IMPACT OF PINATUBO AEROSOLS ON THE PARTITIONING BETWEEN NO2 AND HNO3

Ground based infrared observations of HNO3 column amount at Lauder, Ne w Zealand (45-degrees-S, 170-degrees-E), show significant increases in HNO3 following the arrival of Pinatubo volcanic aerosols. The increas e first became apparent in September 1991 when the HNO3 amount was hig her than that expected from the regular seasonal variation by 2.3 x 10 (15) cm-2, which corresponds to 16 % of the unperturbed value. Between September 1991 and May 1993, the observed HNO3 amounts were systemati cally higher, typically by 1.5 to 3.5 x 10(15) cm-2 or 10 to 30 %. Aft er 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 agreeme nt with those expected from the NO2 decrease when heterogeneous reacti ons on the sulfate aerosols are considered. Model calculations, in whi ch observed aerosol fields have been used, qualitatively agree with th ese observed trends, although the magnitude of the changes have been u nderestimated. These results provide further understanding of the impa ct of heterogeneous chemistry on the partitioning between NO2 and HNO3 .