Boundary layer photochemistry during a total solar eclipse

Simultaneous measurements of radiation, photolysis frequencies, O-3, CO, OH , PAN and NOx species were carried out in the boundary layer, along with pe rtinent meteorological parameters, under total solar eclipse conditions. Th is experiment performed at about 34 degrees solar zenith angle and noontime conditions thus provided a case study about the interactions between radia tion and photochemistry under fast "day-night" and "night-day" transitions, at high solar elevation. The results reveal a close correlation of photoly sis frequencies j(O(1D)) and j(NO2) with the UV radiation flux. All three p arameters show, due to the decreasing fraction of direct radiation at short er wavelengths, much weaker cloud shading effects than global solar radiati on. NO and OH concentrations decrease to essentially zero during totality. Subsequently, NO and OH concentrations increased almost symmetrically to th eir decrease preceding totality. The NO/NO2 ratio was proportional to j(NO2 ) over +/-30 min before and after totality indicating that the partitioning of NOx species is determined by j(NO2). Simple box model simulations show the effect of reduced solar radiation on the photochemical production of O- 3 and PAN.