Potential significance of photoexcited NO₂ on global air quality with the NMMB/BSC chemical transport model

Atmospheric chemists have recently focused on the relevance of the NO 2 * + H 2 O → OH + HONO reaction to local air quality. This chemistry has been considered not relevant for the troposphere from known reaction rates until nowadays. New experiments suggested a rate constant of 1.7 × 10 −13 cm 3 molecule −1 s −1 , which is an order of magnitude faster than the previously estimated upper limit of 1.2 × 10 −14 cm 3 molecule −1 s −1 , determined by Crowley and Carl (1997). Using the new global model, NMMB/BSC Chemical Transport Model (NMMB/BSC‐CTM), simulations are presented that assess the potential significance of this chemistry on global air quality. Results show that if the NO 2 * chemistry is considered following the upper limit kinetics recommended by Crowley and Carl (1997), it produces an enhancement of ozone surface concentrations of 4–6 ppbv in rural areas and 6–15 ppbv in urban locations, reaching a maximum enhancement of 30 ppbv in eastern Asia. Moreover, NO 2 enhancements are minor (<0.01 ppbv) in background regions and reach maximum daytime values of 1–6 ppbv. Similarly, HONO exhibits negligible increases, 8–9 pptv in urban settings. Enhancements in the concentration of OH are around 14–17 × 10 5 molec cm −3 . Decreases in the concentration of O 3 and its precursors are also identified but to a lesser degree. In order to quantify the role of the two kinetic rates measured, model simulations are compared after incorporating both reaction rate constants. Maximum O 3 difference enhancements from 5 to 10 ppbv are modeled over locations where high NO x emissions are present; however, differences are small in most parts of the globe.