Impact of Pinatubo aerosols on the partitioning between NO₂ and HNO₃

Ground based infrared observations of HNO 3 column amount at Lauder, New Zealand (45°S, 170°E), show significant increases in HNO 3 following the arrival of Pinatubo volcanic aerosols. The increase first became apparent in September 1991 when the HNO 3 amount was higher than that expected from the regular seasonal variation by 2.3 × 10 15 cm −2 , which corresponds to 16% of the unperturbed value. Between September 1991 and May 1993, the observed HNO 3 amounts were systematically higher, typically by 1.5 to 3.5 × 10 15 cm −2 or 10 to 30%. After June 1993, the observed HNO 3 amounts were close to the unperturbed values. The decrease in stratospheric NO 2 due to the volcanic aerosols was observed at Lauder after August 1991 [ Johnston et al. , 1992, 1993]. The start of the HNO 3 increase and the general trend are in agreement with those expected from the NO 2 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 NO 2 and HNO 3 .