The large interest in Asian aerosols and their radiative impact is derived in part from the Intergovernmental Panel on Climate Change’s assertion that the largest uncertainties in anthropogenic climate forcing models come from our poor knowledge of aerosol distributions and properties. Our paper describes a highly coordinated, international, interdisciplinary experiment that was directed at the climatic impact of Asian dust and pollution aerosols. Since these particles sometimes travel for thousands of km, their impact can be global. One Asian dust storm we studied, which had national evening-news impacts on visibility in Colorado, was last seen by satellite over the Canary Islands in the Eastern Atlantic. In some regions their climatic impact dwarfs that of even CO2: one of our April clear-sky direct forcing estimates for Asian aerosols in the NW Pacific is -30 W/m2, an order of magnitude larger than all of the greenhouse gases combined and opposite in sign.

Because of this high level of interest in aerosol radiative forcing, we designed a coordinated field program involving multiple aircraft, ships, ground stations, satellites, and models, of the type recommended in a 1966 NRC report. We placed a high priority on intercomparison of measurements from different platforms to ensure that spatial and temporal differences could be evaluated in the context of their measurement uncertainties. We also coordinated all flights and cruises with satellite overpasses, so that our in situ observations could be generalized to larger regions. Most of our observations were designed as closure studies, so that we could identify inconsistencies between measurements and models. (An example is the simultaneous measurement of aerosol light scattering and absorption, alongside the solar fluxes the aerosols should change. If a dust layer extinguished more sunlight than our in situ measurements say it should, either the radiative transfer model or the measurements were in error.) This enabled us to identify those parts of the radiation/climate system where uncertainties are still large and improved measurements or models are needed.

The result is a very broad data set that can support tests of a wide variety of aerosol and radiative transfer models. Authors continue to use the data in new ways and usually cite our paper’s description of the experimental strategies that guided its collection.

Although every large field program produces an overview paper, ours is different from most, in that it focuses on the methodologies that are most likely to produce useful insights and reduce uncertainties in climate models. We describe a variety of observing strategies and the things that can be learned from each. Observations that are not well-posed may offer little improvement in our understanding.

ACE-Asia has produced many new discoveries. One is that mineral dust, which has often been treated as a chemically inert substance, has a large impact on the fate of many air pollutants. For instance, nitric acid and sulfur dioxide are efficiently adsorbed onto large dust particles, thus depriving the submicron aerosol mode of mass and acid substances. Even the light absorption by soot is changed, since these tiny particles are less likely to have their absorptivity increased by a coating of sulfate when dust has taken up much of that sulfur. In other words, the pollution and dust aerosols are not independent modes, but both change each other significantly due to their interaction.

One unique aspect of ACE-Asia was the use of a new low-turbulence inlet (LTI) to sample dust particles from aircraft. Traditional inlets lose most large particles as a result of the high turbulence that accompanies the sudden change from stationary to aircraft speed during sampling. Our profiles of large particle properties are far more defendable as a result of this technological advance in airborne aerosol sampling. We found that dust particles aloft are very different from those at the surface, so surface in situ measurements (the only kind any nation can afford do every day) do not accurately reflect aerosol properties in the column of air above the surface. For instance, dust particles near the surface are much more likely to have been modified by pollution than those in the mid-troposphere, making them more soluble and more likely to grow by taking up water vapor.

Chemical transport models were being improved even before we finished our intensive field operations. Three chemical transport models were used in a forecast mode to guide our flights. When the NSF/NCAR C-130 encountered a huge dust plume over the Yellow Sea that had not been forecast by these models, the modelers quickly identified a missing source: a dried-up lake bed in NE China which (although small compared to the deserts) could cause large impacts on the heavily-populated regions of Eastern Asia. Interestingly, this lake was apparently lost due to pumping of ground water for irrigation, so this dust would meet the IPCC’s definition of an anthropogenic aerosol forcing.

The temperature, rainfall, and storminess of the Earth are controlled in large part by how much of the sun’s light is captured by the Earth. For example the Earth would very rapidly become an oven if clouds and light-colored surfaces did not reflect about 30% of incoming sunlight back to space. Tiny particles (dust and pollution) in the atmosphere also can reflect sunlight back to space or in some cases (diesel soot) absorb sunlight and contribute to warming the Earth. Although we know generally how these particles affect the climate, we can’t predict the exact amount of sunlight change as accurately as we need to, to make expensive policy decisions about pollution controls and greenhouse gas emissions.

In the Aerosol Characterization Experiments, or ACE-Asia program, hundreds of scientists from 13 countries carefully measured dust and pollution particles in Asia at the same time as they measured the changes in sunlight due to these particles. This has made it possible to improve our understanding of the particles’ impact on the climate, so that more accurate predictive models can be created. We found, for instance, that natural dust and man-made pollution mix together and make particles that are different from those produced by either source.

We also found that Asian particles can be transported to North America and beyond, making their impact a global one. Since every nation emits air pollutants, the need to accurately predict their impact is common to all countries. That is why a study in Asia is important to people around the entire globe. Collaborative experiments like ACE-Asia by many countries offer the best chance of understanding the impact of aerosol particles on the Earth on which we live together.