Veerabhadran Ramanathan, a professor of applied ocean sciences and of atmospheric sciences and director of the Center for Atmospheric Sciences at Scripps Institution of Oceanography at the University of California, San Diego, in La Jolla, is investigating the impact of atmospheric brown clouds — clouds loaded with anthropogenic pollutants — on the Earth’s climate. Using miniaturized photonic instruments aboard autonomous unmanned aerial vehicles, he and his colleagues have collected data from clouds over the Indian Ocean that they hope will offer some insight into the effects of particulate pollution on cloud albedo and on the rates of rainfall.Veerabhadran Ramanathan and colleagues have demonstrated that autonomous unmanned aerial vehicles carrying miniaturized photonic payloads can be used to perform in situ studies of atmospheric brown clouds— clouds loaded with pollutants that affect Earth’s climate. In March, the researchers launched three drones a total of 18 times over the Indian Ocean, logging nearly 127 flight hours. The instrument clusters weigh only 5 kg, which Ramanathan noted is less than half the weight of commercial devices. The result of necessity and years of effort, the reduction in mass and volume included redesigning the instruments and crafting custom autonomous aircraft.To perform the cloud studies, the researchers had to fly instruments into the middle of a cloud and a few hundred meters above and below it simultaneously for hours at a time. Because of the length of the flights, using multiple manned aircraft would be too costly, and there were safety issues involved in maintaining the formations. Commercial autonomous unmanned aerial vehicles either were not up to the task or were too expensive. The group, therefore, decided to use custom platforms manufactured by Advanced Ceramics Research Inc. of Tucson, Ariz.The redesign included the repackaging of commercial instruments in-to a lightweight enclosure. The scientists started with instruments from TSI Inc. of Shoreview, Minn., Met One Instruments Inc. of Grants Pass, Ore., Droplet Measurement Technologies of Boulder, Colo., and Li-Cor Biosciences of Lincoln, Neb. Electronics also were miniaturized to fit on one card. Among the devices in the final instrument suites is a condensation particle counter, which uses a 50-mW laser diode operating at 780 nm to measure total aerosol concentration for particles with a diameter of 0.01 to 1.0 μm. To measure concentrations of 0.3- to 10-μm particles, the researchers use an optical particle counter, with a 15-mW laser diode operating at 780 nm. A cloud droplet probe, which uses a 50-mW laser diode operating at 660 nm, handles 1- to 50-μm particles. The instrument trio is joined by a photodiode that measures photon flux from 400 to 700 nm, the photosynthetically active spectral region.In March, the group launched formations of three drones a total of 18 times over the Indian Ocean, logging nearly 127 flight hours. The vehicle immediately below a cloud quantified the pollutants going into it and the light filtering through it. In the cloud, the instruments measured the response to the incoming particles. The vehicle above the cloud measured the amount of sunlight reflected and the export of polluting particles. The 75 to 90 hours of collected data are still being analyzed, but Ramanathan already has plans for the next autonomous unmanned aerial vehicle campaign. He hopes to have hyperspectral imagers covering the 0.3- to 2.0-μm range, infrared spectrometers, cloud droplet and aerosol imagers, and miniature lidar for laser ranging. Contact: Veerabhadran Ramanathan, Scripps Institution of Oceanography, University of California, San Diego, La Jolla; e-mail: vramanathan@ucsd.edu.