If I know the in-situ measured SO2 concentration (pptv), corresponding location, altitude (4 km), and time of measurment, is it possible to calculate a back trajectory of that SO2 plume to know about the spreading over that source region?

If you run a “backward” dispersion calculation from the measurement point you end up with a pattern that looks like concentration but is in fact a source-attribution function. The easiest approach to deposition and chemical conversion of SO2 to SO4 is to assume a fractional rate per hour, perhaps different for day and night. This can easily be reversed to have the value increasing upstream. Use a unit emission rate for the calculation, then after selecting a potential source, divide the measurement by the source-attribution function to obtain the emission rate.

Roland Draxler