Light-scattering data relevant for aerosol radiative forcing

Tellus Series B-chemical and Physical Meteorology, 2004

A B S T R A C T Airborne data are presented on the impact of cloud processing on the aerosol mass light-scattering efficiency. The measurements, on marine stratocumulus, suggest that cloud processing significantly enhanced the mass light-scattering efficiency in three of the five cases analysed. Enhancements were of the order of 10% for air detraining from the cloud deck relative to non-detraining air. A diagnostic modelling analysis suggested that the observed enhancements were consistent with the previously proposed explanation of in-cloud sulfate production in the particle size range for efficient light scattering.

Journal of the Air & Waste Management Association, 2000

The eastern United States national parks experience some of the worst visibility conditions in the nation. To study these conditions, the Southeastern Aerosol and Visibility Study (SEAVS) was undertaken to characterize the size-dependent composition, thermodynamic properties, and optical characteristics of the ambient atmospheric particles. is in turn consistent with the study average sulfate ammoniation corresponding to a molar ratio of NH 4 /SO 4 of approximately one. The f(RH) curve for organics is not significantly different from one, suggesting that organics are weakly to nonhygroscopic.

Journal of the Air & Waste Management Association, 2000

The eastern United States national parks experience some of the worst visibility conditions in the nation. To study these conditions, the Southeastern Aerosol and Visibility Study (SEAVS) was undertaken to characterize the size-dependent composition, thermodynamic properties, and optical characteristics of the ambient atmospheric particles. is in turn consistent with the study average sulfate ammoniation corresponding to a molar ratio of NH 4 /SO 4 of approximately one. The f(RH) curve for organics is not significantly different from one, suggesting that organics are weakly to nonhygroscopic.

Tellus B, 2014

This investigation focuses on the characterisation of the aerosol particle hygroscopicity. Aerosol particle optical properties were measured at Granada, Spain, during winter and spring seasons in 2013. Measured optical properties included particle light-absorption coefficient (s ap) and particle light-scattering coefficient (s sp) at dry conditions and at relative humidity (RH) of 85910%. The scattering enhancement factor, f(RH 085%), had a mean value of 1.590.2 and 1.690.3 for winter and spring campaigns, respectively. Cases of high scattering enhancement were more frequent during the spring campaign with 27% of the f(RH085%) values above 1.8, while during the winter campaign only 8% of the data were above 1.8. A Saharan dust event (SDE), which occurred during the spring campaign, was characterised by a predominance of large particles with low hygroscopicity. For the day when the SDE was more intense, a mean daily value of f(RH085%)01.390.2 was calculated. f(RH 085%) diurnal cycle showed two minima during the morning and afternoon traffic rush hours due to the increase in non-hygroscopic particles such as black carbon and road dust. This was confirmed by small values of the single-scattering albedo and the scattering Å ngstrom exponent. A significant correlation between f(RH 085%) and the fraction of particulate organic matter and sulphate was obtained. Finally, the impact of ambient RH in the aerosol radiative forcing was found to be very small due to the low ambient RH. For high RH values, the hygroscopic effect should be taken into account since the aerosol forcing efficiency changed from (13 W/m 2 at dry conditions to (17 W/m 2 at RH 085%.

Nature Reviews Earth & Environment

Aerosols are small liquid or solid particles suspended in the atmosphere 1. They can be emitted directly (such as dust, sea salt, black carbon (BC) and volcanic aerosols) or formed indirectly through chemical reactions (including sulfate, nitrate, ammonium and secondary organic aerosols). Owing to their relatively short lifetime, aerosol concentrations typically peak near their sources. Desert regions (such as North Africa and the Middle East), industrial regions (such as East and South Asia) and biomass-burning regions (such as South America and South Africa) are, therefore, characterized by high mass concentrations (Fig. 1). Aerosols exhibit complicated compositions and vary substantially in shape and size, typically ranging between 0.01 and 10 μm (reF. 2). Depending on these structural and compositional characteristics, aerosols can scatter and/or absorb shortwave radiation, as quantified through the single-scattering albedo (SSA; Table 1). Purely scattering aerosols include sulfates, nitrates, ammonium and sea-salt particles, whereas absorbing aerosols are primarily BC, with dust and organic carbon partly absorbing in the ultraviolet (UV) spectrum 3. Aerosols have a direct bearing on Earth's energy balance and, therefore, on climate. For instance, aerosol scattering and absorption alters the radiation balance and atmospheric stability through perturbations to the vertical temperature profile. Aerosols can further serve as cloud condensation nuclei (CCN) or ice-nucleating particles (INPs), which modify the reflectivity and lifetime of clouds through microphysical processes. Collectively, these influences are quantified as aerosol forcing: the change of net radiative flux at a specified level of the atmosphere, often assessed relative to estimated pre-industrial conditions 4. Globally, anthropogenic aerosols are estimated to produce a net cooling ~−1.3 ± 0.7 W m −2 at the top of the atmosphere; −0.3 ± 0.3 W m −2 is attributed to the aerosol-radiation interaction (ARI), −1.0 ± 0.7 W m −2 to aerosol-cloud interactions, ~−1.15 W m −2 to total forcing from scattering aerosols and ~+0.12 W m −2 to BC 4. This combined aerosol forcing offsets roughly one-third of the warming from anthropogenic greenhouse gases (GHGs). However, the large spread in the estimated aerosol forcing leads to large discrepancies in climate sensitivity 5,6. Thus, aerosols are considered to be the largest contributor of uncertainty in quantifying present-day climate change 4. Much of this uncertainty in aerosol forcing arises from both the lack of separate global constraints on aerosol optical and microphysical properties (optical depth, size distribution, hygroscopicity and mixing state, among others) and the inaccurate representation of them in climate models 7-10. In particular, aerosol SSA is further

Physical Chemistry Chemical Physics, 2010

The aim of this study is to provide results of the theoretical experiments in order to improve the estimates of indirect effect of aerosol on the cloud albedo and consequently on the radiative forcing. The cloud properties could be changed primarily because of changing of both the aerosol type and concentration in the atmosphere. Only a part of aerosol interacts effectively with water and will, in turn, determine the number concentration of cloud droplets (CDNC). We calculated the CDNC, droplet effective radius (r eff ), cloud optical thickness (τ), cloud albedo and radiative forcing, for various types of aerosol. Our results show into what extent the change of aerosol characteristics (number concentration and chemical composition) on a regional scale can modify the cloud reflectivity. Higher values for cloud albedo in the case of the continental (urban) clouds were obtained.

Journal of Geophysical Research, 2002

Journal of Geophysical Research, 2002

1] Measurements of aerosol optical properties over the atmospheric column (aerosol optical thickness, spectral angular sky radiance (sky brightness), and downwelling hemispheric flux) have been used to derive climate-relevant aerosol parameters such as the phase function, the broadband single-scattering albedo, and the refractive index. These parameters are needed to estimate the direct short-wave radiative forcing by aerosols and to validate aerosol models in the satellite retrieval algorithms. Values of the broadband single-scattering albedo obtained in this study range between w 0 = 0.98 (marine aerosols) and 0.90 (continental pollution aerosols). The columnar ambient broadband refractive index is found to be m = 1.39 ± 0.044 À i (<0.003) for marine conditions and m = 1.48 ± 0.058 À i (0.01 ± 0.003) for polluted continental aerosols. Nonsphericity is shown to be important in the case of marine aerosols. Moreover, aerosol nonsphericity gives an additional contribution to the negative short-wave radiative forcing of marine aerosols under clear-sky conditions, which can be estimated as being 30 up to 50% of the radiative forcing estimated for spherical marine aerosols. In the case of continental polluted aerosols the optical properties can be represented by spherical particles, and no additional shape effect has to be considered. However, the aerosol absorption leads to an increase of about 40% of the radiative forcing estimated for nonabsorbing aerosol of the same size distribution.

Tellus B, 2004

A B S T R A C T A deterministic atmospheric spectral radiative transfer model, that uses comprehensive climatological data, is developed to compute the global distribution of mean monthly clear-sky total direct aerosol radiative forcing in the ultraviolet (UV) and visible, between 0.2-0.85 µm, at the top of the atmosphere (TOA), within the atmosphere and at the Earth's surface for winter and summer conditions. The aerosol data were taken from the Global Aerosol Data Set (GADS), given for various fixed relative humidity values and for 11 wavelengths within the UV-visible range, both for natural and anthropogenic aerosols. We first derive global climatologies of extinction aerosol optical thickness (AOT), singlescattering albedo (ω aer ) and asymmetry factor (g aer ), for actual relative humidity values within the aerosol layer, based on the National Centers for Environmental Prediction and National Center for Atmospheric Research (NCEP/NCAR) Reanalysis Project and the Tiros Operational Vertical Sounder (TOVS) datasets. We include the global distribution of cloud cover using the D2 data from the International Satellite Cloud Climatology Project (ISCCP), to define the clearsky fraction at the pixel level for each month. Supplementary 10-yr climatological data for surface and atmospheric parameters were taken from NCEP/NCAR, ISCCP-D2 and TOVS. Our present analysis allows the aerosol radiative properties and forcings to vary with space, time and wavelength. The computed mean annual global AOT, ω aer and g aer values are found to be 0.08, 0.96 and 0.73, respectively, at 0.5 µm. On a mean monthly 2.5 • pixel resolution, aerosols are found to decrease significantly the downward and the absorbed solar radiation at the surface, by up to 28 and 23 W m −2 , respectively, producing a surface cooling at all latitudes in both winter and summer. Aerosols are found to generally increase the outgoing solar radiation at TOA (planetary cooling) while they increase the solar atmospheric absorption (atmospheric warming). However, the model results indicate that significant planetary warming, by up to 5 W m −2 , can occur regionally, such as over desert areas, due to strong aerosol absorption. A smaller planetary warming (by up to 2 W m −2 ) is also found over highly reflecting ice-or snow-covered areas, such as Antarctica and Greenland, as well as over Eastern Europe, Siberia and North America. In general, the aerosol-induced surface cooling exceeds the induced atmospheric warming, except for regions characterized by strong aerosol absorption (e.g. deserts). On a mean annual global basis, natural plus anthropogenic aerosols are found to cool the Earth by 0.6 W m −2 (they increase the planetary albedo by 0.28%), to heat the atmosphere by 0.8 W m −2 , while they decrease the downward and net surface solar radiation (surface cooling) by about 1.9 and 1.4 W m −2 . maintain the current climate (IPCC, 2001). Inclusion of aerosols is now necessary in climatic change studies that examine changes in surface and atmospheric temperatures, snow-and ice-cover extent, sea-level rise, precipitation changes, frequency and intensity of extreme weather events, and desertification, since aerosols crucially affect the radiation budget at the top of the atmosphere (TOA), in the atmosphere and at the surface, and hence atmospheric dynamics as well as evaporation and surface energy balance. Besides, aerosols are considered to be probably responsible for the disagreement between model estimates and measurements of downward surface solar radiation. Aerosols affect Tellus 56B , 1 51