Measurement of Nitric Acid in Smog

Abstract

The formation of nitric acid (HONO₂) in the polluted atmosphere has been monitored using a newly developed analyzer. The analytical method, the operation and performance of the analyzer and some results of recent applications are described. The analysis consists of sample conditioning in combination with a Mast micro-coulomb detection cell adapted for sensing acids rather than oxi-dants. Results of calibration and interference testing are presented. The sensitivity of the instrument to HONO₂ is about 2 ppb. Application of the analyzer for smog chamber and atmospheric studies is demonstrated. In simulated smog experiments with synthetic auto exhaust, the pattern of HONO₂ formation parallels that of ozone formation. A similar formation pattern was observed during ambient sampling. Evidence is presented which suggest that these reactions,

may both contribute to HONO₂ formation in smog, but the relative importance of the two is yet uncertain. Atmospheric sampling in the Los Angeles area indicated 24 hour average HONO₂ concentrations of about 3 ppb and 1 hour average maximum concentrations of about 10 ppb.