To charge negatively, the teflon tube that is standard on a tribo gun will have to be replaced by a tube made with an electron donor, or else corona charging used instead of tribocharging.
I calculate that to rise in the atmospheric electric field, a particle needs a charge to mass ratio ("specific charge") greater than 50 millicoulomb/kilogram, which may be another factor requiring corona charging.

I guess to get started we will need a telescopic laser range finder to find out how high the particles actually go.

What about building some macroscopic device that uses sky power? In principle, it could fly if it could exceed a specific charge of 50 mC/kg, with payload. Maybe build it to double as a single, large corner-cube reflector pointing down, to facilitate ground-based tracking. Such a form would be mechanically stable while ascending because it places the center of charge above the widest point. If tracking is done at radar wavelengths, the device could be made of an open honeycomb lattice to reduce weight and air resistance, because relatively long wavelengths do not have the resolution necessary to "see" the holes.

The figure of 50 mC/kg was derived by dividing g, the gravitational acceleration at the Earth's surface (about 10 m/s2), by 200 V/m, and multiplying by 1000 to get the units used in studies of powder-coating physics (and the units analysis checks out).

Extrapolating from data in Meng et al., 2008, http://dx.doi.org/10.1088/0022-3727/41/19/195207 , 2.3-micron-diameter sulfur particles corona charged at 90 kV should fly. However, a ten-fold smaller sulfur particle will have a ten-fold greater specific charge, giving some margin to allow for discharging on the way up.

The diameter of the sulfur particle injected into the stratosphere is unrelated to the diameter of the eventual sulfuric acid droplets it produces upon oxidation in the stratosphere, because one reaction intermediate, sulfur dioxide, is gaseous (complication: it’s also a greenhouse gas).