Seminar

Abstract

On average half of the planet is in darkness at any one time. However, from the perspective of atmospheric chemistry, much of the focus is on the day time 'photochemistry'. The 'ROle of Nighttime chemistry in controlling the Oxidising Capacity of the AtmOsphere' (RONOCO) project measured a range of atmospheric constituents, including short lived species such as OH, HO₂, NO₃, and N₂O₅ as well as longer lived species (O₃, CO, VOCs, oVOCs) from the UK large research aircraft (FAAM BAe146) over Northern Europe during the night. These observations complement previous datasets as they were made away from urban centres and so show very low NO concentrations. Analysis of the observations through use of a constrained box model finds a highly complex and little explored area of atmospheric chemistry. Our box model simulations are less successful than our equivalent daytime simulations but they provide significant insight into this unusual chemistry. The model tends to over-predict NO₃ by a factor of 2, whilst under predicting HO₂ by a factor of 3. Hydrocarbon oxidation is dominated by NO₃ chemistry across C=C bonds (on average 1/3 of the oxidation is by O₃ initiated attack, 2/3 from NO₃). This initiates a cascade of complex and badly constrained chemistry. NO₃ also acts to process radicals through RO₂+NO₃ reactions. Thus under these conditions, NO₃ plays a dual role, equivalent in the daytime to the combined role of OH and NO. We will discuss the chemistry in terms of RO₂, NO₃, NO₃x budgets etc and discuss implications for VOC oxidation, nitric acid production etc. We then compare the results of our box model simulation with the complex MCM chemistry with those obtained from equivalent box model simulations using the simplified chemistry scheme from a Chemical Transport Model (GEOS-Chem). We find significant differences between the two chemistry schemes leading to significant failures to simulate HOx at night in the simplified chemistry. In light of the knowledge gained from the observationally constrained model we adapt the chemistry scheme in GEOS-Chem and investigate the global implications of this night time chemistry.