A54B-01 INVITED
Attenuation of Photosynthetically Active Radiation by Aerosols and Characterization of Aerosol Absorption from AERONET
Accurate knowledge of the intensity of incident photosynthetically active radiation (PAR; 400-700 nm) at the surface is important for understanding possible effects on crop productivity and on the net primary productivity of natural ecosystems. Not only the magnitude of the PAR irradiance but also the distribution of that irradiance into direct and diffuse components can have an effect on photosynthesis. In many regions clouds are the primary attenuator of PAR flux, however in some regions there are significant reductions in PAR irradiance at the surface due to aerosol attenuation. The two primary aerosol optical parameters affecting PAR attenuation are the aerosol optical depth which is related to particulate concentration in the total column, and aerosol absorption which is related to the black carbon concentration for pollution and biomass burning aerosols. Model computations show the magnitude of reduction in PAR irradiance as optical depth increases and as aerosol absorption increases, and the increases in diffuse fraction as aerosol optical depth increases and decreases in diffuse fraction as absorption increases. Examples are shown of the retrieval of aerosol absorption (parameterized by the single scattering albedo) from measurements of irradiance and diffuse fraction in conjunction with spectral aerosol optical depth in the PAR wavelength interval. Since PAR flux reduction by aerosols increases as optical path length increases, there is also the effect of greater attenuation in early morning and late afternoon, possibly reducing the effective photoperiod. We also show aerosol absorption retrievals from the new Version 2.0 AERONET retrievals that are more accurate than Version 1.0 due to better characterization of surface reflectance, providing the basic aerosol characterization needed to estimate cloudless sky PAR fluxes.
A54B-02
Mechanistic Response of Terrestrial Plant Productivity and Surface Energy Budget to Routine Aerosol Loading over the Eastern U.S.
The aerosol direct effect not only reduces global irradiance but also increases diffuse radiation. Diffuse radiation is more homogeneously absorbed by the plant canopy and more efficiently utilized for the plant photosynthesis process than direct radiation. Thus, aerosol loading is expected to increase plant productivity (the aerosol diffuse- radiation effect). The study presents the spatio-temoporal variability of the aerosol diffuse-radiation effect over the eastern U.S., using a sun-shade canopy model. First, satellite and model aerosol optical depth (AOD) products are assimilated via an optimal interpolation technique, and a comparison against the ground-based observations shows that the satellite-model assimilated AOD product is superior to either a satellite or model product. Second, surface albedo, surface radiative temperature, CO2 flux, and sensible/latent heat fluxes in a sun-shade canopy model (Unified Land Model: ULM) are compared with corresponding satellite and ground-based observations. Tuning parameters in ULM are constrained by reducing model-observation discrepancies via the Gauss-Maquardt-Levemberg automatic optimization algorithm. Third, the well-calibrated ULM is run in an off-line mode for the warm seasons in 2000 and 2001. Downwelling shortwave radiation is computed with (a control experiment) and without (a potential experiment) assimilated daily AOD in all-sky conditions. The sensitivity experiments (control-potential) show that aerosol loading increases plant productivity in mixed forests and deciduous broadleaf forests in the southeastern U.S., while plant productivity is decreased over the croplands and grasslands. The spatio-temporal variability of aerosol diffuse-radiation effect is well explained by the variability of leaf area index (LAI), cloud optical depth, near-surface atmospheric temperature, and diurnal cycles. Due to the combination of the positive and negative effects, the aerosol diffuse-radiation effect changes plant productivity by only +0.5% in 2001 and -0.09% in 2000 from the potential experiment over the eastern U.S. It should be noted that this is the first regional scale analysis of this process, and the magnitude of sensitivity could vary for a coupled land-atmosphere experiment.
A54B-03 INVITED
The Effects of Aerosol on Atmospheric UV Radiation: Measurements and Modeling from the MILAGRO Field Campaign
The MILAGRO field campaign took place in and near Mexico City 1-30 March 2006. A comprehensive data set was obtained on atmospheric chemical composition (gas and aerosol), aerosol microphysics, spectral radiation, and meteorology from surface-, aircraft-, and satellite-based instruments. For much of this time, the lower atmosphere was laden with large amounts of aerosols originating from urban and industrial sources, biomass fires, and wind-blown dust. Spectral radiation measurements are available from filter radiometers and spectroradiometers, and span ultraviolet (UV) wavelengths important to surface biota and tropospheric photochemistry. By combining the spectral radiation measurements, aerosol composition, optical, and microphysical measurements, and modeling, an assessment is now possible on how aerosols affect surface UV radiation (e.g. DNA damage, erythema, vitamin-D production) and vertical profiles of photolysis frequencies (e.g. JNO2, JO3(O1D), JCH2O, JHONO). Interactions between aerosol-scattered radiation and absorption by gaseous pollutants (esp. O3, SO2, and NO2) can also be evaluated. Implications for human health and photochemical oxidant formation will be discussed.
A54B-04
Effect of Aerosols on Surface Radiation and Air Quality in the Central American Region Estimated Using Satellite UV Instruments
Solar radiation reaching the Earth's surface is reduced by both aerosol scattering and aerosol absorption. Over many parts of the world the latter effect can be as large or larger than the former effect, and small changes in the aerosol single scattering albedo can either cancel the former effect or enhance it. In addition, absorbing aerosols embedded in clouds can greatly reduce the amount of radiation reaching the surface by multiple scattering. Though the potential climatic effects of absorbing aerosols have received considerable attention lately, their effect on surface UV, photosynthesis, and photochemistry can be equally important for our environment and may affect human health and agricultural productivity. Absorption of all aerosols commonly found in the Earth's atmosphere becomes larger in the UV and blue wavelengths and has a relatively strong wavelength dependence. This is particularly true of mineral dust and organic aerosols. However, these effects have been very difficult to estimate on a global basis since the satellite instruments that operate in the visible are primarily sensitive to aerosol scattering. A notable exception is the UV Aerosol Index (AI), first produced using NASA's Nimbus-7 TOMS data. AI provides a direct measure of the effect of aerosol absorption on the backscattered UV radiation in both clear and cloudy conditions, as well as over snow/ice. Although many types of aerosols produce a distinct color cast in the visible images, and aerosols absorption over clouds and snow/ice could, in principle be detected from their color, so far this technique has worked well only in the UV. In this talk we will discuss what we have learned from the long-term record of AI produced from TOMS and Aura/OMI about the possible role of aerosols on surface radiation and air quality in the Central American region.
A54B-05 INVITED
Impact of Ozone Depletion on Biological Doses
After the discovery of ozone depletion, the study of the variability of the UV-B radiation at the earth's surface became a topic of interest for the atmospheric community. Also, biologists, physicians and epidemiologists, working in this field, require values of UV irradiance and dose in their studies. The variation of UV radiation at the earth's surface is not easily derived from ozone variation, since UV radiation is affected by several other factors, such as solar zenith angle, cloud cover, aerosols, albedo, altitude and earth-sun distance. The response of biological systems to UV radiation is wavelength dependant (action spectrum), then, to perform UV biological studies, the spectral distribution of solar radiation is a key issue. In the last decades, many efforts have been developed to measure UV radiation with spectral or multi-channel instruments. Biologically weighted irradiances are easy to calculate from spectral measurements, but calculation from multi-channel radiometers is not direct. In this study we apply a multi-regressive algorithm to retrieve biologically weighted irradiance from irradiance measured by multi-channel moderate bandwidth radiometers. Climatologies and extreme events of biologically weighted irradiance are analyzed for ten spectral and multichannel instruments, which are part of the NSF Radiation Monitoring Network and the IAI Radiation Network, and are distributed from South Pole to the tropics. As case study spring year 2000 is shown. Since radiation effects on ecological systems may be accumulative we study the accumulated anomalies to evaluate if they can influence this process.
A54B-06
Impact of Surface Solar Dimming and Brightening on the Climate System
The author's institute maintains two databases for the worldwide measured solar radiation incident at the Eart's surface, the Global Energy Balance Archive (GEBA), and the Baseline Surface Radiation Network (BSRN). An analysis of these comprehensive datasets suggests that solar radiation incident at the surface has not been stable over time but underwent significant decadal variations. This is seen in a decline in surface solar radiation from the 1960s to the 1980s („global dimming"), with a recovery thereafter at widespread locations („global brightening"). As potential explanation for these variations, both changes in clouds and aerosol have been put forward. An analysis of synop and satellite-based cloud data over Europe suggests, that changes in cloud amount cannot explain the distinct reversal from solar dimming to brightening seen over Europe. This points to aerosol direct/indirect effects as a key explanation for the dimming to brightening transition. The reversal from dimming to brightening may be related to reduced aerosol concentration due to more effective air pollution measures and the breakdown of the economy in the former communist countries, leading to cleaner and more transparent atmospheres. Changes in surface solar radiation may also leave their imprint in the diurnal temperature range. Trends in diurnal temperature ranges show, after decades of decline, a distinct tendency to level off since the mid 1980s. They suggest that surface solar dimming was effective in masking greenhouse warming, but only up to the 1980s, when dimming gradually transformed into brightening. With this transition, the uncovered greenhouse effect started to reveal its full dimension, as manifested in a rapid temperature rise. The reversal from surface solar dimming to brightening may also affect biomass productivity in energy-limited environments, and should leave a significant imprint on vegetation. Wild, M et al. 2005: From dimming to brightening: Decadal changes in solar radiation at the Earth's surface. Science, 308, 847-850. Wild, M., Ohmura A., Makowski, K., 2007: Impact of global dimming and brightening on global warming. GRL 34, L04702. Norris, J.R., and Wild, M., 2007: Trends in direct and indirect aerosol radiative effects over Europe inferred from observed solar "dimming" and "brightening", JGR (in press).
A54B-07
Aerosol and its Radiative Impact on Surface Solar Radiation in China
As a fast developing country covering a large territory, China is experiencing rapid environmental changes. High
concentrations of aerosols with diverse properties are emitted in the region, providing a unique opportunity for
understanding the impact of environmental changes on climate. Until very recently, few observational studies
were conducted in this important source regions. The East Asian Study of Tropospheric Aerosols: an International
Regional Experiment (EAST-AIRE) attempts to characterize the physical, optical and chemical properties of the
aerosols and their effects on climate over China. Some preliminary results will be presented using continuous
high-quality measurements of aerosol, cloud and radiative quantities made at the EAST-AIRE baseline stations in
northern and southern China. Both regions are often covered by a thick layer of haze (with a yearly mean aerosol
optical depth 0.7-0.8) due primarily to anthropogenic emissions of moderately strong absorbing aerosols,
leading exceptionally large aerosol radiative effect at the surface in broadband, PAR and ultraviolet radiation. The
boundary atmosphere is thus heated dramatically during the daytime, which may affect atmospheric stability and
cloud formation.
http:www.atmos.umd.edu/~zli
A54B-08
Impact of Arctic Aerosols on Solar Radiation at the Surface: Modeled versus Measured
Aerosols in the Arctic affect the disposition of solar radiation within the lower atmosphere and at the surface in complex ways, attenuating solar and terrestrial radiation directly, while interacting indirectly with clouds. Aerosol radiative impacts depend on their chemical, physical and optical properties as well as solar geometry and properties of the intervening atmosphere and surface. While the Arctic atmosphere is generally very clean, incursions of both natural and anthropogenic aerosols produce perturbations that vary latitudinally and seasonally. To make accurate climate assessments it is essential to quantify the radiative forcing by the different aerosol types relative to normally clean background conditions. With the complement of instruments now operating at the NOAA and DOE facilities near Barrow, Alaska, it is possible to characterize Arctic aerosols, infer their optical properties and evaluate their radiative impact on the surface-atmosphere system. Successful closure experiments have been conducted in which empirical results have been used to corroborate model simulations. The approach has been applied to estimate the direct radiative forcing by dust from Asia and by smoke from boreal forest fires. The dust event occurred in April 2002 when snow covered the surface, while the smoke event took place in July 2004 after the snow had melted; thus, permitting evaluations to be made over a range of surface and atmospheric conditions. Given the good agreement between the measured and modeled forcings, it then becomes possible to model the impact of similar aerosols over a wider range of ambient and solar conditions, including characterization of the radiation that most affects photo-chemical and photo-biological processes.