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

Recently, deteriorating ozone (O₃) pollution in China brought the precise diagnosis of O₃ sensitive chemistry to the forefront. As a dominant precursor of OH radicals, atmospheric nitrous acid (HONO) plays an important role in O₃ production. However, its measurement unavailability in many regions especially for second- and third-tier cities may lead to the misjudgment of the O₃ sensitivity regime derived from observation-based models. Here, we systematically assess the potential impact of HONO on diagnosing the sensitivity of O₃ 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 ∼87% of observed HONO levels, leading to an obvious decrease (∼19%) of net O₃ production in the morning, which was in line with the previous studies. The unconstrained HONO in the model was found to significantly push O₃ production toward the VOC-sensitive regime. Additionally, it is unrealistic to change NO_x but constrain HONO in the model due to the dependence of HONO formation on NO_x. Assuming that HONO varied proportionally with NO_x, a stronger NO_x-sensitive condition could be achieved. Therefore, effective reduction of NO_x should be given more attention together with VOC emission control for O₃ mitigation.