Heterogeneous photochemical uptake of NO2 on the soil surface as an important ground-level HONO source.

Abstract

Nitrous acid (HONO) production from the heterogeneous photochemical reaction of NO2 on several Chinese soils was performed in a cylindrical reactor at atmospheric pressure. The NO2 uptake coefficient (γ) and HONO yield (YHONO) on different soils were (0.42-5.16)×10-5 and 6.3%-69.6%, respectively. Although the photo-enhanced uptake of NO2 on different soils was observed, light could either enhance or inhibit the conversion efficiency of NO2 to HONO, depending on the properties of the soils. Soils with lower pH generally had larger γ and YHONO. Soil organics played a key role in HONO formation through the photochemical uptake of NO2 on soil surfaces. The γ showed a positive correlation with irradiation and temperature, while it exhibited a negative relationship with relative humidity (RH). YHONO inversely depended on the soil mass (0.32-3.25mgcm-2), and it positively relied on the irradiance and RH (7%-22%). There was a maximum value for YHONO at 298K. Based on the experimental results, HONO source strengths from heterogeneous photochemical reaction of NO2 on the soil surfaces were estimated to be 0.2-2.7ppbh-1 for a mixing layer height of 100m, which could account for the missing daytime HONO sources in most areas.