Using a parcel remixing scheme, Seinfeld demonstrated that a lack of mixing in the atmosphere can lead to deviations from photostationarity. This note shows that if the photostationary-state assumption is valid within each parcel of air, then the correlation between nitric oxide and ozone in the air parcels directly determines the apparent deviation from photostationarity without having to invoke parcel remixing. The normalized photostationary-state ratio, θ, can be greater than, equal to, or less than unity when the correlation is less than, equal to or greater than zero. That is, θ tends to be less than one in the presence of pockets of clean (low in NO and O3) and polluted (high in NO and O3) air, and greater than one in the presence of NO-rich and O3-rich parcels. In order to rigorously test the local photostationary-state assumption, it is necessary to make simultaneous measurements of O3, NO, and NO2, using sensors with a response time less than the time in which different air parcels are encountered. © 1979.
