Santiago Gassó

Asian dust events of April 1998

On April 15 and 19, 1998, two intense dust storms were generated over the Gobi desert by springtime low-pressure systems descending from the northwest. The windblown dust was detected and its evolution followed by its yellow color on SeaWiFS satellite images, routine surface-based monitoring, and through serendipitous observations. The April 15 dust cloud was recirculating, and it was removed by a precipitating weather system over east Asia. The April 19 dust cloud crossed the Pacific Ocean in 5 days, subsided to the surface along the mountain ranges between British Columbia and California, and impacted severely the optical and the concentration environments of the region. In east Asia the dust clouds increased the albedo over the cloudless ocean and land by up to 10–20%, but it reduced the near-UV cloud reflectance, causing a yellow coloration of all surfaces. The yellow colored backscattering by the dust eludes a plausible explanation using simple Mie theory with constant refractive index. Over the West Coast the dust layer has increased the spectrally uniform optical depth to about 0.4, reduced the direct solar radiation by 30–40%, doubled the diffuse radiation, and caused a whitish discoloration of the blue sky. On April 29 the average excess surface-level dust aerosol concentration over the valleys of the West Coast was about 20–50 μg/m3 with local peaks >100 μg/m3. The dust mass mean diameter was 2–3 μm, and the dust chemical fingerprints were evident throughout the West Coast and extended to Minnesota. The April 1998 dust event has impacted the surface aerosol concentration 2–4 times more than any other dust event since 1988. The dust events were observed and interpreted by an ad hoc international web-based virtual community. It would be useful to set up a community-supported web-based infrastructure to monitor the global aerosol pattern for such extreme aerosol events, to alert and to inform the interested communities, and to facilitate collaborative analysis for improved air quality and disaster management.

Influence of Humidity On the Aerosol Scattering Coefficient and Its Effect on the Upwelling Radiance During ACE-2

Aerosol scattering coefficients (σsp) have been measured over the ocean at different relative humidities (RH) as a function of altitude in the region surrounding the Canary Islands during the Second Aerosol Characterization Experiment (ACE-2) in June and July 1997. The data were collected by the University of Washington passive humidigraph (UWPH) mounted on the Pelican research aircraft. Concurrently, particle size distributions, absorption coefficients and aerosol optical depth were measured throughout 17 flights. A parameterization of σsp as a function of RH was utilized to assess the impact of aerosol hydration on the upwelling radiance (normalized to the solar constant and cosine of zenith angle). The top of the atmosphere radiance signal was simulated at wavelengths corresponding to visible and near-infrared bands of the EOS-AM )“Terra” (detectors, MODIS and MISR. The UWPH measured σsp at 2 RHs, one below and the other above ambient conditions. Ambient σsp was obtained by interpolation of these 2 measurements. The data were stratified in terms of 3 types of aerosols: Saharan dust, clean marine (marine boundary layer background) and polluted marine aerosols (i.e., 2- or 1-day old polluted aerosols advected from Europe). An empirical relation for the dependence of σsp on RH, defined by σsp(RH)=k. (1−RH/100)−γ, was used with the hygroscopic exponent γ derived from the data. The following γ values were obtained for the 3 aerosol types: γ(dust)=0.23±0.05, γ(clean marine)= 0.69±0.06 and γ(polluted marine)=0.57±0.06. Based on the measured γs, the above equation was utilized to derive aerosol models with different hygroscopicities. The satellite simulation signal code 6S was used to compute the upwelling radiance corresponding to each of those aerosol models at several ambient humidities. For the pre-launch estimated precision of the sensors and the assumed viewing geometry of the instrument, the simulations suggest that the spectral and angular dependence of the reflectance measured by MISR is not sufficient to distinguish aerosol models with various different combinations of values for dry composition, γ and ambient RH. A similar behavior is observed for MODIS at visible wavelengths. However, the 2100 nm band of MODIS appears to be able to differentiate between at least same aerosol models with different aerosol hygroscopicity given the MODIS calibration error requirements. This result suggests the possibility of retrieval of aerosol hygroscopicity by MODIS.

In situ aerosol-size distributions and clear-column radiative closure during ACE-2

As part of the second Aerosol Characterization Experiment (ACE-2) during June and July of 1997, aerosol-size distributions were measured on board the CIRPAS Pelican aircraft through the use of a DMA and 2 OPCs. During the campaign, the boundary-layer aerosol typically possessed characteristics representative of a background marine aerosol or a continentally influenced aerosol, while the free-tropospheric aerosol was characterized by the presence or absence of a Saharan dust layer. A range of radiative closure comparisons were made using the data obtained during vertical profiles flown on 4 missions. Of particular interest here are the comparisons made between the optical properties as determined through the use of measured aerosol-size distributions and those measured directly by an airborne 14-wavelength sunphotometer and 3 nephelometers. Variations in the relative humidity associated with each of the direct measurements required consideration of the hygroscopic properties of the aerosol for size-distribution-based calculations. Simultaneous comparison with such a wide range of directly-measured optical parameters not only offers evidence of the validity of the physicochemical description of the aerosol when closure is achieved, but also provides insight into potential sources of error when some or all of the comparisons result in disagreement. Agreement between the derived and directly-measured optical properties varied for different measurements and for different cases. Averaged over the 4 case studies, the derived extinction coefficient at 525 nm exceeded that measured by the sunphotometer by 2.5% in the clean boundary layer, but underestimated measurements by 13% during pollution events. For measurements within the free troposphere, the mean derived extinction coefficient was 3.3% and 17% less than that measured by the sunphotometer during dusty and non-dusty conditions, respectively. Likewise, averaged discrepancies between the derived and measured scattering coefficient were −9.6%, +4.7%, +17%, and −41% for measurements within the clean boundary layer, polluted boundary layer, free troposphere with a dust layer, and free troposphere without a dust layer, respectively. Each of these quantities, as well as the majority of the >100 individual comparisons from which they were averaged, were within estimated uncertainties.

Urban/industrial aerosol: Ground‐based Sun/sky radiometer and airborne in situ measurements

Both airborne in situ and ground-based remote sensing methods are used to measure the properties of urban/industrial aerosols during the Sulfate Clouds and Radiation—Atlantic (SCAR-A) experiment in 1993. Airborne in situ methods directly measure aerosol characteristics such as size distribution and scattering coefficient at a particular altitude and infer the total column optical properties, such as optical thickness. Ground-based remote sensing is sensitive to the aerosol optical properties of the entire column and infers the physical properties from inversion of sky radiance. Comparison of optical thickness measurements are encouraging but inconclusive because of measured profiles which extend no higher than 2 km. By comparing aerosol volume size distributions we find that the two systems are in agreement in the radius size range 0.05–2 μm, after the stratospheric aerosol mode is removed from the remote sensing data. At larger aerosol sizes both systems suffer from greater uncertainty, and the larger aerosols themselves are less spatially uniform because of their short lifetimes. The combination of factors makes the comparison at larger radii impossible. The disadvantages of the in situ systems are that there is a measuring efficiency for each device which is dependent on aerosol size and that airborne in situ measurements are rare events in time and space. Also, in situ instruments dry the aerosol before measurement. Automatic remote sensing procedures measure the total column ambient aerosol unaffected by drying or sampling issues, and these instruments can be installed globally to make observations many times per day. However, the disadvantages to remote sensing are that the inferred physical properties are dependent on the assumptions and numerical limitations of the inversion procedures. The favorable comparison between the two types of measurement systems suggests that these drawbacks are manageable in both cases.

Aerosol measurements in the Arctic relevant to direct and indirect radiative forcing

Airborne measurements in the Arctic in June permit calculation of some of the parameters needed to assess both the direct and indirect radiative forcing by aerosols in the region. Values for the single-scattering albedo of the aerosols suggest that in June the direct effect will produce a net cooling, in contrast to winter arctic hazes. Internal closure calculations comparing aerosol size distribution measurements with directly measured light scattering by aerosols over the first 4 km of the atmosphere show good agreement. Measurements of cloud condensation nucleus (CCN) activation spectra show steeper slopes than previous measurements in the Arctic in winter and early spring. Based on the CCN and collated cloud microphysical measurements, the susceptibility of the clouds encountered in this project to aerosol-induced albedo modification appears quite high.

Regional aerosol optical depth characteristics from satellite observations: ACE-1, TARFOX and ACE-2 results

Analysis of the aerosol properties during 3 recent international field campaigns (ACE-1, TARFOX and ACE-2) are described using satellite retrievals from NOAA AVHRR data. Validation of the satellite retrieval procedure is performed with airborne, shipboard, and land-based sunphotometry during ACE-2. The intercomparison between satellite and surface optical depths has a correlation coefficient of 0.93 for 630 nm wavelength and 0.92 for 860 nm wavelength. The standard error of estimate is 0.025 for 630 nm wavelength and 0.023 for 860 nm wavelength. Regional aerosol properties are examined in composite analysis of aerosol optical properties from the ACE-1, TARFOX and ACE-2 regions. ACE-1 and ACE-2 regions have strong modes in the distribution of optical depth around 0.1, but the ACE-2 tails toward higher values yielding an average of 0.16 consistent with pollution and dust aerosol intrusions. The TARFOX region has a noticeable mode of 0.2, but has significant spread of aerosol optical depth values consistent with the varied continental aerosol constituents off the eastern North American Coast.

Shipboard Sunphotometer Measurements of Aerosol Optical Depth Spectra and Columnar Water Vapor During ACE-2, and Comparison with Selected Land, Ship, Aircraft, and Satellite Measurements

Analyses of aerosol optical depth (AOD) and columnar water vapor (CWV) measurements acquired with NASA Ames Research Center’s 6-channel Airborne Tracking Sunphotometer (AATS-6) operated aboard the R/V Professor Vodyanitskiy during the 2nd Aerosol Characterization Experiment (ACE-2) are discussed. Data are compared with various in situ and remote measurements for selected cases. The focus is on 10 July, when the Pelican airplane flew within 70 km of the ship near the time of a NOAA-14/AVHRR satellite overpass and AOD measurements with the 14−channel Ames Airborne Tracking Sunphotometer (AATS-14) above the marine boundary layer (MBL) permitted calculation of AOD within the MBL from the AATS-6 measurements. A detailed column closure test is performed for MBL AOD on 10 July by comparing the AATS-6 MBL AODs with corresponding values calculated by combining shipboard particle size distribution measurements with models of hygroscopic growth and radiosonde humidity profiles (plus assumptions on the vertical profile of the dry particle size distribution and composition). Large differences (30−80% in the mid-visible) between measured and reconstructed AODs are obtained, in large part because of the high sensitivity of the closure methodology to hygroscopic growth models, which vary considerably and have not been validated over the necessary range of particle size/composition distributions. The wavelength dependence of AATS-6 AODs is compared with the corresponding dependence of aerosol extinction calculated from shipboard measurements of aerosol size distribution and of total scattering measured by a shipboard integrating nephelometer for several days. Results are highly variable, illustrating further the great difficulty of deriving column values from point measurements. AATS-6 CWV values are shown to agree well with corresponding values derived from radiosonde measurements during 8 soundings on 7 days and also with values calculated from measurements taken on 10 July with the AATS-14 and the University of Washington Passive Humidigraph aboard the Pelican.

Comparison of Columnar Aerosol Optical Properties Measured by the MODIS Airborne Simulator with In Situ Measurements: A Case Study

Abstract Airborne in situ and airborne remote sensing methods are used to measure the aerosol optical properties of a biomass burning plume which extended over the ocean during the Sulfate, Clouds And Radiation (SCAR) field experiment in the summer of 1994. The University of Washington Convair C131 in situ instrumentation directly measured aerosol characteristics such as size distribution and scattering coefficient at a particular altitude and also measured the optical depth below the aircraft by a lidar instrument mounted on the plane. Almost simultaneously with the C131 pass, radiances were measured by the MODIS Airborne Simulator mounted in the ER-2 NASA aircraft and used to compute optical depths and particle size distribution parameters. These were compared with the same measurements taken by the in situ platform. The effective radii derived by the MAS retrieval algorithm (0.06–0.08 μm) are consistently smaller than those from the in situ samples taken in this study (0.10–0.14 μm). The comparison of MAS optical depths and lidar optical depths measured from the in situ platform indicates that the MAS optical depths agree within the range 0.4 MAS