Atmospheric Nitrous Acid Measurement in the French Landes Forest

Nitrous acid (HONO) is a major hydroxyl radical (^•OH) source, which plays a central role in atmospheric chemistry. In this study, continuous measurements of HONO and other related parameters were conducted from July 5 to July 25, 2018, in the French Landes forest. The mean daytime and nighttime HONO concentrations were 60 and 139 ppt, respectively, while the mean daily ^•OH production rate from HONO photolysis below the forest canopy was estimated to be 7.6 × 10⁴ molecules cm^–3 s^–1, much less than those from ozone (O₃) photolysis in the presence of water vapor (3.1 × 10⁵ molecules cm^–3 s^–1) and alkene ozonolysis (4.4 × 10⁶ molecules cm^–3 s^–1). Interestingly, the HONO levels were peaking daily between 18:00 and 24:00, which cannot be explained by the known chemical formation pathways. Analysis of collocated measurements suggested that the accumulation of HONO during the evening might be explained by micrometeorological processes. Hence creation of a stable decoupled layer retaining the HONO emission from the soil was associated with the formation of a convective decoupled layer. Increasing relative humidity within this period enhanced HONO gaseous deposition leading to its fast depletion.