MLS OBSERVATIONS OF CLO AND HNO3 IN THE 1996-97 ARCTIC POLAR VORTEX

Microwave Limb Sounder (MLS) measurements of lower stratospheric ClO a nd gas-phase HNO3 are presented for the 1996-97 Arctic winter. The hor izontal and vertical extent of enhanced ClO were smaller in 1997 than in 1996, as was the degree of enhancement. This is consistent with dif ferences in the evolution and vertical structure of temperatures in th e two years. Gas-phase HNO3 abundances, which increased in early winte r due to diabatic descent, exhibited a decreasing trend from mid to la te winter in all six Arctic winters observed by MLS, probably due to i ncreasing HNO3 photolysis throughout this period.