ATMOSPHERIC NO(3) .4. VERTICAL PROFILES AT MIDDLE AND POLAR LATITUDES AT SUNRISE

Ground-based measurements of NO3 absorption in the band near 662 nm were carried out on four occasions using the Moon as a light source during sunrise at both middle and polar latitudes. As the Sun rose, the observed slant column abundance of atmospheric NO3 decreased systematically. The observed time dependent decrease is due to the progression of the solar terminator down through the atmosphere and provides a basis for inferring the vertical profile of NO3 at sunrise. The inferred profiles are sensitive to the adopted NO3 absorption cross sections and photolysis rates, and these sensitivities are investigated. Two sets of measurements made in Colorado during the summer display a large contribution to the total column from the troposphere, while two data sets obtained during the Antarctic spring demonstrate that the column is almost entirely located in the stratosphere there. The NO3 abundances obtained in Antarctica are much smaller than those measured over Colorado and show that NO3 evolves only at altitudes where the temperature is high enough to allow its formation via the reaction of NO2 and O3.