A13F-01 INVITED
Climate-Relevant Properties of Atmospheric Aerosols Over Megacity Regions in China
We conducted measurements of optical and cloud-nucleating properties of the atmospheric aerosol over urban and peri-urban regions in northern and southern China (Beijing and Guangzhou areas). We measured light scattering with a nephelometer at several wavelengths and light absorption with a photoacoustic spectrometer at 532 nm. Because the latter instrument measures the absorption on airborne particles, it is free from filter artefacts and provides high accuracy. The cloud-nucleating properties of size-selected aerosol were determined with a DMT CCN counter. Urban aerosols were found to be highly absorbing, as a result of strong emissions of soot particles mostly from vehicular sources. Regional aerosols were somewhat less absorbing due to the continued production of secondary inorganic (sulfate, nitrate) and organic aerosol. Aerosol from biomass burning also made substantial contributions to the atmospheric burden. Absolute values of the scattering and absorption coefficients were very high, especially during the northeastern monsoon in Guangzhou. A negative correlation between single scattering albedo and backscatter fraction was observed and found to have a strong non-linear effect on aerosol radiative forcing efficiency. High CCN concentrations were present over both regions. In spite of the relatively young age of the aerosols, most of the particles contained enough soluble material to be able to activate at intermediate supersaturations. The hygroscopicity of the particles was in general similar or higher than that observed in regional aerosol from Europe or other polluted regions.
A13F-02 INVITED
Progress and Challenge in Using Satellite Remote Sensing Measurements for Quantifying the Aerosol Direct Radiative Forcing
Over the past decade, satellite aerosol retrievals have become increasingly sophisticated. Now, passive satellite sensors measure the angular dependence of polarization and radiance in multiple wavelengths in the ultraviolet through the infrared at fine temporal and spatial resolution. From these observations, retrieved aerosol products include not only optical depth at one wavelength, but spectral optical depth and particle size over both ocean and land, as well as more direct measurements of polarization and phase function. Complementary to the passive sensors, active remote sensing from space is also making promising progress. Furthermore, the constellation of six afternoon-overpass spacecrafts, so-called A-Train makes it possible for the first time to conduct near simultaneous measurements of aerosols, clouds, and radiative fluxes in multiple dimensions with sensors with complementary capabilities. As demonstrated in recent studies, the high accuracy of aerosol products from new and enhanced sensors, together with improvements in characterizing the earth's surface and clouds, has helped to reduce the uncertainties associated with the direct radiative effect by both natural and anthropogenic aerosols. Although satellite instruments do not measure the aerosol chemical composition needed to discriminate anthropogenic from natural aerosol components, the retrieved aerosol size parameters (e.g., fine-mode fraction) provides a feasible way to conduct measurement-based estimates of the radiative forcing of anthropogenic aerosols because anthropogenic aerosols are predominately submicron. In this talk, I will review recent progress in such estimates and discuss possibilities and challenges of using emerging A-Train data to improve estimates of the aerosol direct radiative forcing.
A13F-03
Global Aerosol Radiative Forcing using Satellite and Surface Measurements
Over the industrial period, aerosols have increased due to human activities and their effects on climate are the largest source of uncertainty in the current IPCC estimates of global climate forcing due to human activities. Inhomogeneous distribution of aerosols in space and time poses a challenge in their characterization and requires global measurements to assess their effects and reduce the associated uncertainties. In this paper we use global measurements from both satellite and ground based observations for one year time period to estimate the shortwave aerosol radiative forcing (SWARF) at the top-of-atmosphere (TOA) and discuss the associated uncertainties. For this, aerosol properties (optical depth) derived from AErosol RObotic NETwork (AERONET), a federation of ground-based remote sensing instruments, are used in this paper in conjunction with measurements of the TOA shortwave flux from CERES instrument (onboard Terra satellite). High spectral and spatial resolution observations from Imager (MODIS) will be used to identify clear sky conditions within CERES foot print and GOCART results will also be used for separating aerosol types. Global aerosol forcing and corresponding radiative forcing efficiencies will be presented as a function of major aerosol types [including anthropogenic (sulfate, soot, black carbon) and natural (dust) aerosols], region and season. This study should serve as a useful constraint for both numerical modeling simulations and satellite based estimates of SWARF.
A13F-04
Optical Remote Sensing of CCN
We show a new retrieval algorithm to obtain either CCN concentrations from optical extinction at multiple wavelengths. This is known to be a difficult retrieval problem. We show that it is possible to partially orthogonalize the retrieval against large-particle contributions, which helps substantially. Nonetheless further constraint is necessary. We show the statistics of many dry-aerosol size distributions observed at a rual ridgeline site in eastern New York (Pinnacle Peak). These data are analyzed by fitting them with a distribution dN/dr = C rn/(R + r)k . The results show that this distribution can fit the observed distributions well, and that we see statistical correlations of the coefficients in these data which we can use to provide climatological constraints for the CCN retrieval. We apply the retrieval with this climatological constraint to four years of extinction data taken at the Atmospheric Radiation Measurement (ARM) program central facility. This retrieval generates plausible CCN numbers most of the time. This argues that the parameterization developed from the aerosol distributions seen at Pinnacle Peak, and thus the climatological constraints fed into the retrieval, are useful at the ARM SGP site, and perhaps more generally to mid continental sites.
A13F-05
Patterns and connections between aerosols, clouds and vegetation in the Amazon as seen by the twin MODIS sensors aboard Terra and Aqua
In this study, twin Moderate Resolution Imaging Spectroradiometer (MODIS) sensors aboard NASA's Terra and Aqua satellites are used for characterization of cloud development and identification of processes affecting cloud formation. We find that much of the development of microphysical properties of water clouds over the Brazilian Legal Amazon can be characterized by the simple difference between those properties observed at the two times of MODIS overpass, only 3 hours apart. The time window is small enough that observed differences in cloud properties are primarily associated with the local events; therefore, it is ideal for exploring the effects of plant transpiration, biomass burning and Secondary Organic Aerosol (SOA) formation on regional cloud properties. In this region we find that the effective cloud droplet radius observed in the afternoon by Aqua-MODIS is systematically higher than the effective radii observed in the morning by Terra-MODIS. The difference corresponds to the invigoration of convection in the afternoon with the corresponding growth of droplet size. The monthly mean difference is 1 to 2 um, depending on season, but the overall pattern of the difference prevails throughout the Amazon, is repeated over other tropical rain forest regions globally, and is strikingly different from other types of cloud systems around the globe. Furthermore, we find that the effective radius difference found in the Amazon is inversely correlated to measures of evapotranspiration and all-sky solar radiation at the surface, but is not well-correlated to precipitation. The picture that emerges is a complex one that intertwines a light- limited forest, aerosols (both biogenic and anthropogenic) and cloud development.
A13F-06
Radiative Forcing by Anthropogenic Aerosol - Best Estimates and Uncertainties
The impact of a modified element in the Earth's Atmosphere is commonly quantified by changes it imposes on the Earth's radiative energy balance, also termed its ‘radiative forcing'. Of particular interest, in understanding current and future climates is the radiative forcing that can be attributed to man-made atmospheric modifications. However, its quantification requires proper atmospheric representations not only for current conditions but also for a (human-) undisturbed reference state. Since the reference usually cannot be measured our understanding is largely derived from model simulations: Enhancements to greenhouse gas concentrations have retained (to the Earth-Atmosphere-System) radiative energy, to ‘warm' our planet. The impact of enhanced anthropogenic aerosol, in contrast, is less clear and spatially much more diverse. On a global annual basis (anthropogenic) aerosol is expected to slow greenhouse gas warming. Even when excluding indirect (feedback) effects on clouds or the hydrological cycle, the impact uncertainty solely due to the presence of anthropogenic aerosol has remained large. This ‘direct forcing' uncertainty, however, results not only from difficulties to quantify anthropogenic aerosol but also from the difficulties to quantify environmental properties at sufficient accuracy (e.g. clouds, surface below). Based on a hybrid approach, which incorporates quality data-sets on aerosol and environmental properties into typical properties offered by global modeling, new estimates for the aerosol direct radiative forcing estimates are provided (e.g. -0.4W/m2 at the top of the atmosphere, -2.4W/m2 at the surface). This approach then serves as the basis for sensitivity studies to study the impact of uncertainty to individual input properties, which are eventually summarized to provide estimates for the overall uncertainty of the aerosol ‘direct forcing'. These sensitivity studies are further applied to provide insights on biases introduced by simplifying assumptions, in particular frequent assumptions associated with the terms ‘anthropogenic' and ‘all-sky'.
A13F-07 INVITED
The spectral absorption by aerosols from 350-2500nm and its radiative forcing implications
The absorption properties of aerosol particles are still one of the largest uncertainties on the aerosol forcing of the Climate. Global measurements of BC are not available and badly needed. In situ aerosol absorption measurements are often inaccurate, neglect some important properties, and usually cover a narrow spectral range missing significant absorption features. This work presents results of spectral measurements of aerosol absorption efficiency in a broad spectral range (350-2500nm) highlighting some characteristics of BC and other aerosol absorbers. Implications of these results on the aerosol radiative forcing over different surface types will also be discussed. It is well known that BC is the main absorbing material in atmospheric aerosol but it is not the only one. Soil dust absorbs light in the UV and visible, some organic materials absorb in the UV, and there are recent evidences that some organic materials may also absorb light in longer wavelengths. The absorption spectral dependence by aerosol particles is directly influenced by the chemical composition (refractive indices), the relative size of the absorbers versus the wavelength, and by mixtures between the absorbing and non-absorbing materials. Our spectral absorption measurements covering a broad spectral range (350-2500nm) produce enough information to separate these effects determining the absorption contribution by the different aerosol constituents and the identifying the potential mixture between fine and coarse particles. Results from spectral absorption measurements will be presented for Brazil (Sao Paulo and the Amazon), Mexico, China, UAE, Israel, Bodele, and the US. These results were used on radiative forcing calculations for these aerosols and on the determination of their potential to heat the atmospheric column and affect cloud formation and cloud life time. Our calculations show that in many circumstances, the often neglected wide spectral range, and the absorption by coarse mode particles are essential for an accurate determination of the aerosol forcing.