An efficient photolysis system for fast‐response NO2 measurements

A new photolytic converter for NO 2 measurement is described and its performance assessed using laboratory, ground‐, and aircraft‐based field data. Focusing the output of a 200‐W short‐arc Hg lamp into a photolytic cell attains conversion fractions of NO 2 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 NO 2 , affording a peroxyacetyl nitrate conversion fraction of less than 0.006 and negligible conversion of HNO 3 . Unwanted (artifact) signal in clean synthetic air is also greatly minimized through the use of optical filters. Fast instrument response is achieved by minimizing NO 2 inlet line and photolysis cell residence times. NO and NO 2 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 NO 2 calculated by difference at high time resolution. Induced change in the ambient ratio of NO to NO 2 , 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 NO 2 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 NO 2 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.