Effects of relative humidity on the characterization of a photochemical smog chamber

Water vapor plays an important role in many atmospheric chemical reactions. A self-made indoor environmental smog chamber was used to investigate the effects of relative humidity (RH) on its characterization, which included the wall effects of reactive species such as O3 and NOx, and the determination of chamber-dependent OH radicals in terms of CO-NOx irradiation experiments. Results showed that the rate constant of O3 wall losses increased with increasing RH, and that their relationship was linearly significant. Although RH affected the rate constant of NOx wall losses, their relationship was not statistically significant. Background air generated a small amount of ozone at both high and low RH. When RH varied from 5% to 79%, the apparent rate constant kNO2→HONO for the conversion of NO2 into gas phase HONO was estimated in the range of 0.70×10−3−2.5×10−3 min−1. A linear relationship between kNO2→HONO and RH was obtained as kNO2→HONO (10−3 min−1) = −0.0255RH + 2.64, with R2 and P value being 0.978 and <0.01. To our knowledge, this is the first report on their relationship. The generation mechanism for HONO and OH was also discussed in this work.