Variations of NO, NO2 and O3 concentrations downwind of a Los Angeles freeway

Abstract

sampling study was conducted to quantify the relationships of NO, NO2 and O3 concentrations with distance downwind of the San Diego freeway in Los Angeles. By continuously, monitoring at a site upwind (background) of the freeway and at selected downwind sites, patterns of NO, NO2 and O3 concentrations were detailed. Minimal separation distances of the samplers from the roadway to eliminate measurable influence were estimated to be approx 400–500 m for NO, NO2 and O3. A spatial model was fitted to the empirical NO and NO2 data, which incorporated the effects of dilution, reaction and background level on measured downwind concentration. This model fit the experimental data closely and indicated that: 1. (1) the decrease of NOx downwind of the freeway can be expressed by a simple exponential dilution, 2. (2) the reaction of NO and O3 downwind of the freeway is not well mixed and deviates from the ideal photostationary state, and 3. (3) the simple rate equations for the reaction of NO with O3 and the photodissociation of NO2 could be combined with a simple exponential dilution term to define the measured concentrations of NO2.