| 1. |
- Ekman, Annica M. L., et al.
(författare)
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Sub-micrometer aerosol particles in the upper troposphere/lowermost stratosphere as measured by CARIBIC and modeled using the MIT-CAM3 global climate model
- 2012
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 117, s. D11202-
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Tidskriftsartikel (refereegranskat)abstract
- In this study, we compare modeled (MIT-CAM3) and observed (CARIBIC) sub-micrometer nucleation (N4-12, 4 <= d <= 12 nm) and Aitken mode (N-12, d > 12 nm) particle number concentrations in the upper troposphere and lowermost stratosphere (UT/LMS). Modeled and observed global median N4-12 and N-12 agree fairly well (within a factor of two) indicating that the relatively simplified binary H2SO4-H2O nucleation parameterization applied in the model produces reasonable results in the UT/LMS. However, a comparison of the spatiotemporal distribution of sub-micrometer particles displays a number of discrepancies between MIT-CAM3 and CARIBIC data: N4-12 is underestimated by the model in the tropics and overestimated in the extra-topics. N-12 is in general overestimated by the model, in particular in the tropics and during summer months. The modeled seasonal variability of N4-12 is in poor agreement with CARIBIC data whereas it agrees rather well for N-12. Modeled particle frequency distributions are in general narrower than the observed ones. The model biases indicate an insufficient diffusive mixing in MIT-CAM3 and a too large vertical transport of carbonaceous aerosols. The overestimated transport is most likely caused by the constant supersaturation threshold applied in the model for the activation of particles into cloud droplets. The annually constant SO2 emissions in the model may also partly explain the poor representation of the N4-12 seasonal cycle. Comparing the MIT-CAM3 with CARIBIC data, it is also clear that care has to be taken regarding the representativeness of the measurement data and the time frequency of the model output.
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| 2. |
- Kim, Dongchul, et al.
(författare)
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Distribution and direct radiative forcing of carbonaceous and sulfate aerosols in an interactive size-resolving aerosol–climate model
- 2008
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Ingår i: Journal of geophysical research: Atmospheres. ; 113:D16, s. D16309-
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Tidskriftsartikel (refereegranskat)abstract
- A multimode, two-moment aerosol model has been incorporated in the NCAR CAM3 to develop an interactive aerosol–climate model and to study the impact of anthropogenic aerosols on the global climate system. Currently, seven aerosol modes, namely three for external sulfate and one each for external black carbon (BC), external organic carbon (OC), sulfate/BC mixture (MBS; with BC core coated by sulfate shell), and sulfate/OC mixture (MOS; a uniform mixture of OC and sulfate) are included in the model. Both mass and number concentrations of each aerosol mode, as well as the mass of carbonaceous species in the mixed modes, are predicted by the model so that the chemical, physical, and radiative processes of various aerosols can be formulated depending on aerosol's size, chemical composition, and mixing state. Comparisons of modeled surface and vertical aerosol concentrations, as well as the optical depth of aerosols with available observations and previous model estimates, are in general agreement. However, some discrepancies do exist, likely caused by the coarse model resolution or the constant rates of anthropogenic emissions used to test the model. Comparing to the widely used mass-only method with prescribed geometric size of particles (one-moment scheme), the use of prognostic size distributions of aerosols based on a two-moment scheme in our model leads to a significant reduction in optical depth and thus the radiative forcing at the top of the atmosphere (TOA) of particularly external sulfate aerosols. The inclusion of two types of mixed aerosols alters the mass partitioning of carbonaceous and sulfate aerosol constituents: about 35.5%, 48.5%, and 32.2% of BC, OC, and sulfate mass, respectively, are found in the mixed aerosols. This also brings in competing effects in aerosol radiative forcing including a reduction in atmospheric abundance of BC and OC due to the shorter lifetime of internal mixtures (cooling), a mass loss of external sulfate to mixtures (warming), and an enhancement in atmospheric heating per BC mass due to the stronger absorption extinction of the MBS than external BC (warming). The combined result of including a prognostic size distribution and the mixed aerosols in the model is a much smaller total negative TOA forcing (−0.12 W m−2) of all carbonaceous and sulfate aerosol compounds compared to the cases using one-moment scheme either excluding or including internal mixtures (−0.42 and −0.71 W m−2, respectively). In addition, the global mean all-sky TOA direct forcing of aerosols is significantly more positive than the clear-sky value due to the existence of low clouds beneath the absorbing (external BC and MBS) aerosol layer, particularly over a dark surface. An emission reduction of about 44% for BC and 38% of primary OC is found to effectively change the TOA radiative forcing of the entire aerosol family by −0.14 W m−2 for clear-sky and −0.29 W m−2 for all-sky.
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| 3. |
- Kim, Dongchul, et al.
(författare)
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The responses of cloudiness to the direct radiative effect of sulfate and carbonaceous aerosols
- 2014
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Ingår i: Journal of Geophysical Research: Atmospheres. - 2169-897X. ; 119:3, s. 1172-1185
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Tidskriftsartikel (refereegranskat)abstract
- This study investigates the responses of the direct radiative effect of light absorbing and scattering carbonaceous and sulfate aerosols on cloudiness and associated radiative fluxes using an interactive aerosol-climate model coupled with a slab ocean model. We find that without including the impact of aerosols on cloud microphysics in the model (indirect effect), the direct radiative effect of aerosols alone can cause a change in cloud coverage and thus in cloud flux change which is consistent with several previous studies. More notably, our result indicates that the direct radiative effect of absorbing aerosols can cause changes in both low-level and high-level clouds with opposite signs. As a result, the global mean cloud radiation response to absorbing aerosols has a rather small value. The change of cloud solar radiative response (all-sky effect minus clear-sky effect) at the top of the atmosphere due to the existence of direct radiative effect of scattering, absorbing, and both types of aerosols is 0.72, 0.08, and 0.81Wm(-2), respectively, all are comparable in quantity to the current estimation of aerosol direct radiative forcing. The cloud response due to the longwave radiative effect is 0.09, 0.18, and 0.27Wm(-2), respectively. The global means of the radiative flux and cloud radiative responses appear to be linearly additive; however, this is definitely not the case for the zonal mean or at the regional scale. Key Points The effect of absorbing and scattering aerosols with an aerosol-climate model Cloud responses on the direct radiative are examined Nonlinearity from absorbing and scattering aerosols exists
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| 4. |
- Wang, Chien, et al.
(författare)
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Impact of anthropogenic aerosols on Indian summer monsoon
- 2009
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Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 36, s. L21704-
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Tidskriftsartikel (refereegranskat)abstract
- Using an interactive aerosol-climate model we find that absorbing anthropogenic aerosols, whether coexisting with scattering aerosols or not, can significantly affect the Indian summer monsoon system. We also show that the influence is reflected in a perturbation to the moist static energy in the sub-cloud layer, initiated as a heating by absorbing aerosols to the planetary boundary layer. The perturbation appears mostly over land, extending from just north of the Arabian Sea to northern India along the southern slope of the Tibetan Plateau. As a result, during the summer monsoon season, modeled convective precipitation experiences a clear northward shift, coincidently in general agreement with observed monsoon precipitation changes in recent decades particularly during the onset season. We demonstrate that the sub-cloud layer moist static energy is a useful quantity for determining the impact of aerosols on the northward extent and to a certain degree the strength of monsoon convection.
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