Light-Enhanced Heterogeneous Conversion of NO₂to HONO on Solid Films Consisting of Fluorene and Fluorene/Na₂SO₄: An Impact on Urban and Indoor Atmosphere

Polycyclic aromatic hydrocarbons (PAHs) as constituents of urban grime and indoor surfaces can impact the photochemical conversion of nitrogen dioxide (NO2) to nitrous acid (HONO) thereby impacting the oxidation capacity of the atmosphere. In this study we investigate the effect of relative humidity (RH%), light intensity, and NO2 concentrations on uptake coefficients (γ) of NO2 on solid film consisting of fluorene (FL) and a mixture of FL and Na2SO4 as a proxy for urban and indoor grime at ambient pressure and temperature. γ(NO2) on solid FL increased markedly from (5.7 ± 1.7) × 10-7 at 0% RH to (4.6 ± 1.0) × 10-6 at 90% RH. The NO2 to HONO conversion yield, (ΔHONO/ΔNO2)%, increases with RH from 40% at 0% RH up to 80% at 60-90% RH, indicating that the water molecules favor the formation of HONO up to 60% RH. These results suggest that the heterogeneous photochemical reaction of NO2 on FL and FL/Na2SO4 can be an important source of HONO in the urban environment and indoor atmosphere and should be considered in photochemical models.