Global modeling of the nitrate radical (NO₃) for present andpre-industrial scenarios

8Increasing the complexity of the chemistry scheme in the global chemistry transport model STOCHEM to STOCHEMCRI (Utembe et al., 2010) leads to an increase in NO_x as well as ozone resulting in higher NO₃ production overforested regions and regions impacted by anthropogenic emission. Peak NO₃ is located over the continents nearNO_x emission sources. NO₃ is formed in the main by the reaction of NO₂ with O₃, and the significant losses of NO₃are due to the photolysis and the reactions with NO and VOCs. Isoprene is an important biogenic VOC, and thepossibility of HO_x recycling via isoprene chemistry and other mechanisms such as the reaction of RO₂ with HO₂has been investigated previously (Archibald et al., 2010a). The importance of including HO_x recycling processeson the global budget of NO₃ for present and pre-industrial scenarios has been studied using STOCHEM-CRI, andthe results are compared. The large increase (up to 60% for present and up to 80% for pre-industrial) in NO₃ is drivenby the reduced lifetime of emitted VOCs because of the increase in the HO_x concentration. Themaximum concentrationchanges (up to 15 ppt) for NO₃ from pre-industrial to present day are found at the surface between 30^oN and60^oN because of the increase in NO_x concentrations in the present day integrations.