The aerosol budget in the upper troposphere and lower stratosphere may be perturbed by subsonic aviation through direct particle emission or, indirectly, following oxidation of emitted SO2 into sulfuric acid. These additional (or larger) particles produce a radiative forcing (RF) of climate by interacting with solar and planetary radiation, and may also favour formation of high-altitude clouds <Fahey et al., 1999>. The present modeling study focuses on an additional indirect effect of aircraft generated sulfate, namely heterogeneous chemistry on liquid sulfuric acid particles. For the 1992 subsonic aviation case we show that the NOx-driven RF decreases globally from 0.018 Wm-2 to about zero when emission of SO2 and sulfuric acid particles is also included. This decrease is mostly produced by: (1) a smaller ozone increase when the enhancement of sulfuric acid particle surface area density is taken into account (70%), and (2) a direct negative solar RF of aircraft generated sulfate mass (30%). No indirect effects on high-altitude clouds and contrails are considered here.