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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.

Patange, OS, Ramanathan N, Rehman IH, Tripathi SN, Misra A, Kar A, Graham E, Singh L, Bahadur R, Ramanathan V.  2015.  Reductions in indoor black carbon concentrations from improved biomass stoves in rural India. Environmental Science & Technology. 49:4749-4756.   10.1021/es506208x   AbstractWebsite
Feng, Y, Ramanathan V, Kotamarthi VR.  2013.  Brown carbon: A significant atmospheric absorber of solar radiation? Atmospheric Chemistry and Physics. 13:8607-8621.   10.5194/acp-13-8607-2013   AbstractWebsite

Several recent observational studies have shown organic carbon aerosols to be a significant source of absorption of solar radiation. The absorbing part of organic aerosols is referred to as "brown" carbon (BrC). Using a global chemical transport model and a radiative transfer model, we estimate for the first time the enhanced absorption of solar radiation due to BrC in a global model. The simulated wavelength dependence of aerosol absorption, as measured by the absorption Angstrolm exponent (AAE), increases from 0.9 for non-absorbing organic carbon to 1.2 (1.0) for strongly (moderately) absorbing BrC. The calculated AAE for the strongly absorbing BrC agrees with AERONET spectral observations at 440-870 nm over most regions but overpredicts for the biomass burning-dominated South America and southern Africa, in which the inclusion of moderately absorbing BrC has better agreement. The resulting aerosol absorption optical depth increases by 18% (3 %) at 550 nm and 56% (38 %) at 380 nm for strongly (moderately) absorbing BrC. The global simulations suggest that the strongly absorbing BrC contributes up to +0.25 Wm(-2) or 19% of the absorption by anthropogenic aerosols, while 72% is attributed to black carbon, and 9% is due to sulfate and non-absorbing organic aerosols coated on black carbon. Like black carbon, the absorption of BrC (moderately to strongly) inserts a warming effect at the top of the atmosphere (TOA) (0.04 to 0.11 Wm(-2)), while the effect at the surface is a reduction (-0.06 to -0.14 Wm(-2)). Inclusion of the strongly absorption of BrC in our model causes the direct radiative forcing (global mean) of organic carbon aerosols at the TOA to change from cooling (-0.08 Wm(-2)) to warming (+0.025 Wm(-2)). Over source regions and above clouds, the absorption of BrC is higher and thus can play an important role in photochemistry and the hydrologic cycle.

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.

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.

Praveen, PS, Ahmed T, Kar A, Rehman IH, Ramanathan V.  2012.  Link between local scale BC emissions in the Indo-Gangetic Plains and large scale atmospheric solar absorption. Atmospheric Chemistry and Physics. 12:1173-1187.   10.5194/acp-12-1173-2012   AbstractWebsite

Project Surya has documented indoor and outdoor concentrations of black carbon (BC) from traditional biomass burning cook stoves in a rural village located in the Indo-Gangetic Plains (IGP) region of N. India from November 2009-September 2010. In this paper, we systematically document the link between local scale aerosol properties and column averaged regional aerosol optical properties and atmospheric radiative forcing. We document observations from the first phase of Project Surya and estimate the source dependent (biomass and fossil fuels) aerosol optical properties from local to regional scale. Data were collected using surface based observations of BC, organic carbon (OC), aerosol light absorption, scattering coefficient at the Surya village (SVI_1) located in IGP region and integrated with satellite and AERONET observations at the regional scale (IGP). The daily mean BC concentrations at SVI_1 showed a large increase of BC during the dry season (December to February) with values reaching 35 mu g m(-3). Space based LIDAR data revealed how the biomass smoke was trapped within the first kilometer during the dry season and extended to above 5 km during the pre-monsoon season. As a result, during the dry season, the variance in the daily mean single scattering albedo (SSA), the ratio of scattering to extinction coefficient, and column aerosol optical properties at the local IGP site correlated (with slopes in the range of 0.85 to 1.06 and R-2 > 0.4) well with the "IGP_AERONET" (mean of six AERONET sites). The statistically significant correlation suggested that in-situ observations can be used to derive spatial mean forcing, at least for the dry season. The atmospheric forcing due to BC and OC exceeded 20 Wm(-2) during all months from November to May, supporting the deduction that elimination of cook stove smoke emissions through clean cooking technologies will likely have a major positive impact not only on human health but also on regional climate.

Feng, Y, Ramanathan V.  2010.  Investigation of aerosol-cloud interactions using a chemical transport model constrained by satellite observations. Tellus Series B-Chemical and Physical Meteorology. 62:69-86.   10.1111/j.1600-0889.2009.00444.x   AbstractWebsite

This study simulates optical depth of marine warm clouds for year 2001 based on interactively predicted aerosol concentrations with a global chemical transport model (CTM) driven by the ERA-40 re-analysis meteorological data. The simulated aerosol and cloud droplet number concentrations (CDNC) largely reproduce the variations between polluted and pristine marine environment as revealed by surface and aircraft measurements. By constraining cloud liquid water path (CLWP) with satellite microwave measurements, the simulated global and southern hemispheric aerosol optical depth (AOD) and cloud optical depth (COD) are well within 10% of the observed values. As a result of larger anthropogenic aerosol loadings over the northern oceans, the simulated CDNC and COD are, respectively, by 51 and 18% higher than those over the southern oceans, while the column-averaged droplet effective radius is 13% smaller. These simulated interhemispheric differences, while qualitatively consistent with satellite observations, are larger than the observations. Inclusion of drizzle effect improved the disparities but not entirely. The constrained CTM generally captures the seasonality in AOD and CLWP observations, and demonstrates that annual cycle of COD is dominated by CLWP. During winter monsoon the simulated and observed COD correlate more strongly with changes in AOD over the N. Indian Ocean.

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.

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.

Corrigan, CE, Roberts GC, Ramana MV, Kim D, Ramanathan V.  2008.  Capturing vertical profiles of aerosols and black carbon over the Indian Ocean using autonomous unmanned aerial vehicles. Atmospheric Chemistry and Physics. 8:737-747. AbstractWebsite

Measurements of the vertical distribution of aerosol properties provide essential information for generating more accurate model estimates of radiative forcing and atmospheric heating rates compared with employing remotely sensed column averaged properties. A month long campaign over the Indian Ocean during March 2006 investigated the interaction of aerosol, clouds, and radiative effects. Routine vertical profiles of aerosol and water vapor were determined using autonomous unmanned aerial vehicles equipped with miniaturized instruments. Comparisons of these airborne instruments with established ground-based instruments and in aircraft-to-aircraft comparisons demonstrated an agreement within 10%. Aerosol absorption optical depths measured directly using the unmanned aircraft differed from columnar AERONET sun-photometer results by only 20%. Measurements of total particle concentration, particle size distributions, aerosol absorption and black carbon concentrations are presented along with the trade wind thermodynamic structure from the surface to 3000 m above sea level. Early March revealed a well-mixed layer up to the cloud base at 500 m above mean sea level (m a.s.l.), followed by a decrease of aerosol concentrations with altitude. The second half of March saw the arrival of a high altitude plume existing above the mixed layer that originated from a continental source and increased aerosol concentrations by more than tenfold, yet the surface air mass showed little change in aerosol concentrations and was still predominantly influenced by marine sources. Black carbon concentrations at 1500 m above sea level increased from 70 ng/m(3) to more than 800 ng/m(3) with the arrival of this polluted plume. The absorption aerosol optical depth increased from as low as 0.005 to as much as 0.035 over the same period. The spectral dependence of the aerosol absorption revealed an absorption Angstrom exponent of 1.0, which is typical of an aerosol with most of its absorption attributed to black carbon and generally indicates the absorbing component originated from fossil fuel sources and other high-temperature combustion sources. The results indicate that surface measurements do not represent the aerosol properties within the elevated layers, especially if these layers are influenced by long range transport.

Nakajima, T, Yoon SC, Ramanathan V, Shi GY, Takemura T, Higurashi A, Takamura T, Aoki K, Sohn BJ, Kim SW, Tsuruta H, Sugimoto N, Shimizu A, Tanimoto H, Sawa Y, Lin NH, Lee CT, Goto D, Schutgens N.  2007.  Overview of the Atmospheric Brown Cloud East Asian Regional Experiment 2005 and a study of the aerosol direct radiative forcing in east Asia. Journal of Geophysical Research-Atmospheres. 112   10.1029/2007jd009009   AbstractWebsite

This article introduces an international regional experiment, East Asian Regional Experiment 2005 (EAREX 2005), carried out in March-April 2005 in the east Asian region, as one of the first phase regional experiments under the UNEP Atmospheric Brown Cloud (ABC) project, and discusses some outstanding features of aerosol characteristics and its direct radiative forcing in the east Asian region, with some comparison with the results obtained in another ABC early phase regional experiment, ABC Maldives Monsoon Experiment (APMEX) conducted in the south Asian region. Time series of aerosol optical thickness (AOT), single scattering albedo (SSA), aerosol extinction cross section profile and CO concentration shows that air pollutants and mineral dust were transported every 5 to 7 days in the EAREX region to produce SSA values at wavelength of 700 nm from 0.86 to 0.96 and large clear-sky shortwave forcing efficiency at 500 nm from 60 W m(-2) to 90 W m(-2), though there are some unexplained inconsistencies depending on the evaluation method. The simulated whole-sky total forcing in the EAREX region is -1 to -2 W m(-2) at TOA and -2 to -10 W m(-2) at surface in March 2005 which is smaller in magnitude than in the APMEX region, mainly because of large cloud fraction in this region (0.70 at Gosan versus 0.51 at Hanimadhoo in the ISCCP total cloud fraction). We suggest there may be an underestimation of the forcing due to overestimation of the simulated cloudiness and aerosol scale height. On the other hand, the possible error in the simulated surface albedo may cause an overestimation of the magnitude of the forcing over the land area. We also propose simple formulae for shortwave radiative forcing to understand the role of aerosol parameters and surface condition to determine the aerosol forcing. Such simple formulae are useful to check the consistency among the observed quantities.

Ramana, MV, Ramanathan V.  2006.  Abrupt transition from natural to anthropogenic aerosol radiative forcing: Observations at the ABC-Maldives Climate Observatory. Journal of Geophysical Research-Atmospheres. 111   10.1029/2006jd007063   AbstractWebsite

[1] Using aerosol-radiation observations over the north Indian Ocean, we show how the monsoon transition from southwest to northeast flow gives rise to a similar transition in the direct aerosol radiative forcing from natural to anthropogenic forcing. These observations were taken at the newly built aerosol-radiation-climate observatory at the island of Hanimaadhoo (6.776 degrees N, 73.183 degrees E) in the Republic of Maldives. This observatory is established as a part of Project Atmospheric Brown Clouds (ABC) and is referred to as the ABC-Maldives Climate Observatory at Hanimaadhoo (ABC_MCOH). The transition from the southwest monsoon during October to the northeast monsoon flow during early November occurs abruptly over a period of few weeks over ABC-MCOH and reveals a dramatic contrast between the natural marine aerosols transported from the south Indian Ocean by the southwest monsoon and that of the polluted aerosols transported from the south and Southeast Asian region by the northeast monsoon. We document the change in the microphysical properties and the irradiance at the surface, to identify the human signature on aerosol radiative forcing. We first establish the precision of surface radiometric observations by comparing simultaneous observations using calibrated Kipp & Zonen and Eppley pyrheliometers and pyranometers for direct, diffuse and global solar radiation. We show that the direct, diffuse and global radiation can be measured within a precision of about 3 to 5 Wm(-2). Furthermore, when we include the observed aerosol optical properties as input into the Monte Carlo Aerosol Cloud Radiation (MACR) model (developed by us using Indian Ocean Experiment data), the simulated fluxes agree with the observed direct, diffuse and global fluxes within the measurement accuracy. A steady southwest monsoon flow of about 5 to 7 ms(-1) persists until middle of October which switches to an abrupt change in direction to northeast flow of similar speeds bringing in polluted air from south Asia. However, it is not until end of November that a steady northeasterly flow is well established. The abrupt transition is accompanied by a large increase in aerosol optical depth from about 0.1 in October to as high as 0.4 during January, the SSA decreases from 1 to about 0.9, and the Angstrom coefficient increases from about 0.5 (suggesting large particles > 1 micron) to about 1.2 in January (submicron particles) and an increase in aerosol extinction below 3 km altitude. These changes are consistent with the transport of continental pollution from south and Southeast Asia (about 1000 to several 1000 km away from ABC_MCOH) to the north Indian Ocean during the northeast monsoon. The direct aerosol forcing, determined solely from radiometric observations without resorting to models, changes from -5 Wm(-2) during October to -22 Wm(-2) during January. About 50% of this forcing occurs in the photosynthetically active part of the solar spectrum (0.4 to 0.7 micron). MACR shows that the decrease in SSA from 1 to 0.9 changes the aerosol forcing efficiency by a factor of about 2 from about -40 Wm(-2) (per AOD) in October to -80 Wm(-2) (per AOD) in January. Thus the arrival of the brown clouds from south and Southeast Asia has a large seasonal dimming effect over remote parts of the north Indian Ocean. The observational results presented here should be used for validating climate models that attempt to simulate the anthropogenic effects of aerosol forcing on climate. The observational and model results presented in his study shows how near continuous surface based observations can be used to differentiate the human impact on aerosol forcing which is a major challenge for models.

Roberts, G, Mauger G, Hadley O, Ramanathan V.  2006.  North American and Asian aerosols over the eastern Pacific Ocean and their role in regulating cloud condensation nuclei. Journal of Geophysical Research-Atmospheres. 111   10.1029/2005jd006661   AbstractWebsite

[ 1] Measurements of aerosol and cloud properties in the Eastern Pacific Ocean were taken during an airborne experiment on the University of Wyoming's King Air during April 2004 as part of the Cloud Indirect Forcing Experiment (CIFEX). We observed a wide variety of aerosols, including those of long-range transport from Asia, clean marine boundary layer, and North American emissions. These aerosols, classified by their size distribution and history, were found in stratified layers between 500 to 7500 m above sea level and thicknesses from 100 to 3000 m. A comparison of the aerosol size distributions to measurements of cloud condensation nuclei (CCN) provides insight to the CCN activity of the different aerosol types. The overall ratio of measured to predicted CCN concentration (NCCN) is 0.56 +/- 0.41 with a relationship of N-CCN,N- measured = N-CCN, predicted(0.846 +/- 0.002) for 23 research flights and 1884 comparisons. Such a relationship does not accurately describe a CCN closure; however, it is consistent with our measurements that high CCN concentrations are more influenced by anthropogenic sources, which are less CCN active. While other CCN closures have obtained results closer to the expected 1: 1 relationship, the different aerosol types ( and presumably differences in aerosol chemistry) are responsible for the discrepancy. The measured N-CCN at 0.3% supersaturation (S-c) ranged from 20 cm(-3) (pristine) to 350 cm(-3) ( anthropogenic) with an average of 106 +/- 54 cm(-3) over the experiment. The inferred supersaturation in the clouds sampled during this experiment is similar to 0.3%. CCN concentrations of cloud-processed aerosol were well predicted using an ammonium sulfate approximation for S-c <= 0.4%. Predicted N-CCN for other aerosol types (i.e., Asian and North American aerosols) were high compared to measured values indicating a less CCN active aerosol. This study highlights the importance of chemical effects on CCN measurements and introduces a CCN activation index as a method of classifying the efficiency of an aerosol to serve as CCN relative to an ammonium sulfate particle. This index ranged from close to unity for cloud processed aerosols to as low as 0.31 for aged aerosols transported from Asia. We also compare the performance of two CCN instruments ( static thermal diffusion chamber and streamwise continuous flow chamber) on a 45 minute level leg where we observe an aged layer and a nucleation event. More than 50% of the aged aerosol served as CCN at 0.2% S-c, primarily owing to their large size, while CCN concentrations during the nucleation event were close to 0 cm(-3). CCN concentrations from both instruments agreed within instrument errors; however, the continuous flow chamber effectively captured the rapid transition in aerosol properties.

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.

Li, F, Vogelmann AM, Ramanathan V.  2004.  Saharan dust aerosol radiative forcing measured from space. Journal of Climate. 17:2558-2571.   10.1175/1520-0442(2004)017<2558:sdarfm>;2   AbstractWebsite

This study uses data collected from the Clouds and the Earth's Radiant Energy System (CERES) and the Moderate Resolution Imaging Spectroradiometer ( MODIS) instruments to determine Saharan dust broadband shortwave aerosol radiative forcing over the Atlantic Ocean near the African coast (15degrees-25degreesN, 45degrees-15degreesW). The clear-sky aerosol forcing is derived directly from these data, without requiring detailed information about the aerosol properties that are not routinely observed such as chemical composition, microphysical properties, and their height variations. To determine the diurnally averaged Saharan dust radiative forcing efficiency (i.e., broadband shortwave forcing per unit optical depth at 550 nm, W m(-2) tau(alpha)(-1)), two extreme seasons are juxtaposed: the high-dust months [June-August (JJA)] and the low-dust months [November-January (NDJ)]. It is found that the top-of-atmosphere (TOA) diurnal mean forcing efficiency is -35 +/- 3 W m(-2) tau(alpha)(-1) for JJA, and -26 +/- 3 W m(-2) tau(alpha)(-1) for NDJ. These efficiencies can be fit by reducing the spectrally varying aerosol single-scattering albedo such that its value at 550 nm is reduced from 0.95 +/- 0.04 for JJA to about 0.86 +/- 0.04 for NDJ. The lower value for the low-dust months might be influenced by biomass-burning aerosols that were transported into the study region from equatorial Africa. Although the high-dust season has a greater ( absolute value of the) TOA forcing efficiency, the low-dust season may have a greater surface forcing efficiency. Extrapolations based on model calculations suggest the surface forcing efficiencies to be about -65 W m(-2) tau(alpha)(-1) for the high-dust season versus -81 W m(-2) tau(alpha)(-1)for the low-dust season. These observations indicate that the aerosol character within a region can be readily modified, even immediately adjacent to a powerful source region such as the Sahara. This study provides important observational constraints for models of dust radiative forcing.

Podgorny, IA, Li F, Ramanathan V.  2003.  Large aerosol radiative forcing due to the 1997 Indonesian forest fire. Geophysical Research Letters. 30   10.1029/2002gl015979   AbstractWebsite

[1] During the last decade, the feedback between El Nino and biomass burning caused the Indonesia's forest fire aerosols to be the second most significant source of anthropogenic aerosol over the tropical Indian Ocean after the South Asian Haze. In this paper, the estimates of the radiative forcing during the 1997 Indonesia's forest fire have been obtained by integrating satellite derived aerosol optical depths and cloud cover with in-situ observations of single scattering albedo and a Monte-Carlo Aerosol-Cloud radiation model. The haze reduced the seasonal average solar radiation absorbed by the equatorial Indian ocean by as much as 30 to 60 W m(-2) during September to November 1997, and increased the atmospheric solar heating by as much as 50% to 100% within the first 3 kilometers. The radiative forcing at the top of the atmosphere (TOA) was in the range of 5 to 15 W m(-2) under cloudy skies. The significance of such large radiative flux changes to the tropical ocean-atmosphere heat budget and climate needs to be examined with climate models.

Markowicz, KM, Flatau PJ, Ramana MV, Crutzen PJ, Ramanathan V.  2002.  Absorbing mediterranean aerosols lead to a large reduction in the solar radiation at the surface. Geophysical Research Letters. 29   10.1029/2002gl015767   AbstractWebsite

[1] We present direct radiometric observations of aerosol radiative forcing taken during the MINOS experiment (2001) at Finokalia Sampling Station located on North-Eastern shores of Crete, Greece. The mean value of aerosol optical thickness was 0.21 at 500 nm. Aerosols, mostly of anthropogenic origin, lead to a diurnal average reduction of 17.9 W m(-2) in the surface solar radiation, an increase of 11.3 W m(-2) in the atmospheric solar absorption, and an increase of 6.6 W m(-2) in the reflected solar radiation at the top-of-the atmosphere. Thus, the present data gives observational proof for the large role of absorbing aerosols in the Mediterranean. The negative surface forcing and large positive atmospheric forcing values observed for the Mediterranean aerosols is nearly identical to the highly absorbing south Asian haze observed over the Arabian Sea.

Satheesh, SK, Ramanathan V, Holben BN, Moorthy KK, Loeb NG, Maring H, Prospero JM, Savoie D.  2002.  Chemical, microphysical, and radiative effects of Indian Ocean aerosols. Journal of Geophysical Research-Atmospheres. 107   10.1029/2002jd002463   AbstractWebsite

[1] Extensive and long-term multistation measurements of aerosol properties and radiative fluxes were carried out in the haze plume off the South Asian continent. These experiments are carried out at Kaashidhoo Climate Observatory (KCO) (4.95 degreesN, 73.5 degreesE), Minicoy (8.5 degreesN, 73.0 degreesE), and Trivandrum (8.5 degreesN, 77.0 degreesE). In addition, the top of the atmosphere fluxes were measured simultaneously by the CERES radiation budget instrument. Long-term observations (more than 15 years) over Trivandrum show that there is a gradual increase in aerosol visible optical depth from similar to0.2 in 1986 to similar to0.4 in 1999. Pre- and post-monsoon aerosol characteristics are examined to study the seasonal variations. The impact of aerosols on short-wave radiation budget is estimated using direct observations of solar radiation using several independent ground-based radiometers and satellite data as well as from modeled aerosol properties. It was observed that "excess absorption'' is not needed to model diffuse fluxes. The lower atmospheric heating due to absorbing aerosols was as high as similar to20 W m(-2) which translates to a heating rate perturbation of similar to0.5degreesK/day. The effect of aerosol mixing state (internally and externally) on aerosol forcing appears to be negligible. A sensitivity study of the effect of aerosols over land in contrast to that over the ocean shows an enhancement in lower atmosphere heating by about 40% simultaneous with a reduction of similar to33% in surface cooling. Increasing sea-surface winds increase aerosol cooling due to increased sea salt aerosol concentrations, which can partly offset the heating effect due to absorbing aerosols.

Lelieveld, J, Crutzen PJ, Ramanathan V, Andreae MO, Brenninkmeijer CAM, Campos T, Cass GR, Dickerson RR, Fischer H, de Gouw JA, Hansel A, Jefferson A, Kley D, de Laat ATJ, Lal S, Lawrence MG, Lobert JM, Mayol-Bracero OL, Mitra AP, Novakov T, Oltmans SJ, Prather KA, Reiner T, Rodhe H, Scheeren HA, Sikka D, Williams J.  2001.  The Indian Ocean Experiment: Widespread air pollution from South and Southeast Asia. Science. 291:1031-1036.   10.1126/science.1057103   AbstractWebsite

The Indian Ocean Experiment (INDOEX) was an international, multiplatform field campaign to measure Long-range transport of air pollution from South and Southeast Asia toward the Indian Ocean during the dry monsoon season in January to March 1999. Surprisingly high pollution Levels were observed over the entire northern Indian Ocean toward the Intertropical Convergence Zone at about 6 degreesS. We show that agricultural burning and especially biofuel use enhance carbon monoxide concentrations. Fossil fuel combustion and biomass burning cause a high aerosol Loading. The growing pollution in this region gives rise to extensive air quality degradation with Local, regional, and global implications, including a reduction of the oxidizing power of the atmosphere.

Jayaraman, A, Satheesh SK, Mitra AP, Ramanathan V.  2001.  Latitude gradient in aerosol properties across the inter tropical convergence zone: Results from the joint Indo-US study onboard Sagar Kanya. Current Science. 80:128-137. AbstractWebsite

As part of the Indian Ocean Experiment (INDOEX) Intensive Field Phase (IFP), a cruise by ORV Sagar Kanya was conducted in the Arabian Sea and the Indian Ocean from 20 January to 12 March 1999. Measurements on aerosol properties such as optical depth, mass concentration, size distribution, scattering and absorption coefficients were measured using instruments such as sun-photometer, quartz crystal microbalance, nephelometer and particle-soot absorption photometer. One of the important findings is the large north-south asymmetry in the aerosol characteristics. Aerosol optical depth values were very high, exceeding 0.4, close to the west coast of India and the Arabian Sea, which is greater than by a factor of 4 or more, compared to the values south of the ITCZ. The wavelength exponent alpha is found to be in the range of 1.3 to 1.7 in the high optical depth region and is in the range of 0.5 to 0.7 over the pristine region. Aerosol mass concentration data show that the nucleation mode aerosols (radius < 0.1 mum) are systematically higher over the Arabian Sea, with values ranging from 20 to 50 mug/m(3). Correlating the aerosol mass with the scattering coefficient, we get a scattering to mass concentration ratio of 2.27 m(2)/g, for the Arabian Sea region, which is in between the values reported by other workers, 3.3 m(2)/g for the continent and 1.67 m(2)/g for the marine regions, elsewhere in the world. The single scattering albedo, omega derived from the scattering and absorption data, is around 0.9 for aerosols found over the Arabian Sea, while near the coastal regions the values are as low as 0.8. Low omega and high optical depth found in the coastal region and Arabian Sea indicate large absorption by aerosols. The results undoubtedly show a large spatial difference in aerosol characteristics between north and south of the ITCZ which could lead to a large asymmetry in aerosol radiative forcing between the two regions.