Sorting drops in a bucket.

Sorting drops in a bucket

The acidity of raindrops falling through polluted air seems to depend on their size, according to recent measurements made in New Zealand. This result provides a valuable check on theoretical calculations that attempt to predict how effectively raindrops scavenge pollutants and that model the chemical reactions occurring inside droplets.

These field measurements are made possible by a new instrument that, for the first time, allows the size and acidity of raindrops to be measured at the same time. Previously, researchers could measure the acidity only of bulk rainwater samples or of single droplets of unknown size.

The instrument, as described in the June 26 NATURE, consists of two disks separated by a small, vertical rotating shaft (see diagram). The upper disk has a wedge-shaped opening through which rain passes, while the lower disk is divided into sectors separated by walls. Because drops of different sizes fall at different rates, larger, faster drops tend to fall into sectors that are closer to the sector directly below the aperture. It takes only a few minutes to collect samples large enough for testing.

Using this instrument, Stephen J. Adams and his colleagues at the University of Auckland in New Zealand found that the greatest acidity occurs in droplets about 0.5 millimeter across. Because the raindrops were relatively clean to begin with and picked up pollutants as they fell, the researchers conclude that drops with radii close to 0.5 mm scavenge more efficiently than drops of other sizes.

This instrument could open up new areas of acid rain research, says Nurtan A. Esmen of the University of Pittsburgh. For example, the interaction of a gas like sulfur dioxide with a drizzle's small drops may be quite different from its interaction with a shower's mainly large drops.