Nitrous acid in a street canyon environment: Sources and contributions to local oxidation capacity

Abstract Nitrous acid (HONO) plays an important role in radical formation and photochemical oxidation processes in the boundary layer. However, its impact on the chemistry in a street canyon microenvironment has not been thoroughly investigated. In this study, we measured HONO in a street canyon in urban Hong Kong and used an observation-based box model (OBM) with the Master Chemical Mechanism (MCM v3.3.1) to investigate the contribution of HONO to local oxidation chemistry. The observed HONO mixing ratios were in the range of 0.4–13.9 ppbv, with an average of 3.91 ppbv in the daytime and 2.86 ppbv at night. A mean HONO/NO x emission ratio of 1.0% (±0.5%) from vehicle traffic was derived. OBM simulations constrained by the observed HONO showed that the maximum concentrations of OH, HO 2 , and RO 2 reached 4.65 × 10 6 , 4.40 × 10 6 , and 1.83 × 10 6 molecules cm −3 , which were 7.9, 5.0, and 7.5 times, respectively, the results in the case without HONO constrained. Photolysis of HONO contributed to 86.5% of the total primary radical production rates and led to efficient NO 2 and O 3 production under the condition of weak regional transport of O 3 . The formation of HNO 3 contributed to 98.4% of the total radical termination rates. Our results suggest that HONO could significantly increase the atmospheric oxidation capacity in a street canyon and enhance the secondary formation of HNO 3 and HCHO, which can damage outdoor building materials and pose health risks to pedestrians.