A sensitive spectral radiometer system capable of measuring the atmospheric emission in the 6.5&mgr;m to 13.0&mgr;m region has been constructed for use on the WB57F aircraft used on the Department of Transporation Climatic Impact Assessment Program. The data obtained with this instrument on a number of flights have been analyzed to determine the total HNO3 column density above the aricraft at various latitudes and seasons. Examination of these data indicate a definite variation of HNO3 column density with latitude and possible seasonal variations.