The application of cavity ring-down spectroscopy to determine nitrate radical concentrations in the atmosphere

Abstract

The nitrate radical, NO₃, is a potent atmospheric oxidant. Current NO₃ measurement methods average NO₃ concentrations over several kilometer pathlengths. Recent modeling studies predict that NO₃'s distribution is vertically inhomogenous, causing previous NO₃ observations to not be representative of its atmospheric chemistry. In-situ measurements offer a test of these modeling predictions and a better understanding of NO₃ chemistry. In this thesis, we describe construction of an instrument capable of detecting NO₃ in-situ. This sensor is based upon cavity ring-down spectroscopy. Our initial work demonstrated that CRDS could sensitively detect NO₃. We then built and tested a field prototype during June 2001, successfully detecting NO₃ in the field. CRDS observations were compared to path averaged NO₃ observations. Similar time behavior of both signals indicated that the techniques were observing NO₃. A consistent difference in signal levels indicated that the two techniques were not sampling a homogeneous air mass.