Enhancement of O₃–CO ratios at tropospheric subtropical latitudes: Photochemistry and stratospheric influence

Abstract. The subtropics are influenced by stratosphere-troposphere exchange processes through the subtropical jet streams and tropopause folding events, which are commonly identified by the opposing gradients of ozone (O₃) and carbon monoxide (CO) and thus their ratio. Here, we used airborne observations of CO and O₃, as well as the global three-dimensional ECHAM5/MESSy Atmospheric Chemistry (EMAC) model, to investigate whether there is another important mechanism that conditions the subtropics. We show that high O₃–CO ratios extend deeply into the troposphere in the subtropics, which is evident in both in situ observations and model results. Tropospheric photochemistry leads to similar O₃–CO ratios as those for stratospheric air diluted into the troposphere. In the upper tropical troposphere, frequent deep convective events produce lightning that leads to high concentrations of nitrogen oxides (NO_x ≡ NO+NO₂), which drive O₃ production and which further catalyze the recycling of hydroxyl (OH) radicals, which reduces CO. These lightning-affected air masses can be transported from the tropics into the subtropics via the Hadley circulation. We have excluded NO production through lightning in a sensitivity run of the EMAC model and see an annual relative reduction of the O₃–CO ratio of up to almost 50 % in the tropics and up to 40 % in the northern subtropics, with even larger seasonal variability and major effects on the vertical profiles of O₃ and CO. We therefore show that photochemistry is an additional key factor alongside stratosphere-troposphere mixing in determining O₃-rich and CO-poor air masses in the troposphere.