An efficient photolysis system for fast-response NO2 measurements

A new photolytic converter for NO2 measurement is described and its performance assessed using laboratory, ground-, and aircraft-based field data. Focusing the output of a 200-W short-are Hg lamp into a photolytic cell attains conversion fractions of NO2 to NO greater than 0.70 in cell residence times of less than a second. Limiting lamp output to wavelengths greater than 350 nm by means of optical filters increases specificity for NO2, affording a peroxyacetyl nitrate conversion fraction of less than 0.006 and negligible conversion of HNO3. Unwanted (artifact) signal in clean synthetic air is also greatly minimized through the use of optical filters. Fast instrument response is achieved by minimizing NO2 inlet line and photolysis cell residence times, NO and NO2 sample residence times are matched in a multichannel instrument so that signal from ambient NO may be easily subtracted from the total signal and ambient NO2 calculated by difference at high time resolution. Induced change in the ambient ratio of NO to NO2, due to reaction of ozone and other oxidants with NO during sampling, is minimized in the new design. This configuration permits simple and accurate retrieval of NO2 concentrations in aircraft transects of power plant plumes, where ambient NO concentrations can change over several orders of magnitude in seconds. at lower concentrations found in the planetary boundary layer, agreement between calculated and observed NO2 is within +/-(40 pptv + 7%) for a 10-s average. The new converter consumes less power, is more efficient, and is less expensive to operate than previous photolysis designs.