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Lasers Track Air Pollution

Photonics Spectra
Jan 1998
Kate Leggett

Chemical emissions in the Earth's atmosphere profoundly affect the weather. When dust, soot, ice or sea salt act as catalysts, atmospheric chemicals can reduce or contribute to smog, global warming and the ozone hole. Scientists studying climate changes, air pollution and other environmental problems now can use a laser-based system that provides clear information about these particles and the chemical changes they engender.
Until recently, scientists had no rapid method for collecting and analyzing atmospheric samples. They typically collected air samples at altitude, then examined them using off-line analytical techniques. Not only is this procedure time-consuming, but particles can react with one another, skewing results.

A mobile, rugged unit
Kimberly Prather, a researcher at the University of California at Riverside, has designed a system to monitor atmospheric particle chemistry in real time. This system is a turnkey, mobile unit rugged enough to be mounted in the back of a truck and driven to a test site. Prather and her team have used the device to track changes in the chemistry of sea salt particles as they traveled from coastal Long Beach, Calif., to Riverside, about 45 miles inland.
The system sucks outside air into a tube, forcing it through a nozzle at supersonic speeds. Then, as the particle-laden air expands into a high- vacuum chamber, friction disperses the particles. The lighter particles travel quickly through the chamber, while the heavier ones travel more slowly.
As they move, the particles cross the beams of two continuous-wave, argon-ion lasers from Omnichrome. These lasers clock the particles' speeds and extrapolate their size and shape. A third, pulsed Nd:YAG laser, the Minilite from Continuum, dissociates the particles into positively and negatively charged fragments, which are measured by a time-of-flight mass spectrometer from R.M. Jordan Co. Inc.
After analyzing the airborne particles of sodium chloride at three ground-based sites, Prather found that as the sea salt particles travel east from the Pacific Ocean, they react with nitric acid -- an air pollutant formed from car exhaust and OH (hydroxyl) radicals -- creating sodium nitrate. This reaction decreases the amount of nitric acid in the atmosphere, ultimately reducing acid rain.
Prather and her group now are studying the chemical effects of various kinds of particles on the amount of sunlight reaching the Earth's surface. Scientists know that particles containing sulfates scatter sunlight. This cools our environment, possibly counteracting the temperature rise caused by greenhouse gases.



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