Ground based infrared observations of HNO3 column amount at Lauder, New Zealand (45°S, 170°E), show significant increases in HNO3 following the arrival of Pinatubo volcanic aerosols. The increase first became apparent in September 1991 when the HNO3 amount was higher than that expected from the regular seasonal variation by 2.3 × 1015 cm−2, which corresponds to 16% of the unperturbed value. Between September 1991 and May 1993, the observed HNO3 amounts were systematically higher, typically by 1.5 to 3.5 × 1015 cm−2 or 10 to 30%. After June 1993, the observed HNO3 amounts were close to the unperturbed values. The decrease in stratospheric NO2 due to the volcanic aerosols was observed at Lauder after August 1991 [Johnston et al., 1992, 1993]. The start of the HNO3 increase and the general trend are in agreement with those expected from the NO2 decrease when heterogeneous reactions on the sulfate aerosols are considered. Model calculations, in which observed aerosol fields have been used, qualitatively agree with these observed trends, although the magnitude of the changes have been underestimated. These results provide further understanding of the impact of heterogeneous chemistry on the partitioning between NO2 and HNO3. Copyright 1994 by the American Geophysical Union.