Cooperative Institute for Research in Environmental Sciences

Abstracts: 5

Cavity Ring-Down Spectroscopy: A New Approach for Difficult-to-Measure Atmospheric Trace Gases

Steven Brown

Observation of the absorption of light is an age-old method for measuring concentration of important compounds in the atmosphere. Cavity ring-down spectroscopy (CaRDS) is a novel technique that greatly enhances the sensitivity of such measurements. A pulse of laser light is introduced into a cell formed by two highly reflective mirrors - the "optical cavity." Because there is a small intensity loss upon each encounter with the mirrors, the light intensity inside of the cavity slowly decreases in time as the light passes back and forth, in much the same way that the sound decreases after ringing a bell. If light absorption takes place inside the cavity, the light intensity decreases more rapidly, just as the sound from a bell decreases more quickly if one lightly touches the bell. The measurement of the rate of the light intensity decrease in the presence and absence of the absorption gives a direct and very sensitive determination of the absorber's concentration. The sensitivity comes about because the light can travel nearly 100 km inside of the cavity during this process, and also because changes in the intensity of the laser light source do not affect the measurement.

The nitrate radical, NO3, and dinitrogen pentoxide, N2O5, are a closely linked pair of chemical compounds that readily lend themselves to observation via CaRDS. They occur only in the nighttime atmosphere because they are readily broken down by sunlight during the day. The nitrate radical is present at concentrations of only a few molecules in a trillion molecules of air, yet its chemistry is important; it acts as a natural "detergent" that removes pollutants (such as hydrocarbons), and it is important to the chemistry that leads to ozone pollution in the lower atmosphere. Dinitrogen pentoxide is similarly important to ozone formation, and it can also lead to the formation of acid rain. Until recently, measurement of NO3 was cumbersome and observation of N2O5 in the lower atmosphere was impossible. Development of a CaRDS instrument, however, has made the measurement of these two important compounds sensitive, convenient, accurate, rapid and portable. For example, we have recently demonstrated simultaneous measurement of both compounds with a sensitivity better than a molecule in two trillion molecules of air. During a recent four-week field campaign, the New England Air Quality Study, the instrument made measurements over the ocean off the coast of New England aboard the U.S.S. Ron Brown. The success of this campaign will allow new insights into the role that nighttime chemistry plays in regional air quality.