Open path spectrometers for atmospheric monitoring

Open path monitors are a type of air monitor used to identify and determine the concentration of gases in the atmosphere based on their optical absorption properties. These instruments project a beam of light through a long open air path and spectrally analyze the transmitted beam of light. The beam of light is totally open to the atmosphere so that all possibility of sampling artifacts is eliminated. This work describes two new open path monitors. These instruments make intensity measurements at hundreds to thousands of different wavelengths over the spectral region being monitored. The instruments differ in that one is designed for the ultraviolet (UV) region while the other operates in the infrared (IR) spectral region. A detailed description of each of these instruments is presented.;The open path UV instrument has been used to make the first open path measurements of aromatic hydrocarbons in this spectral region. The UV instrument was shown to be much more sensitive than the IR system for measurement of many of these aromatic hydrocarbons, in particular benzene and toluene.;The UV and IR open path monitors have been used both separately and in combination to make emission rate measurements using atmospheric tracer techniques. The path integrated concentrations measured by the open path monitors were found to be much more useful than data from a single point sampler when applied to this type of emission rate monitoring.;The open path monitors have been compared to a standard analytical technique for measuring the concentration of air pollutants. The agreement between the open path systems and the standard analytical technique was very good, with the average difference being less than 15 and 5% for the UV and IR systems respectively. The precision of the instruments under typical field conditions was found to be better than 10%.