A51B-01
Impacts of Atmospheric Aerosols on Regional Climate Changes
In this study observed changes in climate parameters of Taiwan, and in certain cases those of China, are examined with emphasis on changes likely caused by anthropogenic aerosols, particularly those due to the indirect effect of aerosols. As a newly developed country with high population density and extremely large number of factories, Taiwan has one of the highest concentrations of anthropogenic aerosols in the world. Moreover, being an island Taiwan is relatively removed from the direct influence of emissions in other regions of Asia. Since Taiwan has kept a relatively complete, good quality record of climate parameters for the last 100 years, it provides an excellent, unique opportunity to examine the interactions between anthropogenic aerosols and climate changes. We have found two significant changes in climate parameters that are likely caused by anthropogenic aerosols. The first is that the average yearly sunshine duration has decreased by about 15% since 1970's. The other change that has occurred around the same time period is a significant decreasing trend in light precipitation (< 5 mm/hr). We will show that a highly likely cause of the reduction in sunshine duration is an increase in synoptic scale clouds and/or cloud albedo as a result of increased anthropogenic aerosols over Asia. The decreasing trend in light precipitation may also be due to the indirect effect of anthropogenic aerosols. However, other factors and/or processes, e.g. land-use changes and direct effect of aerosols may also play a non-negligible role.
A51B-02
Characterization of Fine and Coarse Modes of Atmospheric Aerosols Using MFRSR Measurements
Ground-based sun-photometry provides an important source of information for characterization of atmospheric aerosols, as well as a validation tool for global satellite aerosol retrievals. While Multi-Filter Rotating Shadow- band Radiometers (MFRSRs) are widely used for measurements of aerosol optical thickness (AOT), the information content of MFRSR data is substantially larger. Our recently updated MFRSR data analysis algorithm allows us also to partition the spectral AOT into fine and coarse modes and to retrieve the fine mode effective radius. Our sensitivity study demonstrated that for a typical accuracy 0.01 of AOT measurements the trade-offs between the spectral aerosol extinction in visible range and NO2absorption effectively prevent conclusive estimation of NO2column from MFRSR data, and may bias aerosol size retrievals. This prompted us to adopt a constrained retrieval technique which relies on climatological amounts of NO2and uses ozone columns from TOMS satellite measurements. To demonstrate geographical and seasonal variability of aerosol properties we present the results of application of the described algorithm to a multi-year dataset from the local MFRSR network at the Southern Great Plains (SGP) site operated by the U.S. Department of Energy Atmospheric Radiation Measurement (ARM) Program. The network consists of 21 instruments located at SGP's Central and Extended Facilities and covers the area of approximately 3 by 4 degrees in northern Oklahoma and southern Kansas. We also present a detailed inter-comparison of our retrievals of total, fine, and coarse AOT, and fine mode effective radius with the correlative AERONET almucantar scan analysis results (Version 2) derived from a CIMEL sun-photometer co-located with two MFRSRs at the SGP's Central Facility.
A51B-03
Effects of Solar Radiation on the Optical Quality of Dissolved Organic Material in a Tidal Marsh Estuarine Ecosystem
Photochemical alteration of colored dissolved organic matter (CDOM) during exposure to solar UV and Visible radiation generates a variety of photoproducts, including reactive oxygen species, atmospherically important trace gases, and microbially labile carbonyl compounds. Sunlight-induced changes in CDOM chemical structure are reflected in changes of its optical properties that provide a first order measure of the photoreactivity of CDOM in a water body of interest. Here, we examined the effects of solar exposure on the photochemical degradation of CDOM derived from different sources in a tidal marsh-estuarine ecosystem. Consistent with changes in fluorescence emission, absorption loss upon exposure to different portions of the solar spectrum (i.e. different long-pass cut-off filters) occurred across the entire spectrum but the wavelength of maximum photobleaching decreased as the cut-off wavelength of the filter decreased. Our results illustrate that solar exposure can cause either an increase or a decrease in the CDOM absorption spectral slope depending on the spectral quality of irradiation and, thus, on the parameters (e.g. aerosol and ozone amounts) that affect the spectral characteristics of the light to which CDOM is exposed. Quantitative description of this response, as needed to model optical quality in coastal waters, is hampered by the lack of models that predict both absorbance and spectral changes. We have developed a simple spectral model for describing the effects of solar radiation on CDOM optical quality. The model accurately, and consistently, predicted the observed dependence of CDOM photobleaching on the amount and spectral quality of solar exposure.
A51B-04
Airborne Spectral Measurements of Surface-Atmosphere Anisotropy Over Different Surfaces in Mexico
During the Intercontinental Chemical Transport Experiment-Phase B/Megacity Initiative: Local and Global
Research Observations (INTEX-B/MILAGRO) the NASA's Cloud Absorption Radiometer (CAR) flew aboard the
Jetstream-31 and measured the spectral and angular distribution of scattered light by clouds and aerosols, and
provided bidirectional reflectance of various surfaces, and imagery of cloud and Earth surface features. From
these measurements, we have selected several cases including Mexico City for determination of aerosol optical
properties, surface bidirectional reflectance distribution function (BRDF), and spectral albedo from a combination
of several datasets: CAR, AERONET and Ames Airborne Tracking Sunphotometer (AATS).
http:car.gsfc.nasa.gov/data/
A51B-05
MAIAC - Multi-Angle Implementation of Atmospheric Correction for MODIS
A new Multi-Angle Implementation of Atmospheric Correction (MAIAC) algorithm has been developed for processing of MODIS measurements. MAIAC is a generic algorithm which simultaneously retrieves the aerosol optical thickness and surface bi-directional reflectance using the 16-day time series of gridded MODIS L1B measurements over land. The new algorithm is generic and works globally over all surface types, including bright deserts, with the temporary exception of snow. MAIAC products include cloud mask, water vapor, aerosol optical thickness at 0.47 mm and 0.66 mm and Angstrom parameter, surface spectral bidirectional reflectance factor and albedo for the reflective land and ocean MODIS bands. All products are generated uniformly at 1 km resolution in gridded format. The initial comparison of the aerosol optical thickness with AERONET measurements for different types of aerosol, including urban/continental (USA, Europe), biomass burning (Africa, Brazil), dust (Saudi Arabia, China), shows a very good agreement.
A51B-06
Estimates of Shortwave Aerosol Forcing From the Data of Ground-Based Networks With Account of Coarse Particles
Beginning with approximately late 1990-th, a surplus (up to 0.1-0.2) in magnitudes of Single Scattering Albedo (SSA) for smoke and dust aerosols has been observed in comparison with the estimates of former years. According to the new data, shortwave aerosol forcing has changed its magnitude and even the sign. For example, as it was underlined in papers published in 2001-2002 by our colleagues from the NASA, a dust plume of the Saharan aerosol absorbs sunlight much less than it had been considered earlier. According to their estimates obtained on the basis of independent ground and space observations, the dust absorbs only 1-5 % of the incident solar radiation instead of 10-15 % as it was considered before. Such decrease in estimates of solar absorption can be attributed to the SSA value of dust aerosol which increased up to 0.95-0.98 in the visible spectrum. One of the possible explanations of this difference in the former and new results could be the incorrect account of optical properties inherent in coarse particles in the determination of aerosol parameters and in subsequent calculations of radiative fluxes. Coarse particles in combination with large optical thickness, typical for fire smokes and dust storms, require a modification of the existing algorithms for processing the data, received from ground-based measuring networks such as AERONET and USDA to provide stability of retrieval of aerosol optical and microphysical parameters. The existing methods of processing the measurements of these networks do not consider particles with radiuses greater than 15 mkm. Such modifications have been made using the analysis of the accessible simultaneous network data by CIMEL and MFRSR photometers. The obtained estimates of shortwave aerosol forcing show that the total solar radiation absorbed by the atmosphere and sandy surface during the storm is greater than it is in the quiet atmosphere. This result is in agreement with the "historic" data obtained, for example, during the USSR-US experiment in Tajikistan in 1989.
A51B-07
Comparison of Asian aerosol's radiative effect in Seoul and Gosan, Korea
Seasonal variations of aerosol optical properties as well as their direct radiative effects were investigated using the ground-based aerosol measurements and an optical model calculation in Seoul, a mega city, and Gosan, a background rural island, Korea. From the yearly AERONET dataset, our analysis of seasonal and monthly cycle of aerosol optical depth (AOD) shows that AODs in Seoul are higher than those in Gosan because of the higher concentrations of water soluble ions in Seoul. Especially, seasonal AOD differences of both sites becomes maximum in Summer due to the synoptic meteorological patterns and hygroscopic growth of anthropogenic aerosols. OPAC (Optical Properties of Aerosol and Clouds) model and Fu-Liou RTM (Radiative Transfer Model) were employed for the evaluation of aerosol direct radiative forcing (ADRF) at surface and top of the atmosphere (TOA). A method of determining the values of aerosol optical properties as input parameters was utilized for the ADRF calculation from the AERONET dataset. In each season, the mean ADRF in Seoul turned out to be larger than that of Gosan. The ADRF in Gosan from RTM calculation was compared with the ground-based radiation measurements during the ACE-Asia IOP in 2001 and the ABC-EAREX2005 IOP in 2005. In addition, ADRF contributions by chemical compositions were compared in this study between Seoul and Gosan.
A51B-08
Spectral Measurements of Aerosol Absorption from UV to VISIBLE
Amount of solar radiation reaching the Earth's surface can be strongly influenced by aerosol absorption. The aerosol absorption optical thickness (AAOT) in the visible and near IR (440 nm- 1020nm) is routinely produced from almucantar measurements made by the CIMEL instruments in the AERONET network. AAOT in the UV (300nm- 368nm) have been derived from the total and diffuse hemispherical flux measurements made by UV- Multifilter Rotating Shadowband Radiometer (UV-MFRSR, Yankee Environmental Systems, Inc.) instruments. However, no direct comparisons between these two methods exist because the CIMEL wavelengths (used in almucantar retrievals) do not overlap with the UV-MFRSR wavelengths. To enable direct comparisons between the two techniques, we have modified our UV-MFRSR, part of USDA UVB Monitoring and Research Network, by replacing standard 300nm filter with 440nm filter used in AERONET network. The instrument has been deployed at Mauna Loa Observatory, at NASA GSFC in Greenbelt, MD (July 2005 - June 2006) and during SCOUT-03 field campaign in Thessaloniki, Greece in July 2006. During these deployments the instrument's calibration was monitored daily using co-located AERONET and BREWER direct sun measurements of aerosol extinction optical thickness (AOT). Between the deployments the instrument was thoroughly calibrated at the NOAA Central UV Calibration Facility in Boulder, Colorado. We find that the UV-MSFRSR instrument is highly susceptible to calibration drifts. However, these drifts can be accurately assessed using AERONET and BREWER direct sun data. After correcting for these calibration changes, the AAOT was inferred by fitting the measurements of global and diffuse atmospheric transmittances with the forward RT model independently at each spectral channel. The AOT data and ancillary measurements of aerosol column particle size distribution and refractive index in the visible wavelengths (by CIMEL sun-sky almucantar inversions), direct -sun column NO2 and ozone (by Brewer mk III spectrometer), surface pressure and albedo constrained the forward radiative transfer model input. Derived AAOT and SSA at 440nm were inter- compared with AERONET almucantar inversions at the same wavelength and with Brewer SSA measurements at 340nm during SCOUT-03 campaign. These measurements constitute the first continuous UV-VIS spectra of AAOT and SSA at two different locations in Maryland, USA and Thessaloniki, Greece.
A51B-09
OMI measurements of aerosol absorption over Central America
The Ozone Monitoring Instrument (OMI) on the Eos-Aura satellite retrieves information on column aerosol absorption using measurements of backscattered radiation in the near UV. OMI aerosol products include the UV Aerosol Index as well as estimates of aerosol extinction and absorption optical depth. Spring season biomass burning is the main source of absorbing aerosols in Central America. The horizontal spread of the Central American carbonaceous aerosol plume may extend over the oceans and as far north as NE United States depending on the meteorological conditions. Pollution aerosols from local sources as well as mineral aerosol from local and distant sources are also frequently detected by OMI over this region. The spatial and temporal distribution of aerosol absorption over Central America as derived from OMI will be discussed.
A51B-10
Clouds modulate terrestrial carbon uptake in a mid-latitude hardwood forest
13 years of turbulent exchange and radiation measurements in a mid-latitude hardwood forest show that clouds enhance radiation use efficiency of carbon uptake (RUE), and that maximum carbon uptake occurs under moderate cloud cover. We find that both cyclic and secular variability of a simple observable metric of cloudiness (transmittance index) is the best statistical predictor of the interannual variability of both net ecosystem exchange (NEE) and gross ecosystem exchange (GEE) seen in our dataset. In contrast other factors analyzed show much weaker relationships with the terrestrial carbon uptake. This suggests that clouds play a pivotal role in driving the interannual variability of terrestrial carbon uptake by this forest and are an important mechanism of carbon cycle/climate interaction.