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Surabi Menon, V. K. Saxena, and B. D. Logie

Abstract

Variations in the chemical composition of cloud droplets of different sizes are predicted in models. Measurements made in natural clouds to verify this prediction are extremely limited, however. During the spring of 1995 and the summers of 1995 and 1996, a size-fractionating version of the California Institute of Technology active-strand cloud water collector was operated on a mountaintop platform in Mount Mitchell State Park, North Carolina (35°44′05"N, 82°17′15"W), to examine differences in drop chemistry between large and small cloud droplets. The size-fractionated measurements also were compared with the chemical composition collected from a passive string–type collector that collects bulk samples. Back-trajectory analysis was used to categorize the source of cloud-forming air masses that arrived at the site as polluted continental, continental, and marine. The differences in cloud drop acidity and chemical constituents were investigated for these different air masses. On average, smaller drops were more enriched in SO42−, NO3, NH4+, and H+; larger droplets had higher values of Na+, Ca2+, and Mg2+. Samples were collected for which the reverse was true, however. In this study, cloud droplet chemical inhomogeneity between droplet sizes and the effect of airmass origin on variations in the chemical composition were examined. Smaller droplets were found to be more acidic than were larger droplets for both marine and polluted continental air masses. The sodium content was the highest in the larger drops for marine events. The sulfate content in both the larger and smaller droplets was the highest for air masses that were from the polluted continental sector. Slightly higher solute concentrations for the larger droplet size range were found for events caused by orographic lifting; for cloud events influenced by frontal activity, higher solute concentrations were found for the smaller drop size range.

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Surabi Menon, Anthony D. Del Genio, Dorothy Koch, and George Tselioudis

Abstract

In this paper the coupling of the Goddard Institute for Space Studies (GISS) general circulation model (GCM) to an online sulfur chemistry model and source models for organic matter and sea salt that is used to estimate the aerosol indirect effect is described. The cloud droplet number concentration is diagnosed empirically from field experiment datasets over land and ocean that observe droplet number and all three aerosol types simultaneously; corrections are made for implied variations in cloud turbulence levels. The resulting cloud droplet number is used to calculate variations in droplet effective radius, which in turn allows one to predict aerosol effects on cloud optical thickness and microphysical process rates. The aerosol indirect effect is calculated by differencing the top-of-the-atmosphere net cloud radiative forcing for simulations with present-day versus preindustrial emissions. Both the first and second indirect effects are explored. The sensitivity of the results presented here to cloud parameterization assumptions that control the vertical distribution of cloud occurrence, the autoconversion rate, and the aerosol scavenging rate, each of which feeds back significantly on the model aerosol burden, are tested. The global mean aerosol indirect effect for all three aerosol types ranges from −1.55 to −4.36 W m−2 in the simulations. The results are quite sensitive to the preindustrial background aerosol burden, with low preindustrial burdens giving strong indirect effects, and to a lesser extent to the anthropogenic aerosol burden, with large burdens giving somewhat larger indirect effects. Because of this dependence on the background aerosol, model diagnostics such as albedo-particle size correlations and column cloud susceptibility, for which satellite validation products are available, are not good predictors of the resulting indirect effect.

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Dorothy Koch, Surabi Menon, Anthony Del Genio, Reto Ruedy, Igor Alienov, and Gavin A. Schmidt

Abstract

Aerosol direct (DE), indirect (IE), and black carbon–snow albedo (BAE) effects on climate between 1890 and 1995 are compared using equilibrium aerosol–climate simulations in the Goddard Institute for Space Studies General Circulation Model coupled to a mixed layer ocean. Pairs of control (1890)–perturbation (1995) with successive aerosol effects allow isolation of each effect. The experiments are conducted both with and without concurrent changes in greenhouse gases (GHG). A new scheme allowing dependence of snow albedo on black carbon snow concentration is introduced. The fixed GHG experiments global surface air temperature (SAT) changed by −0.2°, −1.0°, and +0.2°C from the DE, IE, and BAE. Ice and snow cover increased 1% from the IE and decreased 0.3% from the BAE. These changes were a factor of 4 larger in the Arctic. Global cloud cover increased by 0.5% from the IE. Net aerosol cooling effects are about half as large as the GHG warming, and their combined climate effects are smaller than the sum of their individual effects. Increasing GHG did not affect the IE impact on cloud cover, however they decreased aerosol effects on SAT by 20%, and on snow/ice cover by 50%; they also obscure the BAE on snow/ice cover. Arctic snow, ice, cloud, and shortwave forcing changes occur mostly during summer–fall, but SAT, sea level pressure, and longwave forcing changes occur during winter. An explanation is that aerosols impact the cryosphere during the warm season but the associated SAT effect is delayed until winter.

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Dorothy Koch, Susanne E. Bauer, Anthony Del Genio, Greg Faluvegi, Joseph R. McConnell, Surabi Menon, Ronald L. Miller, David Rind, Reto Ruedy, Gavin A. Schmidt, and Drew Shindell

Abstract

The authors simulate transient twentieth-century climate in the Goddard Institute for Space Studies (GISS) GCM, with aerosol and ozone chemistry fully coupled to one another and to climate including a full dynamic ocean. Aerosols include sulfate, black carbon (BC), organic carbon, nitrate, sea salt, and dust. Direct and BC-snow-albedo radiative effects are included. Model BC and sulfur trends agree fairly well with records from Greenland and European ice cores and with sulfur deposition in North America; however, the model underestimates the sulfur decline at the end of the century in Greenland. Global BC effects peak early in the century (1940s); afterward the BC effects decrease at high latitudes of the Northern Hemisphere but continue to increase at lower latitudes. The largest increase in aerosol optical depth occurs in the middle of the century (1940s–80s) when sulfate forcing peaks and causes global dimming. After this, aerosols decrease in eastern North America and northern Eurasia leading to regional positive forcing changes and brightening. These surface forcing changes have the correct trend but are too weak. Over the century, the net aerosol direct effect is −0.41 W m−2, the BC-albedo effect is −0.02 W m−2, and the net ozone forcing is +0.24 W m−2. The model polar stratospheric ozone depletion develops, beginning in the 1970s. Concurrently, the sea salt load and negative radiative flux increase over the oceans around Antarctica. Net warming over the century is modeled fairly well; however, the model fails to capture the dynamics of the observed midcentury cooling followed by the late century warming. Over the century, 20% of Arctic warming and snow–ice cover loss is attributed to the BC-albedo effect. However, the decrease in this effect at the end of the century contributes to Arctic cooling.

To test the climate responses to sulfate and BC pollution, two experiments were branched from 1970 that removed all pollution sulfate or BC. Averaged over 1970–2000, the respective radiative forcings relative to the full experiment were +0.3 and −0.3 W m−2; the average surface air temperature changes were +0.2° and −0.03°C. The small impact of BC reduction on surface temperature resulted from reduced stability and loss of low-level clouds.

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