Enhancement of O ₃ –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 (NOx≡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.