Measurement of Nitrous Acid (HONO) During PROPHET 2000 Summer Intensive

Ambient nitrous acid (HONO) was measured during the PROPHET (Program for Research on Oxidants: PHotochemistry, Emission and Transport) 2000 Summer Intensive at Pellston, Michigan from July 22 to August 15, 2000. Mixing ratios of HONO were in the range of < 5 - 740 pptv with a median of 51 pptv and a mean of 77 pptv within the forest canopy, and in the range of 12 - 390 pptv with a median of 65 pptv and a mean of 80 pptv above the canopy. Diurnal patterns of HONO concentration and relative humidity within canopy indicate that dew droplets on leaf surfaces may play an important role in controlling ambient HONO levels. The data of gradient measurement suggest that heterogeneous reactions occurring on the leaf surfaces were a source of HONO for most of the time when they were free of excess moisture. However, deposition of HONO on the leaf surfaces occurred around and after midnight as the dew droplets formed. High level of morning HONO indicates that the trapped nitrous acid/nitrite was then released to the atmosphere when dew droplets were evaporated after sunrise. As the solar radiation increased, photolysis of ambient HONO overcame its formation, therefore, its observed concentration decreased throughout the day. The observed daytime HONO concentration was significantly higher than that expected from photo-steady state with OH and NO and from its production based on its nighttime accumulation rate, indicating a strong daytime source for HONO. Photolysis of HNO₃ accumulated on leaf surfaces via dry deposition may be this 'missing' source. Our observation has significant implications in the atmospheric chemistry of the boundary layer in terms of radical dynamics and of 're-NOx-ification' of nitric acid. Calculated photolysis rate of HONO at 3.0 ppbv/day contributed about 30% of the daily total radical production.