Ravi M. Varma

Long optical cavities for open-path monitoring of atmospheric trace gases and aerosol extinction

An incoherent broadband cavity-enhanced absorption spectroscopy setup employing a 20 m long optical cavity is described for sensitive in situ measurements of light extinction between 630 and 690 nm. The setup was installed at the SAPHIR atmospheric simulation chamber during an intercomparison of instruments for nitrate (NO(3)) radical detection. The long cavity was stable for the entire duration of the two week campaign. A detection limit of approximately 2 pptv for NO(3) in an acquisition time of 5 s was established during that time. In addition to monitoring NO(3), nitrogen dioxide (NO(2)) concentrations were simultaneously retrieved and compared against concurrent measurements by a chemiluminescence detector. Some results from the campaign are presented to demonstrate the performance of the instrument in an atmosphere containing water vapor and inorganic aerosol. The spectral analysis of NO(3) and NO(2), the concentration dependence of the water absorption cross sections, and the retrieval of aerosol extinction are discussed. The first deployment of the setup in the field is also briefly described.

A novel, broadband spectroscopic method to measure the extinction coefficient of aerosols in the near-ultraviolet

Few in situ methods can currently measure the optical properties of aerosols in the near-ultraviolet spectral region. Such measurements are necessary to determine the radiative forcing of different particle types and the subsequent effect on local photochemistry, especially in and around large urban centers. In this work, we demonstrate a system based on incoherent broadband cavity-enhanced absorption spectroscopy (IBBCEAS) to measure the aerosol extinction coefficient with high sensitivity. The system was applied across an atmosphere simulation chamber and had continuous spectral coverage from 320 to 410 nm. Systems studied were ammonium sulfate aerosol and secondary organic aerosol (SOA) formed from the photolysis of 1-nitronaphthalene, an important nitro-polycyclic aromatic hydrocarbon.