Relationship between large-scale atmospheric states, subsidence, static stability and ground-level ozone in Illinois, USA

We analyze ground level ozone pollution in Illinois. We find that during elevated ground level ozone conditions, Illinois is located to the south-west of a high pressure system centered over south-eastern Canada. This state causes weakening of the climatological westerlies, near-surface southerlies and upper troposphere northerlies. This results in corresponding weakening of both lower troposphere heating and upper troposphere cooling by lateral advective heat flux divergence. Heat balance is restored primarily by enhanced subsidence, most notably between the surface and similar to 400 mb. The strengthened subsidence suppresses boundary layer deepening by upward surface heat fluxes, resulting in a shallower than normal boundary layer during times of elevated ground level ozone. We argue that subsidence induced boundary layer suppression, aided by elevated temperatures, is the primary cause of the elevated ground level ozone events. The enhanced subsidence during those times also dries the lower troposphere, reducing the probability of moist convection, and thus slowing boundary layer ventilation.