Impact of Pinatubo aerosols on the partitioning between NO₂ and HNO₃

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.