A13G-01 INVITED
Characteristics and Cloud Interactions of Ice Nucleating Aerosols From Asian Continental Emissions During the Pacific Dust Experiment
In contemplating the potential indirect effect of various natural and anthropogenic aerosol particles on clouds that extend to below the freezing level, it is understood from previous studies that it is vital to document the transport, transformations, and cloud activation behaviors of desert dust and certain carbonaceous particles in the atmosphere. The Asian continent provides large sources for both of these basic particle species to the atmosphere, with dust production peaking in the spring. Here we report first measurements of the ice nucleation behavior of aerosols exiting the Asian continent at various stages and levels of transport across the Pacific Ocean. These measurements were made during the Pacific Dust Experiment (PACDEX), a project that utilized the special capabilities of the NSF HIAPER aircraft for extended deployment over intercontinental scales. The study occurred during the months of April and May 2007. We will describe ice nuclei (IN) measurement results and relate these to other measurements of aerosol properties, including the aerosol size distribution and single particle compositions. IN concentrations varied over the full range ever measured in the free troposphere, and in general proportion with the concentrations of larger aerosol particles, presumably indicating the strong influence of dust particle plumes in the free troposphere. Aerosol processing during transport was associated with apparent negative impacts on IN concentrations. Conclusions regarding the potential role of carbonaceous particles as ice nuclei will require more careful analyses due to frequent mixing with dust particles. We will also discuss some preliminary results regarding aerosol-cold cloud interactions using additional measurements of the ice nucleation ability of residues of cloud particles sampled with a counterflow virtual impactor inlet.
A13G-02 INVITED
Three Dimensional Modeling Analysis of the Transpacific Transport of Aerosols During PACDEX
Mineral dust and aerosols emitted from Asia are known to traverse long distances across the Pacific Ocean and can reach North America within a few days. A pilot field study, the PACific Dust Experiment (PACDEX), was carried out in April and May of 2007, during the peak East Asian dust emission season. The NSF/NCAR-HIAPER (High Performance Instrumented Airborne Platform for Environmental Research) platform allowed for sampling the evolution of mineral aerosol/pollution plumes and their physical and chemical characteristics as they traverse the Pacific Ocean and interact with the Pacific cloud systems en route to North America in both the upper and lower troposphere. A comprehensive 3-dimensional regional-scale model developed at The University of Iowa (Sulfur Transport dEposition Model, STEM) has been used for the analysis of aerosol interactions to help define key measurement strategies during the mission and to help interpret observations from the HIAPER platform. In this study we will present model aerosol distribution inter-comparison with cloud fields and aircraft observations. Model analysis provides further insight into cloud/pollution/dust interactions as East Asian emissions transit the Pacific Ocean en route to North America. Trajectory analysis and emission markers are used to help understand the air mass history and aerosol aging processes of the aerosols sampled by the HIAPER platform. Estimates of the fluxes of aerosol dust, BC and sulfate due to transpacific transport will also be presented.
A13G-03
Emissions and transport of air pollutants from China to the Pacific: Major findings from the EAST-AIRE air campaign
Accompanying economic boom over the past few decades, pollutant emissions from China have increased dramatically and raised growing concerns regarding their large-scale impact. Observations over the Pacific Ocean and numerical simulations generally identify mid-latitude cyclones as the major mechanism driving the long-range transport of pollutants off the Chinese coast to downwind areas. Here we present results from the first aircraft campaign of EAST-AIRE (East Asian Study of Tropospheric Aerosols: an International Regional Experiment), carried out over an industrialized region in Northeast China in spring 2005. Prefrontal and postfrontal flights provide vertical distribution of pollutants within different sectors of two mid-latitude cyclones traveling through the area. In consistence with previous studies, both cyclones feature abundant anthropogenic pollutants ahead of cold fronts, and much lower pollutant levels (but with dust) behind cold fronts. Pollutant levels above the planetary boundary layer (PBL) were found substantial in one prefrontal flight (April 5) but low in the other (April 11), showing different potentials for long-range transport. Backward trajectories suggest that in both cases, isentropic upward motions associated with the SW flows in the warm sector were weak and largely constrained within PBL. Synoptic analysis and satellite observations further indicate that upwind dry (non- precipitating) convection may explain the pollutants observed above PBL on the 5th. With the assistance of forward trajectory analysis and chemical transport models, two satellite sensors (OMI and MODIS) successfully tracked the pollution plume associated with the April 5 cyclone, as it propagated into the North Pacific on the next few days. Satellite observed changes in SO2 and aerosol content within the plume are used to qualitatively estimate the conversion from aerosol precursor gases to secondary aerosols, in a semi-Lagrangian way. http://www.atmos.umd.edu/~zli/EAST-AIRE/station.htm
A13G-04
Black carbon in cloud residual nuclei during PACDEX: Combining the single particle soot photometer and the counterflow virtual impactor
The single particle soot photometer (SP2) measures black carbon (BC) using laser incandescence. In the PACific Dust EXperiment (PACDEX), the SP2 was operated downstream of a counterflow virtual impactor (CVI) during flight portions when the aircraft was passing through a cloud. The CVI collects cloud droplets and ice crystals larger than 5 μm and evaporates the water content, so that residual nuclei are sampled. The CVI also concentrates the incoming air-stream by as much as a factor of 30 or more. The combination of the SP2 with the CVI enables BC measurements below a few ng/m3. Preliminary results indicate that compared to aerosol in the surrounding air mass, black carbon concentrations (per unit volume air) were generally lower in cloud, but a greater fraction of cloud residual particles contain BC. Cloud residual nuclei also seem to contain more BC mass/particle than the ambient aerosol. The May 5 flight made a number of passes through a Pacific frontal system. During one such pass at 8.3 km ASL, BC in residual nuclei sampled through the CVI was on average 0.4 ng/m3 with a mean incandescent particle concentration of 0.1 particles/cm3, compared to over 7 ng/m3 and 2.5 particles/cm3 in ambient aerosol behind the front. For total concentrations over 0.2 particles/cm3 as detected by the SP2, the fraction of cloud nuclei that incandesced or contained BC was often greater than the incandescing fraction of the ambient aerosol at the same altitude, with up to 40% of cloud nuclei incandescing compared to ~10-25% for ambient aerosol. BC mass distributions peaked around 5 fg-BC/particle in ambient air. Inside the front, BC mass distributions in cloud nuclei were broader with peaks between 10-23 fg-BC/particle. Possible explanations for these results are that either BC-containing particles are scavenged by clouds, or these particles are good cloud nuclei, with larger aerosol containing more BC mass/particle preferentially forming cloud droplets and ice crystals.
A13G-05 INVITED
The Impact of Black Carbon on Cloud Radiative Forcing
We present an overview of the current level of understanding on how black carbon may modify the shortwave radiative properties of clouds, from the perturbation of the scattering and absorption properties of individual cloud droplets and ice crystals to its influence on energy balance via the transmitted and reflected spectral irradiance fields. Until recently, most of our knowledge resulted from modeling studies. However, the April-May 2007 PACific Dust EXperiment (PACDEX) provided a first of its kind opportunity to investigate the evolution of urban industrial aerosols plumes as they transport from their Eurasian source regions to North America, with subsequent radiative, dynamical, and microphysical impacts on trans-Pacific storm systems. We will highlight measurements of spectral cloud reflectance and transmittance in the presence of pollution and interpret in the context of some of the recent modeling studies on black carbon – cloud interactions.
A13G-06
Spectral Signature of Radiative Forcing by East Asian Dust-Soot Mixture
The Pacific Dust Experiment (PACDEX) provides the first detailed sampling of dust-soot mixtures from the western Pacific to the eastern Pacific Ocean. The data includes down and up spectral irradiance, mixing state of dust and soot, and other aerosol properties. This study attempts to simulate the radiative forcing by dust-soot mixtures during the experimental period. The MODTRAN band model was employed to investigate the spectral signatures of solar irradiance change induced by aerosols at moderate spectral resolutions. For the short wave band (300-1100nm) used in this study, the reduction of downward irradiance at surface by aerosols greatly enhances with increasing wavelength in the UV band (300-400nm), reaches a maximum in the blue band, then gradually decreases toward the red band. In the near-IR band (700-1100nm), irradiance reduction by aerosols shows great fluctuations in the band with center wavelength at around 940nm, 820nm, 720nm, 760nm, 690nm, where the aerosol effect is overwhelmed by the water vapor and O2 absorptions. The spectral pattern of irradiance reduction varies for different aerosol species. The maximum reduction lies at around 450nm for soot, and shifting to about 490nm for East Asian mineral dust. It's worth noting that although soot aerosols reduce more irradiance than East Asian dust in the UV and blue band, the impact of dust to the irradiance exceeds that by soot at the longer wavelength band (i.e. around 550nm). The reduction of irradiance by East Asian dust (soot) in the UV band, visible band, and near-IR accounts for about 6% (10%), 56% (64%), and 38% (26%) of total irradiance reduction. As large amount of soot aerosols are involved during the long range transport of East Asian dust, the optical properties of dust aerosols are modified with different mixing state with soot, the spectral pattern of the irradiance reduction will be changed. The study of aerosol forcing at moderate spectral resolutions has the potential application for research on aerosol mixing state and its climate impacts.
A13G-07
Measurements of Solar Spectral Irradiance During the PACific Dust EXperiment (PACDEX)
The Hiaper Atmospheric Radiation Package (HARP) made its first flight on the NCAR HIAPER aircraft during the PACDEX mission of April and May of 2007, measuring upwelling and downwelling solar spectral irradiance as well as actinic flux. The combination of spectral irradiance, actinic flux, and a host of in situ measurements enables the simultaneous investigation of aerosol and cloud properties and their effects on important radiative quantities like flux divergence and single scattering albedo. For example, flights across the Pacific ocean encountered several examples of high pollution (as identified by elevated black carbon and carbon monoxide levels) encountered both in and out of clouds. Of particular interest are clouds with elevated levels of black carbon where multiple scattering (especially in the case of internal mixing) is thought to enhance absorption. We examine the solar radiative changes associated with pollution plumes with emphasis on the effects of black carbon on clouds and the clear sky.
A13G-08
The Pacific Dust Experiment (PaCDEX) Field Campaign: A summary of accomplishments during the field campaign and examples of early results.
The Pacific Dust Experiment (PACDEX) is a pilot study utilizing quasi-Lagrangian sampling of the Asian dust and pollution plumes as they travel across the Pacific Ocean and interact with clouds and maritime weather systems. The NSF/NCAR G-V aircraft was used, guided by daily chemical forecasts, to follow dust events at various Pacific locations during the eastbound transit towards North America. Objectives of PACDEX include identification of the principal cloud-active and black carbon aerosols in the Asian plume, documenting changes in cloud hydrometeors in regions affected by the plumes, documenting the solar changes in the plume vertical structure in clear and cloudy regions, and examining differences in these variables among Western and Eastern Pacific plumes. Fourteen research flights were conducted in April and May 2007. Asian dust events were followed across much of the Pacific Ocean at altitudes from 11 km to near the surface. Cross-frontal sampling in several extratropical storms was conducted at multiple altitudes in regions where the plumes were found. Ice forming nuclei (IN) activation spectra (at various supersaturations and temperatures) were obtained in several dust regions, polluted regions, and in background air. Early results from the field project suggest that Asian dust serves as an important source of IN to maritime clouds, and that anthropogenic Asian emissions, particularly CO and black carbon, are effectively incorporated into the upper regions of extratropical storm clouds.