peninsula and western Asia (MAE(880 nm) = 5.6m(2) g(-1), MAE(520 nm) = 9.5m(2) g(-1)) or from closer source regions as southern India (MAE(880 nm) = 4.3m(2) g(-1), MAE(520 nm) = 7. 3m(2) g(-1)).

2015
Sambandam, S, Balakrishnan K, Ghosh S, Sadasivam A, Madhav S, Ramasamy R, Samanta M, Mukhopadhyay K, Rehman H, Ramanathan V.  2015.  Can currently available advanced combustion biomass cook-stoves provide health relevant exposure reductions? Results from initial assessment of select commercial models in india Ecohealth. 12:25-41.   10.1007/s10393-014-0976-1   AbstractWebsite

Household air pollution from use of solid fuels is a major contributor to the national burden of disease in India. Currently available models of advanced combustion biomass cook-stoves (ACS) report significantly higher efficiencies and lower emissions in the laboratory when compared to traditional cook-stoves, but relatively little is known about household level exposure reductions, achieved under routine conditions of use. We report results from initial field assessments of six commercial ACS models from the states of Tamil Nadu and Uttar Pradesh in India. We monitored 72 households (divided into six arms to each receive an ACS model) for 24-h kitchen area concentrations of PM2.5 and CO before and (1-6 months) after installation of the new stove together with detailed information on fixed and time-varying household characteristics. Detailed surveys collected information on user perceptions regarding acceptability for routine use. While the median percent reductions in 24-h PM2.5 and CO concentrations ranged from 2 to 71% and 10-66%, respectively, concentrations consistently exceeded WHO air quality guideline values across all models raising questions regarding the health relevance of such reductions. Most models were perceived to be sub-optimally designed for routine use often resulting in inappropriate and inadequate levels of use. Household concentration reductions also run the risk of being compromised by high ambient backgrounds from community level solid-fuel use and contributions from surrounding fossil fuel sources. Results indicate that achieving health relevant exposure reductions in solid-fuel using households will require integration of emissions reductions with ease of use and adoption at community scale, in cook-stove technologies. Imminent efforts are also needed to accelerate the progress towards cleaner fuels.

2013
Cazorla, A, Bahadur R, Suski KJ, Cahill JF, Chand D, Schmid B, Ramanathan V, Prather KA.  2013.  Relating aerosol absorption due to soot, organic carbon, and dust to emission sources determined from in-situ chemical measurements. Atmospheric Chemistry and Physics. 13:9337-9350.   10.5194/acp-13-9337-2013   AbstractWebsite

Estimating the aerosol contribution to the global or regional radiative forcing can take advantage of the relationship between the spectral aerosol optical properties and the size and chemical composition of aerosol. Long term global optical measurements from observational networks or satellites can be used in such studies. Using in-situ chemical mixing state measurements can help us to constrain the limitations of such estimates. In this study, the Absorption Angstrom Exponent (AAE) and the Scattering Angstrom Exponent (SAE) derived from 10 operational AERONET sites in California are combined for deducing chemical speciation based on wavelength dependence of the optical properties. In addition, in-situ optical properties and single particle chemical composition measured during three aircraft field campaigns in California between 2010 and 2011 are combined in order to validate the methodology used for the estimates of aerosol chemistry using spectral optical properties. Results from this study indicate a dominance of mixed types in the classification leading to an underestimation of the primary sources, however secondary sources are better classified. The distinction between carbonaceous aerosols from fossil fuel and biomass burning origins is not clear, since their optical properties are similar. On the other hand, knowledge of the aerosol sources in California from chemical studies help to identify other misclassification such as the dust contribution.

Anenberg, SC, Balakrishnan K, Jetter J, Masera O, Mehta S, Moss J, Ramanathan V.  2013.  Cleaner cooking solutions to achieve health, climate, and economic cobenefits. Environmental Science & Technology. 47:3944-3952.   10.1021/es304942e   AbstractWebsite
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2012
Bahadur, R, Praveen PS, Xu YY, Ramanathan V.  2012.  Solar absorption by elemental and brown carbon determined from spectral observations. Proceedings of the National Academy of Sciences of the United States of America. 109:17366-17371.   10.1073/pnas.1205910109   AbstractWebsite

Black carbon (BC) is functionally defined as the absorbing component of atmospheric total carbonaceous aerosols (TC) and is typically dominated by soot-like elemental carbon (EC). However, organic carbon (OC) has also been shown to absorb strongly at visible to UV wavelengths and the absorbing organics are referred to as brown carbon (BrC), which is typically not represented in climate models. We propose an observationally based analytical method for rigorously partitioning measured absorption aerosol optical depths (AAOD) and single scattering albedo (SSA) among EC and BrC, using multiwavelength measurements of total (EC, OC, and dust) absorption. EC is found to be strongly absorbing (SSA of 0.38) whereas the BrC SSA varies globally between 0.77 and 0.85. The method is applied to the California region. We find TC (EC + BrC) contributes 81% of the total absorption at 675 nm and 84% at 440 nm. The BrC absorption at 440 nm is about 40% of the EC, whereas at 675 nm it is less than 10% of EC. We find an enhanced absorption due to OC in the summer months and in southern California (related to forest fires and secondary OC). The fractions and trends are broadly consistent with aerosol chemical-transport models as well as with regional emission inventories, implying that we have obtained a representative estimate for BrC absorption. The results demonstrate that current climate models that treat OC as nonabsorbing are underestimating the total warming effect of carbonaceous aerosols by neglecting part of the atmospheric heating, particularly over biomass-burning regions that emit BrC.

Chung, CE, Ramanathan V, Decremer D.  2012.  Observationally constrained estimates of carbonaceous aerosol radiative forcing. Proceedings of the National Academy of Sciences of the United States of America. 109:11624-11629.   10.1073/pnas.1203707109   AbstractWebsite

Carbonaceous aerosols (CA) emitted by fossil and biomass fuels consist of black carbon (BC), a strong absorber of solar radiation, and organic matter (OM). OM scatters as well as absorbs solar radiation. The absorbing component of OM, which is ignored in most climate models, is referred to as brown carbon (BrC). Model estimates of the global CA radiative forcing range from 0 to 0.7 Wm(-2), to be compared with the Intergovernmental Panel on Climate Change's estimate for the pre-Industrial to the present net radiative forcing of about 1.6 Wm(-2). This study provides a model-independent, observationally based estimate of the CA direct radiative forcing. Ground-based aerosol network data is integrated with field data and satellite-based aerosol observations to provide a decadal (2001 through 2009) global view of the CA optical properties and direct radiative forcing. The estimated global CA direct radiative effect is about 0.75 Wm(-2) (0.5 to 1.0). This study identifies the global importance of BrC, which is shown to contribute about 20% to 550-nm CA solar absorption globally. Because of the inclusion of BrC, the net effect of OM is close to zero and the CA forcing is nearly equal to that of BC. The CA direct radiative forcing is estimated to be about 0.65 (0.5 to about 0.8) Wm(-2), thus comparable to or exceeding that by methane. Caused in part by BrC absorption, CAs have a net warming effect even over open biomass-burning regions in Africa and the Amazon.

Shindell, D, Kuylenstierna JCI, Vignati E, Van Dingenen R, Amann M, Klimont Z, Anenberg SC, Muller N, Janssens-Maenhout G, Raes F, Schwartz J, Faluvegi G, Pozzoli L, Kupiainen K, Hoglund-Isaksson L, Emberson L, Streets D, Ramanathan V, Hicks K, Oanh NTK, Milly G, Williams M, Demkine V, Fowler D.  2012.  Simultaneously mitigating near-term climate change and improving human health and food security. Science. 335:183-189.   10.1126/science.1210026   AbstractWebsite

Tropospheric ozone and black carbon (BC) contribute to both degraded air quality and global warming. We considered similar to 400 emission control measures to reduce these pollutants by using current technology and experience. We identified 14 measures targeting methane and BC emissions that reduce projected global mean warming similar to 0.5 degrees C by 2050. This strategy avoids 0.7 to 4.7 million annual premature deaths from outdoor air pollution and increases annual crop yields by 30 to 135 million metric tons due to ozone reductions in 2030 and beyond. Benefits of methane emissions reductions are valued at $700 to $5000 per metric ton, which is well above typical marginal abatement costs ( less than $250). The selected controls target different sources and influence climate on shorter time scales than those of carbon dioxide-reduction measures. Implementing both substantially reduces the risks of crossing the 2 degrees C threshold.

Kim, D, Ramanathan V.  2012.  Improved estimates and understanding of global albedo and atmospheric solar absorption. Geophysical Research Letters. 39   10.1029/2012gl053757   AbstractWebsite

This study integrates available surface-based and satellite observations of solar radiation at the surface and the top of the atmosphere (TOA) with a comprehensive set of satellite observations of atmospheric and surface optical properties and a Monte Carlo Aerosol-Cloud-Radiation (MACR) model to estimate the three fundamental components of the planetary solar radiation budget: Albedo at the TOA; atmospheric solar absorption; and surface solar absorption. The MACR incorporates most if not all of our current understanding of the theory of solar radiation physics including modern spectroscopic water vapor data, minor trace gases, absorbing aerosols including its effects inside cloud drops, 3-D cloud scattering effects. The model is subject to a severe test by comparing the simulated solar radiation budget with data from 34 globally distributed state-of-the art BSRN (Baseline Surface Radiation Network) land stations which began data collection in the mid 1990s. The TOA over these sites were obtained from the CERES (Cloud and Earth's Radiant Energy System) satellites. The simulated radiation budget was within 2 Wm(-2) for all three components over the BSRN sites. On the other hand, over these same sites, the IPCC-2007 simulation of atmospheric absorption is smaller by 7-8 Wm(-2). MACR was then used with a comprehensive set of model input from satellites to simulate global solar radiation budget. The simulated planetary albedo of 29.0% confirms the value (28.6%) observed by CERES. We estimate the atmospheric absorption to be 82 +/- 8 Wm(-2) to be compared with the 67Wm(-2) by IPCC models as of 2001 and updated to 76Wm(-2) by IPCC-2007. The primary reasons for the 6 Wm(-2) larger solar absorption in our estimates are: updated water vapor spectroscopic database (similar to 1 Wm(-2)), inclusion of minor gases (similar to 0.5 Wm(-2)), black and brown carbon aerosols (similar to 4Wm(-2)), the inclusion of black carbon in clouds (similar to 1 Wm(-2)) and 3-D effect of clouds (similar to 1 Wm(-2)). The fundamental deduction from our study is the remarkable consistency between satellite measurements of the radiation budget and the parameters (aerosols, clouds and surface reflectivity) which determine the radiation budget. Because of this consistency we can account for and explain the global solar radiation budget of the planet within few Wm(-2). Citation: Kim, D., and V. Ramanathan (2012), Improved estimates and understanding of global albedo and atmospheric solar absorption, Geophys. Res. Lett., 39, L24704, doi:10.1029/2012GL053757.

Kennel, CF, Ramanathan V, Victor DG.  2012.  Coping with climate change in the next half-century. Proceedings of the American Philosophical Society. 156:398-415. AbstractWebsite
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2011
Bahadur, R, Feng Y, Russell LM, Ramanathan V.  2011.  Impact of California's air pollution laws on black carbon and their implications for direct radiative forcing. Atmospheric Environment. 45:1162-1167.   10.1016/j.atmosenv.2010.10.054   AbstractWebsite

We examine the temporal and the spatial trends in the concentrations of black carbon (BC) - recorded by the IMPROVE monitoring network for the past 20 years - in California. Annual average BC concentrations in California have decreased by about 50% from 0.46 mu g m(-3) in 1989 to 0.24 mu gm(-3) in 2008 compared to the corresponding reductions in diesel BC emissions (also about 50%) from a peak of 0.013 Tg Yr(-1) in 1990 to 0.006 Tg Yr(-1) by 2008. We attribute the observed negative trends to the reduction in vehicular emissions due to stringent statewide regulations. Our conclusion that the reduction in diesel emissions is a primary cause of the observed BC reduction is also substantiated by a significant decrease in the ratio of BC to non-BC aerosols. The absorption efficiency of aerosols at visible wavelengths - determined from the observed scattering coefficient and the observed BC - also decreased by about 50% leading to a model-inferred negative direct radiative forcing (a cooling effect) of -1.4 W m(-2) (+/- 60%) over California. (C) 2010 Elsevier Ltd. All rights reserved.

2010
Ramanathan, V, Xu YY.  2010.  The Copenhagen Accord for limiting global warming: Criteria, constraints, and available avenues. Proceedings of the National Academy of Sciences of the United States of America. 107:8055-8062.   10.1073/pnas.1002293107   AbstractWebsite

At last, all the major emitters of greenhouse gases (GHGs) have agreed under the Copenhagen Accord that global average temperature increase should be kept below 2 degrees C. This study develops the criteria for limiting the warming below 2 degrees C, identifies the constraints imposed on policy makers, and explores available mitigation avenues. One important criterion is that the radiant energy added by human activities should not exceed 2.5 (range: 1.7-4) watts per square meter (Wm(-2)) of the Earth's surface. The blanket of man-made GHGs has already added 3 (range: 2.6-3.5) Wm(-2). Even if GHG emissions peak in 2015, the radiant energy barrier will be exceeded by 100%, requiring simultaneous pursuit of three avenues: (i) reduce the rate of thickening of the blanket by stabilizing CO(2) concentration below 441 ppm during this century (a massive decarbonization of the energy sector is necessary to accomplish this Herculean task), (ii) ensure that air pollution laws that reduce the masking effect of cooling aerosols be made radiant energy-neutral by reductions in black carbon and ozone, and (iii) thin the blanket by reducing emissions of short-lived GHGs. Methane and hydrofluorocarbons emerge as the prime targets. These actions, even if we are restricted to available technologies for avenues ii and iii, can reduce the probability of exceeding the 2 degrees C barrier before 2050 to less than 10%, and before 2100 to less than 50%. With such actions, the four decades we have until 2050 should be exploited to develop and scale-up revolutionary technologies to restrict the warming to less than 1.5 degrees C.

Chung, CE, Ramanathan V, Carmichael G, Kulkarni S, Tang Y, Adhikary B, Leung LR, Qian Y.  2010.  Anthropogenic aerosol radiative forcing in Asia derived from regional models with atmospheric and aerosol data assimilation. Atmospheric Chemistry and Physics. 10:6007-6024.   10.5194/acp-10-6007-2010   AbstractWebsite

An estimate of monthly 3-D aerosol solar heating rates and surface solar fluxes in Asia from 2001 to 2004 is described here. This product stems from an Asian aerosol assimilation project, in which a) the PNNL regional model bounded by the NCEP reanalyses was used to provide meteorology, b) MODIS and AERONET data were integrated for aerosol observations, c) the Iowa aerosol/chemistry model STEM-2K1 used the PNNL meteorology and assimilated aerosol observations, and d) 3-D (X-Y-Z) aerosol simulations from the STEM-2K1 were used in the Scripps Monte-Carlo Aerosol Cloud Radiation (MACR) model to produce total and anthropogenic aerosol direct solar forcing for average cloudy skies. The MACR model and STEM-2K1 both used the PNNL model resolution of 0.45 degrees x 0.4 degrees in the horizontal and of 23 layers in the troposphere. The 2001-2004 averaged anthropogenic all-sky aerosol forcing is -1.3 Wm(-2) (TOA), +7.3 Wm(-2) (atmosphere) and -8.6 Wm(-2) (surface) averaged in Asia (60-138 degrees E and Equator -45 degrees N). In the absence of AERONET SSA assimilation, absorbing aerosol concentration (especially BC aerosol) is much smaller, giving -2.3 Wm(-2) (TOA), +4.5 Wm(-2) (atmosphere) and -6.8 Wm(-2) (surface), averaged in Asia. In the vertical, monthly forcing is mainly concentrated below 600 hPa with maximum around 800 hPa. Seasonally, low-level forcing is far larger in dry season than in wet season in South Asia, whereas the wet season forcing exceeds the dry season forcing in East Asia. The anthropogenic forcing in the present study is similar to that in Chung et al. (2005) in overall magnitude but the former offers fine-scale features and simulated vertical profiles. The interannual variability of the computed anthropogenic forcing is significant and extremely large over major emission outflow areas. Given the interannual variability, the present study's estimate is within the implicated range of the 1999 INDOEX result.

2009
Molina, M, Zaelke D, Sarma KM, Andersen SO, Ramanathan V, Kaniaru D.  2009.  Reducing abrupt climate change risk using the Montreal Protocol and other regulatory actions to complement cuts in CO2 emissions. Proceedings of the National Academy of Sciences of the United States of America. 106:20616-20621.   10.1073/pnas.0902568106   AbstractWebsite

Current emissions of anthropogenic greenhouse gases (GHGs) have already committed the planet to an increase in average surface temperature by the end of the century that may be above the critical threshold for tipping elements of the climate system into abrupt change with potentially irreversible and unmanageable consequences. This would mean that the climate system is close to entering if not already within the zone of "dangerous anthropogenic interference'' (DAI). Scientific and policy literature refers to the need for "early,''"urgent,''"rapid," and "fast-action" mitigation to help avoid DAI and abrupt climate changes. We define "fast-action'' to include regulatory measures that can begin within 2-3 years, be substantially implemented in 5-10 years, and produce a climate response within decades. We discuss strategies for short-lived non-CO(2) GHGs and particles, where existing agreements can be used to accomplish mitigation objectives. Policy makers can amend the Montreal Protocol to phase down the production and consumption of hydrofluorocarbons (HFCs) with high global warming potential. Other fast-action strategies can reduce emissions of black carbon particles and precursor gases that lead to ozone formation in the lower atmosphere, and increase biosequestration, including through biochar. These and other fast-action strategies may reduce the risk of abrupt climate change in the next few decades by complementing cuts in CO(2) emissions.

Flanner, MG, Zender CS, Hess PG, Mahowald NM, Painter TH, Ramanathan V, Rasch PJ.  2009.  Springtime warming and reduced snow cover from carbonaceous particles. Atmospheric Chemistry and Physics. 9:2481-2497. AbstractWebsite

Boreal spring climate is uniquely susceptible to solar warming mechanisms because it has expansive snow cover and receives relatively strong insolation. Carbonaceous particles can influence snow coverage by warming the atmosphere, reducing surface-incident solar energy (dimming), and reducing snow reflectance after deposition (darkening). We apply a range of models and observations to explore impacts of these processes on springtime climate, drawing several conclusions: 1) Nearly all atmospheric particles (those with visible-band single-scatter albedo less than 0.999), including all mixtures of black carbon (BC) and organic matter (OM), increase net solar heating of the atmosphere-snow column. 2) Darkening caused by small concentrations of particles within snow exceeds the loss of absorbed energy from concurrent dimming, thus increasing solar heating of snowpack as well (positive net surface forcing). Over global snow, we estimate 6-fold greater surface forcing from darkening than dimming, caused by BC+OM. 3) Equilibrium climate experiments suggest that fossil fuel and biofuel emissions of BC+OM induce 95% as much springtime snow cover loss over Eurasia as anthropogenic carbon dioxide, a consequence of strong snow-albedo feedback and large BC+OM emissions from Asia. 4) Of 22 climate models contributing to the IPCC Fourth Assessment Report, 21 underpredict the rapid warming (0.64 degrees C decade(-1)) observed over springtime Eurasia since 1979. Darkening from natural and anthropogenic sources of BC and mineral dust exerts 3-fold greater forcing on spring-time snow over Eurasia (3.9W m(-2)) than North America (1.2 W m(-2)). Inclusion of this forcing significantly improves simulated continental warming trends, but does not reconcile the low bias in rate of Eurasian spring snow cover decline exhibited by all models, likely because BC deposition trends are negative or near-neutral over much of Eurasia. Improved Eurasian warming may therefore relate more to darkening-induced reduction in mean snow cover.

2008
Spencer, MT, Holecek JC, Corrigan CE, Ramanathan V, Prather KA.  2008.  Size-resolved chemical composition of aerosol particles during a monsoonal transition period over the Indian Ocean. Journal of Geophysical Research-Atmospheres. 113   10.1029/2007jd008657   AbstractWebsite

An aerosol time-of-flight mass spectrometer (ATOFMS) was used to measure the size-resolved mixing state of particles over the northern Indian Ocean in October and November 2004. This period was chosen to observe the impact of the monsoonal transition on the size, chemistry, sources, and radiative properties of atmospheric aerosols in the region. Overall, elemental carbon with sulfate (EC-sulfate), biomass/biofuel burning, fresh sea salt (SS), aged sea salt, fly ash, and EC mixed with sea salt were the dominant supermicron particle types, whereas EC-sulfate, biomass/biofuel burning, and fly ash were the dominant submicron particle types. Interestingly, particles composed mostly of aged organic carbon and nitrate were virtually absent during the campaign. This is possibly from low ozone formation in the region or selective scavenging during transport. Notably, during long-range transport periods when an aethalometer measured the highest black carbon concentrations, 77% of submicron particles between 0.5 and 2.5 mm and 71% of EC/soot particles contained an intense (39)K(+) ion (a known tracer for biomass/biofuel combustion). These observations suggest when the air mass originated from India, biofuel combustion represented a significant source of the regional atmospheric brown cloud. The majority (similar to 80%) of EC and biomass/biofuel burning particles were mixed with significant amounts of sulfate due to extensive secondary processing of these particles during transport. EC mixed with sea salt was also observed suggesting the particles had undergone cloud processing and become internally mixed during transport. These measurements support the use of an internal mixture of sulfate with EC/soot and biomass/biofuel burning in models to accurately calculate radiative forcing by aerosols in this region.

2007
Ramanathan, V, Li F, Ramana MV, Praveen PS, Kim D, Corrigan CE, Nguyen H, Stone EA, Schauer JJ, Carmichael GR, Adhikary B, Yoon SC.  2007.  Atmospheric brown clouds: Hemispherical and regional variations in long-range transport, absorption, and radiative forcing. Journal of Geophysical Research-Atmospheres. 112   10.1029/2006jd008124   AbstractWebsite

The study uses satellite observations, global assimilated aerosol data sets, Atmospheric Brown Clouds (ABC) observatories, a Monte Carlo aerosol-cloud-radiation model and a regional chemical transport model (STEM-2K) to characterize the spatial extent of brown clouds, regional and megacity ABC hot spots, chemical composition and the direct radiative forcing. It presents the first annual cycle of aerosol observations and forcing from the ABC observatories in the Indo-Asia-Pacific regions. East Asia, Indo-Gangetic Plains, Indonesian region, southern Africa and the Amazon basin are the regional hot spots defined by the criteria that anthropogenic aerosol optical depths (AODs) should exceed 0.3 and absorbing AOD > 0.03. Over these hot spots, as well as in other polluted oceanic regions, the EC mass exceeds 0.5 mu g m(-3), the OC mass exceeds 2 mg m(-)3 and sulfate mass exceeds 10 mg m -3 from the surface to 3 km. The brown clouds also have strong seasonal dependence. In the tropics the seasonal dependence is driven by pollution accumulating during the dry seasons, December to February in Northern Hemisphere tropics and June to August in Southern Hemisphere tropics. In the extratropics the pollution peaks during the summer. The brown cloud problem is not restricted to the tropical regions. Over the eastern half of US and western Europe the AODs exceeds 0.2 and absorption AODs exceed 0.02. Brown clouds also extend well into the western Pacific Ocean, the Indian Ocean reaching as far south as 60 degrees S and the eastern Atlantic Ocean. The largest total SO(2) emission occurs over China and US, while SO(2) emission per unit surface area is maximum over Germany and England. The largest total EC and OC emissions occur over China, but the largest OC emission per unit surface area occur over India. As a result, the maximum negative annual mean TOA direct forcing is over India and Germany. The surface annual-diurnal mean dimming over the regional hot spots is of the order of -10 W m(-2) and -20 W m(-2) over megacity hotpots.

Adhikary, B, Carmichael GR, Tang Y, Leung LR, Qian Y, Schauer JJ, Stone EA, Ramanathan V, Ramana MV.  2007.  Characterization of the seasonal cycle of south Asian aerosols: A regional-scale modeling analysis. Journal of Geophysical Research-Atmospheres. 112   10.1029/2006jd008143   AbstractWebsite

The sulfur transport and deposition model ( STEM) is used to study the aerosol seasonality, distribution, and composition over south Asia from September 2004 to August 2005. Model predictions of sulfate, black carbon, primary organic carbon, other anthropogenic particulate matter, windblown mineral dusts, and sea salt are compared at two sites in south Asia where yearlong experimental observations are available from the Atmospheric Brown Cloud ( ABC) project. The model predictions are able to capture both the magnitude and seasonality of aerosols over Hanimaadhoo Observatory, Maldives. However, the model is not able to explain the seasonality at the Kathmandu Observatory; but the model does capture Kathmandu's observed annual mean concentration. The absence of seasonal brick kiln emissions within Kathmandu valley in the current inventory is a probable reason for this problem. This model study reveals high-anthropogenic aerosol loading over the Ganges valley even in the monsoonal months, which needs to be corroborated by experimental observations. Modeling results also show a high dust loading over south Asia with a distinct seasonality. Model results of aerosol monthly composition are also presented at five cities in south Asia. Total and fine-mode monthly aerosol optical depth along with contribution from each aerosol species is presented; the results show that the anthropogenic fraction dominates in the postmonsoon and the early dry season with major contributions from sulfate and absorbing aerosols. Model sensitivity studies of dry deposition velocity and wet scavenging efficiency show that model improvements are needed in the treatment of carbonaceous aerosol dry and wet removal processes. Modeled SO2 conversion rate constrained with sulfate observations at Hanimaadhoo suggests the need to increase model sulfate production rate during the dry season to account for probable sulfate production via heterogeneous pathways.

2006
Corrigan, CE, Ramanathan V, Schauer JJ.  2006.  Impact of monsoon transitions on the physical and optical properties of aerosols. Journal of Geophysical Research-Atmospheres. 111   10.1029/2005jd006370   AbstractWebsite

Project Atmospheric Brown Cloud ( ABC- Asia) has focused on measuring the anthropogenic influence of aerosols, including black carbon, to determine the extent of sunlight dimming and radiative forcing over the Asian region. As part of this project, an observatory was built in the Republic of Maldives for the long- term monitoring of climate. An inaugural campaign was conducted to investigate the influence of the shifting monsoon seasons on aerosols and climate change. The presence of black carbon and other anthropogenic aerosols over the Indian Ocean varies with the cyclic nature of the Indian Monsoon. Roughly every 6 months, the winds change directions from southwest to northeast or vice versa. From June to October the wet monsoon brings clean air into the region from the Southern Hemisphere. Conversely, the dry monsoon brings polluted air from the Indian subcontinent and Southeast Asia from November through April. As a result, the region becomes charged with black carbon and other anthropogenic pollutants during the dry monsoon. In 2004 the transition between the clean and polluted seasons resulted in nearly an order of magnitude increase of scattering and absorbing aerosols. The change was foreshadowed with small events over a 1 month period prior to the abrupt arrival of pollution over a period of a few days as air from India and Southeast Asia arrived in the Maldives at the surface level. The new, polluted aerosol was characteristically darker since the black carbon concentration increased more substantially than the overall aerosol scattering. As a result, the aerosol coalbedo at a wavelength of 550 nm showed an increase from an average of 0.028 to 0.07. Black carbon mass concentrations increased by an order of magnitude from 0.03 to 0.47 mu g/m(3). These measurements suggest a large increase in the aerosol radiative forcing of the region with the arrival of the dry monsoon.

Mikhailov, EF, Vlasenko SS, Podgorny IA, Ramanathan V, Corrigan CE.  2006.  Optical properties of soot-water drop agglomerates: An experimental study. Journal of Geophysical Research-Atmospheres. 111   10.1029/2005jd006389   AbstractWebsite

[ 1] Black carbon ( BC) and organic carbon (OC) are the largest contributors to the aerosol absorption in the atmosphere, yet the absorption cross sections of BC and OC per unit mass are subject to a large uncertainty due to morphology, physicochemical properties, and the mixing state of carbonaceous particles. Theoretical studies suggest the possibility of an enhanced absorption by soot - cloud drop agglomerates; however, the magnitude of the effect has never been measured directly and remains highly uncertain. This study is a laboratory experiment aimed at the modeling of direct radiation forcing due to soot-water interaction in the presence of glutaric acid, a water-soluble OC. Specifically, we generate, in the laboratory, hydrophobic soot ( acetylene soot) and hydrophilic soot ( mixture of acetylene soot and glutaric acid) and investigate the structural and optical properties of hydrophobic and hydrophilic soot particles in dry and water-saturated air. Hydrophobic soot (HBS) particles do not exhibit any structural or morphological differences under dry and saturated conditions, whereas hydrophilic soot (HLS) particles, i.e., BC with a monolayer of glutaric acid, collapse into globules when relative humidity ( RH) is increased to saturation. The optical properties of HBS show very little dependence on RH while HLS scattering and absorption coefficient increase markedly with RH. For the cases considered here, the maximum enhancement in absorption for a soot - water drop mixture was as much as a factor of 3.5, very similar to theoretical predictions. The data provided in this study should advance the treatment of polluted cloud layers in climate models.

2005
Chung, CE, Ramanathan V, Kim D, Podgorny IA.  2005.  Global anthropogenic aerosol direct forcing derived from satellite and ground-based observations. Journal of Geophysical Research-Atmospheres. 110   10.1029/2005jd006356   AbstractWebsite

[1] A global estimate of the direct effects of anthropogenic aerosols on solar radiation in cloudy skies is obtained by integrating satellite and ground-based observations with models of aerosol chemistry, transport, and radiative transfer. The models adopt global distribution of aerosol optical depths ( from MODIS), clouds, water vapor, ozone, and surface albedo from various satellite climatology. Gaps and errors in satellite derived aerosol optical depths are filled and corrected by surface network (AERONET), and an aerosol chemical-transport model (GOCART) by using statistical techniques. Using these derived aerosol properties and other related variables, we generate climatological monthly mean anthropogenic aerosol forcing for both clear and average cloudy skies. Unless otherwise stated, our estimates are for average cloudy skies, also referred to as all sky conditions. The global annual mean direct forcing is - 0.35 W m(-2) ( range of - 0.6 to - 0.1 Wm(-2)) at the top-of-the atmosphere (TOA), + 3.0 Wm(-2) ( range of + 2.7 to + 3.3Wm(-2)) in the atmosphere, and - 3.4 Wm(-2) ( range of - 3.5 to - 3.3 Wm(-2)) at the surface. The uncertainty of about 10 - 20% in the surface and atmosphere forcing translates into a six fold uncertainty in the TOA forcing because the TOA forcing is a small sum of two large terms ( surface and atmosphere) of opposing signs. Given the current state of observations and modeling, it is very difficult to further reduce the uncertainty in the estimated TOA forcing. The major contributors to the uncertainty in atmospheric absorption are from the uncertainty in the vertical distribution of aerosols and the single scattering albedo of aerosols. The TOA forcing in clear skies is a factor of two different, while the surface and atmosphere forcing terms differ by only about 10 - 25%. Another major finding of this study is that the reduction in the surface solar radiation is a factor of 10 larger than the reduction in net solar ( down minus up) radiation at TOA. The TOA forcing changes sign regionally, whereas the surface forcing is always negative. Thus caution must be exercised against relying too strongly on assessing the aerosol impacts based solely on global mean forcing. Aerosols over the NH contribute about 64% to the global surface forcing. Regionally the populated tropical regions contribute the most to the global surface forcing, with Asia the largest contributor. Roughly 49% of the total surface forcing is over the oceanic regions. Most of the previous global aerosol forcing estimate studies were conducted with a chemical transport model coupled to a general circulation model with model generated aerosols and cloudiness. Thus the present study, which adopts observed aerosol properties and observed three dimensional cloudiness, provides an independent approach for estimating the aerosol forcing.

Ramanathan, V, Chung C, Kim D, Bettge T, Buja L, Kiehl JT, Washington WM, Fu Q, Sikka DR, Wild M.  2005.  Atmospheric brown clouds: Impacts on South Asian climate and hydrological cycle. Proceedings of the National Academy of Sciences of the United States of America. 102:5326-5333.   10.1073/pnas.0500656102   AbstractWebsite

South Asian emissions of fossil fuel SO2 and black carbon increased approximate to 6-fold since 1930, resulting in large atmospheric concentrations of black carbon and other aerosols. This period also witnessed strong negative trends of surface solar radiation, surface evaporation, and summer monsoon rainfall. These changes over India were accompanied by an increase in atmospheric stability and a decrease in sea surface temperature gradients in the Northern Indian Ocean. We conducted an ensemble of coupled ocean-atmosphere simulations from 1930 to 2000 to understand the role of atmospheric brown clouds in the observed trends. The simulations adopt the aerosol radiative forcing from the Indian Ocean experiment observations and also account for global increases in greenhouse gases and sulfate aerosols. The simulated decreases in surface solar radiation, changes in surface and atmospheric temperatures over land and sea, and decreases in monsoon rainfall are similar to the observed trends. We also show that greenhouse gases and sulfates, by themselves, do not account for the magnitude or even the sign in many instances, of the observed trends. Thus, our simulations suggest that absorbing aerosols in atmospheric brown clouds may have played a major role in the observed regional climate and hydrological cycle changes and have masked as much as 50% of the surface warming due to the global increase in greenhouse gases. The simulations also raise the possibility that, if current trends in emissions continue, the subcontinent may experience a doubling of the drought frequency in the coming decades.