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

Abstract

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 ∼87% of observed HONO levels, leading to an obvious decrease (∼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 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 O ₃ mitigation.