particle surface tension; and extend his studies to particles > 200 nm, where aerosols may affect human health and atmospheric visibility. Finally, because nucleation occurs quickly, he requires sampling rates of < 1 min. Time response becomes especially important for researchers collecting atmospheric data from airplanes. "The slowest plane moves at 50 m/s," said Peter Daum of Brookhaven National Laboratory's Environmental Chemistry Division, "and speeds can be over 200 m/s." Thus spatial averages for a single measurement can run on the order of kilometers. A nitric acid filter pack, for instance, requires about 1 h of flying time to collect a sufficient sample, whereas many trace gas instruments have time responses on the order of 1060 s. However, a typical smokestack plume measures 0.5 km, and clouds can have diameters of < 1 km. Obtaining the required spatial resolution requires rapid and sensitive analysis. "The first law of airplane sampling," said Daum, "is that if the sensitivity is high enough then the response time is too slow, and vice versa. The second law is that if there is the desired response and sensitivity, then
it is too big or requires too much power." Most ofthe airborne analytical devices are commercial instruments that have been modified for flight. Air samples are composed of several phases—aerosols, gases, and droplets—that need to be distinguished. One method of separating phases is to pass the sample air through a conventional cyclone wherein the heavier particles are thrown to the sides and drop to the bottom. Alternatively, the axial flow separator spins the air sample via a static vane assembly, throwing particles to the sides of the sampling tube where they may be drawn off. For both these devices adiabatic heating and subsequent evaporation of water droplets is of concern. A newer, more sophisticated collector—the counterflow impactor—has been developed to obtain size-distributed cloud droplet composition. An inlet nozzle lined with a sintered metal tube sticks out of the aircraft. A counterflow of air is fed through this tube at a controlled rate so as to determine the size particles that will pass through the inlet. As the counterflow velocity increases, the smaller particles are excluded from the inlet and only the larger particles are sampled.
The trickiest sample to collect by plane is snow. Usually, says Daum, a cyclone collector is employed. However, placement of the cyclone is critical because snowflakes typically have a low density and tend to follow the flow stream around the plane, bypassing the collector. These sample inlets require careful attention, and a history of their activity must be maintained. Once again, sticky molecules such as HNO3, H2O2, and NH3 cause problems. Given the recent political changes on the international scene that have brought a new awareness of the environment, Daum sees the need for air sampling at more and increasingly remote sites. "We are going to need the techniques to measure lower concentrations from airplanes," said Daum. The symposium was organized by Leonard Newman of Brookhaven National Laboratory's Environmental Chemistry Division and will be published in the ACS Advances in Chemistry series under the title Measurement Challenges in Atmospheric Chemistry. Newman is looking for additional contributions to the volume and invites interested researchers in this field to contact him. Alan R. Newman
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