Publications

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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.

2013
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.

2012
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.

2008
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.

2004
Di Girolamo, L, Bond TC, Bramer D, Diner DJ, Fettinger F, Kahn RA, Martonchik JV, Ramana MV, Ramanathan V, Rasch PJ.  2004.  Analysis of Multi-angle Imaging SpectroRadiometer (MISR) aerosol optical depths over greater India during winter 2001-2004. Geophysical Research Letters. 31   10.1029/2004gl021273   AbstractWebsite

We present the first detailed spatial analysis of a four-year, wintertime visible aerosol optical depth (AOD) climatology from the Multi-angle Imaging SpectroRadiometer (MISR) over greater India. Meteorological fields from the National Centers for Environmental Prediction (NCEP) reanalysis, topographic data, and information related to aerosol source regions are used to explain the spatial patterns in MISR AODs. High AODs are found over much of greater India. The highest AODs are over the northern Indian state of Bihar, where we show that meteorology, topography, and aerosol sources all favor development of a concentrated pool of airborne particles. MISR AODs are validated against five ground-based sites in India and Nepal, revealing similar error characteristics found in other validation studies for the MISR aerosol product.

2001
Eck, TF, Holben BN, Dubovik O, Smirnov A, Slutsker I, Lobert JM, Ramanathan V.  2001.  Column-integrated aerosol optical properties over the Maldives during the northeast monsoon for 1998-2000. Journal of Geophysical Research-Atmospheres. 106:28555-28566.   10.1029/2001jd000786   AbstractWebsite

Measurements made during the Indian Ocean Experiment (INDOEX) have shown the presence of large aerosol loadings over the region of the northern Indian Ocean and Arabian Sea. In recent years there has been significant interannual variability in the magnitude of this aerosol loading during the NE monsoon months of. January-April. Monitoring of the integrated atmospheric column effective aerosol optical properties was initiated in early 1998 and continued in 2000 on the island of Kaashidhoo in the Republic of Maldives. An Aerosol Robotic Network Sun-sky radiometer at the Kaashidhoo Climate Observatory made spectral measurements of the direct Sun and directional sky radiances which were utilized to infer spectral aerosol optical depths tau (a), single scattering albedos, asymmetry factors, and aerosol size distributions. Monthly average aerosol optical depths at 500 nm varied by more than a factor of 2 during January through April for the 3 years that were investigated, 1998-2000. Interannual variations in the monthly mean Angstrom wavelength exponent were also observed, resulting from differences in the bimodal aerosol size distributions. Spectral variations in the Angstrom wavelength exponent were observed, especially at high aerosol optical depths when fine mode aerosols dominated over the optical influence of coarse-mode aerosols. Some differences in spectral single scattering albedo and asymmetry factor were observed for 1999 versus 2000 in the infrared wavelengths, but with relatively little change in the visible wavelengths. The spectral variation in the retrieved single scattering albedo was large, with approximately linear wavelength dependence averaging from 0.91 at 440 nm to 0.83 at 1020 nm for January-March 1999 for observations where tau (a) at 440 nm >0.4.