The Lack of HONO Measurement May Affect the Accurate Diagnosis of Ozone Production Sensitivity

Recently, deteriorating ozone (O-3) pollution in China brought the precise diagnosis of O-3 sensitive chemistry to the forefront. As a dominant precursor of OH radicals, atmospheric nitrous acid (HONO) plays an important role in O-3 production. However, its measurement unavailability in many regions especially for second- and third-tier cities may lead to the misjudgment of the O-3 sensitivity regime derived from observation-based models. Here, we systematically assess the potential impact of HONO on diagnosing the sensitivity of O-3 production using a 0-dimension box model based on a comprehensive summer urban field campaign. The results indicated that the default mode (only the NO + OH reaction is included) in the model could underestimate similar to 87% of observed HONO levels, leading to an obvious decrease (similar to 19%) of net O-3 production in the morning, which was in line with the previous studies. The unconstrained HONO in the model was found to significantly push O-3 production toward the VOC-sensitive regime. Additionally, it is unrealistic to change NOx but constrain HONO in the model due to the dependence of HONO formation on NOx. Assuming that HONO varied proportionally with NOx, a stronger NOx-sensitive condition could be achieved. Therefore, effective reduction of NOx should be given more attention together with VOC emission control for O-3 mitigation.