A two dimensional photochemical model is used to examine the effects of heterogeneous processing by a parameterized lower stratospheric sulfate aerosol layer on model calculations. The single reaction considered is N2O5+H2O→2HNO3, at a rate consistent with recent laboratory measurements. The calculations are compared with data, to see if the heterogeneous chemistry improves the agreement between model calculations and data. The data used are Limb Infrared Monitor of the Stratosphere (LIMS) measurements of HNO3 and NO2, and in situ measurements of NO, NOy, and ClO taken by the NASA ER-2 aircraft. A contradictory picture results. With heterogeneous processing, model HNO3 agrees better with LIMS HNO3; however, LIMS NO2, and NO2/HNO3 agree more closely with the results of a standard gas-phase calculation. High-latitude NO/NOy from in situ measurements in winter also favor standard chemistry, while the seasonal variation of NO and NO2 are better represented when the heterogeneous reaction is included. ClO is also better simulated when the heterogeneous reaction is included. These comparisons suggest that the assumptions made to parameterize the sulfate aerosol chemistry result in a rate of heterogeneous processing that is too vigorous. However, the lack of consistency in the different comparisons leaves the situation ambiguous, and a definite conclusion is unwarranted. ¿ American Geophysical Union 1992