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* + H₂O → 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 cm³ molecule^-1 s^-1, which is an order of magnitude faster than the previously estimated upper limit of 1.2 × 10^-14 cm³ 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 NO₂* 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 × 10⁵ molec cm^-3. Decreases in the concentration of O₃ 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 O₃ 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. 2012. American Geophysical Union. All Rights Reserved.