This paper presents a comprehensive description and benchmark evaluation of the tropospheric gas-phase chemistry component of the Multiscale Online Nonhydrostatic AtmospheRe CHemistry model (NMMB-MONARCH), formerly known as NMMB/BSC-CTM, that can be run on both regional and global domains. Here, we provide an extensive evaluation of a global annual cycle simulation using a variety of background surface stations (EMEP, WDCGG and CASTNET), ozonesondes (WOUDC, CMD and SHADOZ), aircraft data (MOZAIC and several campaigns), and satellite observations (SCIAMACHY and MOPITT). We also include an extensive discussion of our results in comparison to other state-of-the-art models. We note that in this study, we omitted aerosol processes and some natural emissions (lightning and volcano emissions). The model shows a realistic oxidative capacity across the globe. The seasonal cycle for CO is fairly well represented at different locations (correlations around 0.3–0.7 in surface concentrations), although concentrations are underestimated in spring and winter in the Northern Hemisphere, and are overestimated throughout the year at 800 and 500 hPa in the Southern Hemisphere. Nitrogen species are well represented in almost all locations, particularly NO₂ in Europe (root mean square error – RMSE – below 5 ppb). The modeled vertical distributions of NO_x and HNO₃ are in excellent agreement with the observed values and the spatial and seasonal trends of tropospheric NO₂ columns correspond well to observations from SCIAMACHY, capturing the highly polluted areas and the biomass burning cycle throughout the year. Over Asia, the model underestimates NO_x from March to August, probably due to an underestimation of NO_x emissions in the region. Overall, the comparison of the modeled CO and NO₂ with MOPITT and SCIAMACHY observations emphasizes the need for more accurate emission rates from anthropogenic and biomass burning sources (i.e., specification of temporal variability). The resulting ozone (O₃) burden (348 Tg) lies within the range of other state-of-the-art global atmospheric chemistry models. The model generally captures the spatial and seasonal trends of background surface O₃ and its vertical distribution. However, the model tends to overestimate O₃ throughout the troposphere in several stations. This may be attributed to an overestimation of CO concentration over the Southern Hemisphere leading to an excessive production of O₃ or to the lack of specific chemistry (e.g., halogen chemistry, aerosol chemistry). Overall, O₃ correlations range between 0.6 and 0.8 for daily mean values. The overall performance of the NMMB-MONARCH is comparable to that of other state-of-the-art global chemistry models.

Atmospheric chemists have recently focused on the relevance of the NO₂* + 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, NO₂ 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.