Boundary layer photochemistry during a total solar eclipse

Simultaneous measurements of radiation, photolysis frequencies, O₃, CO, OH, PAN and NO_x species were carried out in the boundary layer, along with pertinent meteorological parameters, under total solar eclipse conditions. This experiment performed at about 34° solar zenith angle and noontime conditions thus provided a case study about the interactions between radiation and photochemistry under fast "day-night" and "night-day" transitions, at high solar elevation. The results reveal a close correlation of photolysis frequencies j_O(¹D) and jNO₂ with the UV radiation flux. All three parameters show, due to the decreasing fraction of direct radiation at shorter wavelengths, much weaker cloud shading effects than global solar radiation. NO and OH concentrations decrease to essentially zero during totality. Subsequently, NO and OH concentrations increased almost symmetrically to their decrease preceding totality. The NO/NO₂ ratio was proportional to jNO₂ over ±30 min before and after totality indicating that the partitioning of NO_x species is determined by jNO₂. Simple box model simulations show the effect of reduced solar radiation on the photochemical production of O₃ and PAN.