Contents lists available at ScienceDirect Atmospheric Environment journal homepage: www.elsevier.com/locate/atmosenv Global and local sensitivity analysis of urban background ozone modelled with a simplified photochemical scheme Andrea L. Pineda Rojas a,* , Damián E. Bikiel b a Centro de Investigaciones del Mar y la Atmósfera, UMI-IFAECI/CNRS, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, CONICET, UBA, Buenos Aires, Argentina b Instituto de Química Física de los Materiales, Medio Ambiente y Energía, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, CONICET, UBA, Buenos Aires, Argentina ARTICLE INFO Keywords: Air quality models Generic reaction set Ozone Sensitivity analysis Urban scale ABSTRACT The Generic Reaction Set (GRS) is a simplified photochemical scheme that allows estimation of ozone (O 3 ) concentrations resulting from nitrogen oxides (NO x ) and reactive organic compounds (ROC) emissions in an urban area. Due to its simplicity and relative good performance, it has been included in the algorithms of many atmospheric dispersion models. However, few works assess quantitatively the sensitivity of simulated O 3 con- centrations to the uncertainty in the GRS input variables. In order to do so, in this work we carry out two sensitivity analyses: global (applying an atmospheric dispersion model to a representative real case) and local (using a box-GRS model under a wide range of hypothetical conditions). In the global study, a Monte Carlo analysis is performed to estimate the uncertainty of maximum O 3 concentrations that is caused by possible errors in the GRS input variables, applying the DAUMOD-GRS model in the Metropolitan Area of Buenos Aires. Results show that the initial concentration of O 3 dominates at all analysed receptors while that of NO x can make a non- negligible contribution if its error is relatively large. In order to further analyse the effect of each parameter individually, a local sensitivity analysis is performed using a box-GRS model under a wide range of conditions. Except for the small NO x -limited region, sensitivity indexes computed for ± 1 ppb changes in the initial con- centrations of ROC, NO x and O 3 are in the ranges 0.00–0.35, 0.00–0.81 and 0.05–0.96, respectively, with that of ozone dominating over most of the isopleth diagram space. In turn, its relative impact increases with decreasing ROC initial concentrations. Reaction rate coefficients have also different effects on O 3 peak concentrations de- pending on the initial conditions of the system. Our results show quantitatively the change of dominant variables under different environments. 1. Introduction Tropospheric ozone (O 3 ) is among the air pollutants of increasing concern worldwide. At elevated concentrations, O 3 can lead to re- spiratory problems, triggers asthma, reduces lung function and causes lung diseases (WHO, 2016). In addition, relatively high ozone con- centration levels can produce damage to the environment and vegeta- tion (The Royal Society, 2008; Feng et al., 2015). In the urban atmo- sphere, ozone is formed from nitrogen oxides (NO x ) and volatile organic compounds (VOCs) that are emitted by human activities. Biogenic VOCs can also impact ozone concentrations in certain environments. The oxidation of VOCs is initiated by the reaction of the hydroxyl ra- dical (OH • ). Subsequent reactions involve NO x and light, transforming cyclically NO 2 to NO and producing a complex network of free radicals. Tropospheric ozone is a byproduct of this oxidation. The amount of O 3 formed depends on various factors given that such formation can be achieved through many different reaction pathways depending on the environmental conditions. Air quality models including photochemistry provide a link between NO x and VOCs emissions, atmospheric processes and ground-level O 3 concentrations; and therefore they allow not only to complement data on ozone concentrations at non-sampled areas but also to study the potential impact of any scenario (for example, the effect that would produce a given emissions reduction strategy) on air pollutant levels (e.g., Miranda et al., 2015). Since ozone is a secondary pollutant (i.e., it is formed in the atmosphere), its representation by models depends largely on the chemical module. At present there exist a variety of photochemical schemes that are capable of estimating O 3 https://doi.org/10.1016/j.atmosenv.2019.06.014 Received 22 December 2018; Received in revised form 4 June 2019; Accepted 5 June 2019 * Corresponding author. Centro de Investigaciones del Mar y la Atmósfera (CIMA/CONICET-UBA), Ciudad Universitaria, Pab. II, piso 2, 1428, Buenos Aires, Argentina. E-mail address: pineda@cima.fcen.uba.ar (A.L. Pineda Rojas). Atmospheric Environment 213 (2019) 199–206 Available online 06 June 2019 1352-2310/ © 2019 Elsevier Ltd. All rights reserved. T