ILLUSTRATING DEVIATIONS IN THE BEER-LAMBERT LAW IN AN INSTRUMENTAL ANALYSIS LABORATORY - MEASURING ATMOSPHERIC POLLUTANTS BY DIFFERENTIAL OPTICAL-ABSORPTION SPECTROMETRY

With the recent approval by the American Chemical Society of an underg raduate chemistry degree with an option in environmental chemistry, th ere is a need for relevant environmental experiments in the traditiona l chemistry core curriculum. We report here an experiment designed for an undergraduate junior/senior-level laboratory on instrumental analy sis. It can be used to illustrate the basic components of a W-vis abso rption experiment, its application to the measurement of a number of a tmospheric gases of interest, and the importance of understanding the conditions under which deviations from the simplest form of the Beer-L ambert law occur. A wide variety of gases in both polluted and remote areas play a key role in the chemistry and radiative properties of the atmosphere (1). These include O-3, a toxic air pollutant for which th e current air quality standard in the U.S. is 0.12 ppm for 1 hour (2). O-3 is also a greenhouse gas (3, 4). It is the absorption of UV light by O-3 in the stratosphere that is responsible for the wavelength cut off of light at the earth's surface at lambda - 290 nm. Other gases su ch as nitrogen oxides-NOx (NO + NO2)- are also inextricably linked to the formation and fate of O-3 (1). NO2 is formed via oxidation of NO, directly emitted by combustion sources, by hydroperoxy or alkylperoxy free radicals: NO + HO2 (or RO2) --> NO2 + OH (or RO) (1) This leads t o ozone formation by photolysis of NO2: NO2 + hv --> NO + O((3)p) (2) M O((3)p) + O-2 -->(M) O-3 (3) Reactions 2 and 3 represent the sole kn own significant anthropogenic source of ozone in the troposphere. In a reas with elevated NOx levels such as continental sites, this source d ominates other sources such as transport from the stratosphere.