Every year, from December to April, anthropogenic haze spreads over most of the North Indian Ocean, and South and Southeast Asia. The Indian Ocean Experiment (INDOEX) documented this Indo‐Asian haze ...at scales ranging from individual particles to its contribution to the regional climate forcing. This study integrates the multiplatform observations (satellites, aircraft, ships, surface stations, and balloons) with one‐ and four‐dimensional models to derive the regional aerosol forcing resulting from the direct, the semidirect and the two indirect effects. The haze particles consisted of several inorganic and carbonaceous species, including absorbing black carbon clusters, fly ash, and mineral dust. The most striking result was the large loading of aerosols over most of the South Asian region and the North Indian Ocean. The January to March 1999 visible optical depths were about 0.5 over most of the continent and reached values as large as 0.2 over the equatorial Indian ocean due to long‐range transport. The aerosol layer extended as high as 3 km. Black carbon contributed about 14% to the fine particle mass and 11% to the visible optical depth. The single‐scattering albedo estimated by several independent methods was consistently around 0.9 both inland and over the open ocean. Anthropogenic sources contributed as much as 80% (±10%) to the aerosol loading and the optical depth. The in situ data, which clearly support the existence of the first indirect effect (increased aerosol concentration producing more cloud drops with smaller effective radii), are used to develop a composite indirect effect scheme. The Indo‐Asian aerosols impact the radiative forcing through a complex set of heating (positive forcing) and cooling (negative forcing) processes. Clouds and black carbon emerge as the major players. The dominant factor, however, is the large negative forcing (‐20±4 W m−2) at the surface and the comparably large atmospheric heating. Regionally, the absorbing haze decreased the surface solar radiation by an amount comparable to 50% of the total ocean heat flux and nearly doubled the lower tropospheric solar heating. We demonstrate with a general circulation model how this additional heating significantly perturbs the tropical rainfall patterns and the hydrological cycle with implications to global climate.
Our understanding of the activation of aerosol particles into cloud drops during the formation of warm cumulus clouds presently has a limited observational foundation. Detailed observations of ...aerosol size and composition, cloud microphysics and dynamics, and atmospheric thermodynamic state were collected in a systematic study of 21 cumulus clouds by the Center for Interdisciplinary Remotely‐Piloted Aircraft Studies (CIRPAS) Twin Otter aircraft during NASA's Cirrus Regional Study of Tropical Anvils and Cirrus Layers–Florida Area Cirrus Experiment (CRYSTAL‐FACE). An “aerosol‐cloud” closure study was carried out in which a detailed cloud activation parcel model, which predicts cloud drop concentration using observed aerosol concentration, size distribution, cloud updraft velocity, and thermodynamic state, is evaluated against observations. On average, measured droplet concentration in adiabatic cloud regions is within 15% of the predictions. This agreement is corroborated by independent measurements of aerosol activation carried out by two cloud condensation nucleus (CCN) counters on the aircraft. Variations in aerosol concentration, which ranged from 300 to 3300 cm−3, drives large microphysical differences (250–2300 cm−3) observed among continental and maritime clouds in the South Florida region. This is the first known study in which a cloud parcel model is evaluated in a closure study using a constraining set of data collected from a single platform. Likewise, this is the first known study in which relationships among aerosol size distribution, CCN spectrum, and cloud droplet concentration are all found to be consistent with theory within experimental uncertainties much less than 50%. Vertical profiles of cloud microphysical properties (effective radius, droplet concentration, dispersion) clearly demonstrate the boundary layer aerosol's effect on cloud microphysics throughout the lowest 1 km of cloud depth. Onboard measurements of aerosol hygroscopic growth and the organic to sulfate mass ratio are related to CCN properties. These chemical data are used to quantify the range of uncertainty associated with the simplified treatment of aerosol composition assumed in the closure study.
Vertical profiles of aerosol size, composition, and hygroscopic behavior from Center for Interdisciplinary Remotely Piloted Aircraft Studies (CIRPAS) Twin Otter and National Oceanic and Atmospheric ...Administration R/V Ronald H. Brown observations are used to construct a generic optical model of the Asian Pacific Regional Aerosol Characterization Experiment (ACE‐Asia) aerosol. The model accounts for sulfate, black carbon, organic carbon, sea salt, and mineral dust. The effects of relative humidity and mixing assumptions (internal versus external, coating of dust by pollutants) are explicitly accounted for. The aerosol model is integrated with a Monte Carlo radiative transfer model to compute direct radiative forcing in the solar spectrum. The predicted regional average surface aerosol forcing efficiency (change in clear‐sky radiative flux per unit aerosol optical depth at 500 nm) during the ACE‐Asia intensive period is −65 Wm−2 for pure dust and −60 Wm−2 for pure pollution aerosol (clear skies). A three‐dimensional atmospheric chemical transport model (Chemical Weather Forecast System (CFORS)) is used with the radiative transfer model to derive regional radiative forcing during ACE‐Asia in clear and cloudy skies. Net regional solar direct radiative forcing during the 5–15 April 2001 dust storm period is −3 Wm−2 at the top of the atmosphere and −17 W m−2 at the surface for the region from 20°N to 50°N and 100°E to 150°E when the effects of clouds on the direct forcing are included. The model fluxes and forcing efficiencies are found to be in good agreement with surface radiometric observations made aboard the R.H. Brown. Mean cloud conditions are found to moderate the top of atmosphere (TOA) radiative forcing by a factor of ∼3 compared to clear‐sky calculations, but atmospheric absorption by aerosol is not strongly affected by clouds in this study. The regional aerosol effect at the TOA (“climate forcing”) of −3 Wm−2 is comparable in magnitude, but of opposite sign, to present‐day anthropogenic greenhouse gas forcing. The forcing observed during ACE‐Asia is similar in character to that seen during other major field experiments downwind of industrial and biomass black carbon sources (e.g., the Indian Ocean Experiment (INDOEX)), insofar as the primary effect of aerosol is to redistribute solar heating from the surface to the atmosphere.
ACE-ASIA Seinfeld, John H.; Carmichael, Gregory R.; Arimoto, Richard ...
Bulletin of the American Meteorological Society,
03/2004, Volume:
85, Issue:
3
Journal Article
Peer reviewed
Open access
Although continental-scale plumes of Asian dust and pollution reduce the amount of solar radiation reaching the earth's surface and perturb the chemistry of the atmosphere, our ability to quantify ...these effects has been limited by a lack of critical observations, particularly of layers above the surface. Comprehensive surface, airborne, shipboard, and satellite measurements of Asian aerosol chemical composition, size, optical properties, and radiative impacts were performed during the Asian Pacific Regional Aerosol Characterization Experiment (ACE-Asia) study. Measurements within a massive Chinese dust storm at numerous widely spaced sampling locations revealed the highly complex structure of the atmosphere, in which layers of dust, urban pollution, and biomass-burning smoke may be transported long distances as distinct entities or mixed together. The data allow a first-time assessment of the regional climatic and atmospheric chemical effects of a continental-scale mixture of dust and pollution. Our results show that radiative flux reductions during such episodes are sufficient to cause regional climate change.
This paper presents one of the few quantitative estimates of surface aerosol forcing made directly from surface irradiance observations. The method described within yields estimates of the forcing ...accurate to 20%. The study was conducted from February to March 1998 at the Kaashidhoo Climate Observatory (KCO) during the First Field Phase of the Indian Ocean Experiment (INDOEX‐FFP). For the 400–700 nm region studied here, the forcing is −7.6±1.5 W m−2. The data are obtained from two photodiode radiometers measuring global and diffuse irradiance in five channels in the visible and ultraviolet. The instruments were chosen, calibrated, and deployed specifically for a precise measurement of aerosol forcing. The angular, spectral, and absolute response characteristics of the instruments are determined in the laboratory and used to calibrate the data, as described here. The accuracy in the calibrated data is 2.4% for the global irradiance and 1.8% for the diffuse irradiance. Direct aerosol forcing is obtained from the measured aerosol forcing efficiency, which is determined by two methods: hybrid and differential. The hybrid method uses a radiative transfer model to subtract out the contribution from the aerosol‐free atmosphere. The differential method assumes that changes in 400–700 nm solar flux are forced by changes in aerosol optical depth. By using flux changes, the differential method is not sensitive to the small calibration uncertainties, and is independent of model assumptions about the single‐scatter properties of the aerosol. For this soot‐laden marine region south of India, a 0.1 change in aerosol optical depth produces a −4.0±0.8 W m−2; change in the 400–700 nm surface flux; 55% of this forcing is observed in the 400–540 nm region. The global and diffuse data agree to within 5 W m−2 of results calculated by a Monte Carlo radiative transfer model. The model assumes an aerosol consistent with the spectral optical depth, lidar vertical profiles, and surface optical properties measured simultaneously at KCO.
The accuracy of the 2003 prognostic, physically based aerosol activation parameterization of A. Nenes and J. H. Seinfeld (NS) with modifications introduced by C. Fountoukis and A. Nenes in 2005 ...(modified NS) is evaluated against extensive microphysical data sets collected on board the Center for Interdisciplinary Remotely Piloted Aircraft Studies (CIRPAS) Twin Otter aircraft for cumuliform and stratiform clouds of marine and continental origin. The cumuliform cloud data were collected during NASA's Cirrus Regional Study of Tropical Anvils and Cirrus Layers–Florida Area Cirrus Experiment (CRYSTAL‐FACE, Key West, Florida, July 2002), while the stratiform cloud data were gathered during Coastal Stratocumulus Imposed Perturbation Experiment (CSTRIPE, Monterey, California, July 2003). In situ data sets of aerosol size distribution, chemical composition, and updraft velocities are used as input for the NS parameterization, and the evaluation is carried out by comparing predicted cloud droplet number concentrations (CDNC) with observations. This is the first known study in which a prognostic cloud droplet activation parameterization has been evaluated against a wide range of observations. On average, predicted droplet concentration in adiabatic regions is within ∼20% of observations at the base of cumuliform clouds and ∼30% of observations at different altitudes throughout the stratiform clouds, all within experimental uncertainty. Furthermore, CDNC is well parameterized using either a single mean updraft velocity or by weighting droplet nucleation rates with a Gaussian probability density function of w. This study suggests that for nonprecipitating warm clouds of variable microphysics, aerosol composition, and size distribution the modified NS parameterization can accurately predict cloud droplet activation and can be successfully implemented for describing the aerosol activation process in global climate models.
This work examines the effect of black carbon (BC) radiative heating on cloud droplet formation. Changes in cloud droplet concentration and cloud albedo due to the presence of black carbon are ...computed for different cases of aerosol size distributions, meteorological conditions, BC mixing state, and aerosol composition. We examine the effect of three new mechanisms (that result from BC heating) that can affect cloud droplet number and lifetime. Two of these mechanisms act to increase cloud droplet number or lifetime: (1) the ability of BC to decrease the collection efficiency of giant cloud condensation nuclei (CCN) and (2) the delayed growth of low‐Sc CCN that allow higher‐Sc CCN to form droplets. These two mechanisms complement each other in terms of increasing cloud droplet number, since it is shown that the former is most efficient at strong updrafts and the latter is most efficient at low updraft velocities. A third mechanism identified, gas‐phase heating (which is different from the so‐called “semi‐direct effect”), in our simulations acts to decrease LWC, and thus albedo; however, the droplet number concentration does not change significantly due to dynamic readjustments in cloud supersaturation. The simulations indicate that the mixing state of BC with the CCN population can have an important influence on the effect of BC heating on the droplet population. Although additional work is necessary to fully assess the effects of BC heating on cloud microphysics and climate, this work shows that these effects are more complex than currently thought.
Aerosol light absorption can be intense close to local sources such as wildland and oil fires, with smoke that disperses into the boundary layer and, with enough lift, into the upper atmosphere where ...it may be transported around the globe. Filter‐based methods such as the Particle Soot Absorption Photometer (PSAP) are most commonly used to quantify aerosol light absorption aloft. This paper reports first measurements of aerosol light absorption aloft with photoacoustic instrumentation (PA). Three examples of aerosol light absorption are presented. The first one illustrates a case of detached layers aloft arising from intercontinental, interoceanic transport of smoke from wildland fires in Siberia to the North American continent and the measurement campaign held at the Department of Energy Atmospheric Radiation Measurement Program Climate Research Facility in north central Oklahoma. Then, two examples of intense local fire smoke light absorption from the Coastal Stratocumulus Imposed Perturbation Experiment near Marina, California, USA, are presented. The first local fire was an oil fire burning in a storage tank near Moss Landing, California, USA, and smoke from this fire was very dark, indicating a low single scattering albedo. By contrast, the second local fire was predominantly burning wood, vegetation, and structures near Fort Ord in Marina, California, USA, and the smoke was very bright, indicating a high single scattering albedo. In all examples, PA measurements at 676 nm were compared with those from a PSAP modified to measure at three wavelengths, including 660 nm.
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