A14B-01
Radiative forcing by brown carbon and mixed black carbon: a probabilistic approach
Climate-relevant properties of organic carbon aerosols, including hygroscopicity and light absorption, are not well known. Most sources emit a mix of black (strongly-absorbing) carbon, organic (less-absorbing) carbon, and secondary aerosol precursors. Therefore, the climate-relevant properties of these materials and their mixtures should be known in order to determine the effect of individual sources on radiative forcing. Although direct radiative forcing due to organic carbon might be smaller than that of other aerosol constituents, this contribution might determine whether forcing by individual sources is positive or negative. We recommend UV-visible spectra and hygroscopic properties that are appropriate for representing organic carbon. These values are based on literature synthesis and laboratory experiments. Organic material from some sources absorbs light ("brown carbon"), while other organic carbon does not. We implement these properties in the Community Atmosphere Model (CAM), along with a global emission inventory which identifies the sources from which brown carbon is emitted. In addition, we use new parameterizations of mixing between strongly absorbing aerosol (black carbon) and non-absorbing aerosol. We estimate total radiative forcing by mixed and unmixed black carbon, brown carbon, and non-absorbing carbon. Finally, we present a two-box model used to calculate radiative forcing. Source, transport and sink processes are calibrated to match recent AEROCOM model results. Normalized forcing (forcing per mass) is taken from CAM simulations. We use reported atmospheric and laboratory measurements to constrain uncertainties in emission rates, transport processes, and optical properties and perform Monte Carlo simulations of forcing due to individual sources. We also estimate the contribution of secondary aerosols and indirect effects. Transport processes, particularly wet deposition and convection, are large sources of uncertainty. Despite uncertainties, the probability that aerosols from some emissions sources produce positive radiative forcing is quite high. For other sources, uncertainties prevent unequivocal determination of the sign of radiative forcing.
A14B-02
Atmospheric Elemental Carbon Concentrations in 1835 to 2005
Atmospheric elemental carbon concentrations, {EC}atm, were determined for the past ~170 y in northeastern United States by using a novel technique. Assuming wet and dry deposition is the only source of EC in the remote and high altitude lakes, measured elemental carbon concentrations in the lake sediment, {EC}sed, can be related to {EC}atm, by {EC} = K{EC}atm, where K (m3 g-1) is constant for a given lake and depend upon sedimentation processes. The value of constant K was determined: (1) by measuring {EC}atm in the monthly composites of daily filters collected at Whiteface Mountain, NY from 1978 to 2005 using the thermal optical method; and (2) {EC}sed in four bottom sediment cores from West Pine Pond, a remote lake in the vicinity of Whiteface Mountain. The mean {EC}atm for 1978-1986, 1987-1996, and 1997-2005 periods were 550 ± 150, 225 ± 65, and 66 ± 15 ng m-3, respectively. Thus, the {EC}atm has consistently decreased from ~ 1978, and decreased very sharply in 1997. The core was sectioned, and ages determined using 210Pb technique. The {EC} in the sediment samples were determined by chemically separating EC and measured its concentration using the thermal optical method. The {EC}sed for the 2005 -1978 period mimicked the directly measured {EC}atm in the filters, supporting the hypothesis that the relationship between atmospheric and lake sediment EC concentrations are essentially a constant. From the {EC}atm and {EC}sed K was determined to be 10,400 ± 4,400 m3 g-1. From K and {EC}sed we calculated {EC}atm for the 1978 to 1835 period. {EC}atm varied from 62 to 760 ng m-3 between 1835 and 2005. The {EC}atm show monotonic increase from 1835, peaked around 1920 and then decreased gradually till 1980. After 1980, {EC}Satm decreased sharply to the present. The {EC}atm was compared with Nouvakou et.al.'s [GRL 30, doi: 10.1029/2002GLO16345] black carbon (BC) emissions based on consumption of fossil fuel and showed good correlation.
A14B-03
Sensitivity of sulfate direct climate forcing to the hysteresis of particle phase transitions
We investigate the effect of the hysteresis of solid-aqueous phase transitions of sulfate-ammonium particles on sulfate direct climate forcing (SDCF) by using both a column model and a global chemical transport model. Aqueous particles have a larger mass extinction efficiency but a smaller backscattered fraction than their solid counterparts. The column model shows that hysteresis can result in an uncertainty in the SDCF of 20%. The global chemical transport model explicitly accounts for the relative humidity processing of particles and the associated hysteresis. The model also treats the extent of sulfate neutralization by ammonia. The base case simulation finds that solid particles contribute 41% of the global sulfate burden of the anthropogenic component, 26% of the clear-sky optical thickness, 31% of the clear-sky SDCF, and 37% of the full-sky SDCF, a trend that reflects the correlation of solid particles with clear skies. A perturbation to the model, omitting hysteresis by assuming that all particles are aqueous, results in an overestimate of the SDCF by +8% compared to base case. A converse assumption that crystallization occurs at the deliquescence relative humidity underestimates the SDCF by -8%. A case that assumes that aqueous particles occur whenever the ambient relative humidity exceeds the crystallization relative humidity biases the SDCF by +5%. A case that includes hysteresis but omits the difference in the fraction of radiation backscattered to space by aqueous compared to solid particles changes the SDCF by +15%. Seasonal and regional differences can be much larger. We recommend that the ratio of the sulfate aerosol optical thickness calculated with vs. without consideration of particle hygroscopicity be reported as a standard parameter to facilitate meaningful SDCF intercomparisons among different models.
A14B-04 INVITED
Response of a Vapor Field to Injection of Aerosols: Characteristic Time Scale
What is a good measure of "fast" or "slow" when discussing the impact of aerosol injections on the development of a growing cloud? I suggest simple criteria for answering this question, based on a combination of global and local water vapor balance approximations. This results in a compact expression for a characteristic time scale. While approximate, such relaxation time is simple to calculate and has a transparent physical meaning. Several illustrations using realistic scenarios for aerosol loading and updraft speed are discussed.
A14B-05
Error in Empirical Measures and Calculated Radiative Forcing of the First Indirect Effect
Aerosol effects on cloud albedo are the most uncertain climate forcing addressed in the recent Intergovernmental Panel on Climate Change report. The first aerosol indirect effect, or Twomey effect, relates a change in aerosol concentration to changes in cloud microphysical properties, and thus cloud albedo, that modify the radiative balance for clouds of similar liquid water content. These relationships are commonly expressed as a variable, IE, that is physically limited to values between 0 and 0.33. A survey of recently published observations of IE shows that values span almost this entire range. To explore the error that uncertainty in measures of IE may produce in climate models, we have used a radiative transfer model to calculate radiative forcings for a range of cloud properties and aerosol concentrations associated with anthropogenic activities. For each 0.05 change in IE, the error in radiative forcing at the top of the atmosphere can range from -3 to -10 W m-2 depending on the level of aerosol perturbation and cloud liquid water path. Because there is so much uncertainty in IE with considerable impact on radiative forcing, we have undertaken an extensive analysis of aerosol-cloud interactions based on surface-based in-situ and remote sensing data. Primary questions addressed are the extent to which the range in reported values is physical, rather than due to measurement artifacts, and what are the most accurate and robust approaches for observing aerosol indirect effects.
A14B-06
Simulated Global and Regional Sensitivities of Aerosol Indirect Effects
Global estimates of aerosol radiative effects, particularly indirect effects, are highly uncertain and difficult to constrain with observations. We present estimates of indirect effects in a new version of the NCAR community atmosphere model. Results indicate large global aerosol indirect effects concentrated in particular regions, mostly in the northern hemisphere mid-latitude storm tracks, downwind of major industrial regions. The sensitivity of aerosol effects to the simulated aerosol and cloud climatology of the model and clouds is analyzed. Detailed single column model analyses of different regions are conducted to gauge the sensitivity of aerosol effects on clouds in different regions and cloud regimes (stratocumulus, cumulus, mid-latitude cyclones) to aerosol loading and cloud climatology.
A14B-07
Evidence for Enhanced Aerosol Indirect Effects in Low-Level Arctic Mixed-Phase Clouds
Several decades of research has greatly broadened our understanding of aerosol indirect effects on cloud radiative forcings regionally and globally, especially for low-level marine clouds. However, little work has yet focused on aerosol indirect effects on radiative forcing by low-level mixed-phase clouds. Here we present evidence that glaciation enhances aerosol indirect effects on cloud radiative forcing in the Arctic, based on our analysis of large-eddy simulations with mixed-phase size-resolved microphysics, using data from three field campaigns---the 1984 Beaufort Arctic Storms Experiment (BASE), the 1998 Surface Heat Budget of the Arctic (SHEBA) experiment, and the 2004 Mixed-Phase Arctic Cloud Experiment (MPACE). Where the aerosol indirect effect is primarily positive (associated with surface warming), owing to unique regional conditions of low- emissivity cloud cover over sea ice at low solar zenith angles, the tendency of increasing aerosol numbers to decrease glaciation (in addition to increasing cloud emissivity) leads to enhanced surface warming. Similarly, where the aerosol indirect effect is primarily negative (associated with surface cooling), owing to high-emissivity cloud cover over open ocean at higher solar zenith angles, the tendency of increasing aerosols to decrease glaciation (in addition to increasing cloud optical depth) leads to enhanced surface cooling. While ice formation mechanisms remain highly uncertain in the moderately supercooled clouds that occurred during all three field experiments and are typical of many Arctic environments, we use field data to constrain our model performance under three cases: clean conditions over ice with low-emissivity clouds (during BASE), polluted conditions over ice with low-emissivity clouds (during SHEBA), and clean conditions over open ocean with high-emissivity clouds (during M-PACE). We then perform sensitivity tests in which aerosol conditions are switched for each field experiment in order to evaluate the strength of the indirect effect. In low-emissivity clouds under polluted conditions, we find that reduced glaciation dominates the total aerosol indirect effect because liquid water path and cloud emissivity are significantly increased when desiccation by precipitating ice is reduced.
A14B-08
On the dependence of cloud droplet spectral dispersion on aerosol populations in shallow marine convection
The so-called Twomey effect is amplified or diminished when increasing droplet concentrations increase or decrease the relative dispersion of cloud droplet size distributions. Results from a number of observational and theoretical studies conflict on whether relative dispersion increases or decreases as droplet concentrations increase. We present results from large-eddy simulations with bin microphysics that target this issue. The simulations are based on case studies of marine stratocumulus and trade cumulus idealized from field projects. Beyond our consideration of environmental factors, we also assess the impact of ignoring the droplet curvature and solute effects on droplet condensational growth, as commonly done in previous studies.