In converting DN values to radiance as well as reflectance, do we use the same equations in raw satellite imagery for pansharpened products? I saw this DigitalGlobe technical note for DN to radiance conversion where "the information applies to all Quickbird products EXCEPT pansharpened" imagery. So may I know then how pansharpened imagery are atmospherically corrected, i.e from DN to radiance to reflectance?
There are multiple things to respond to in your question.
1. The title of your question refers to atmospheric correction, while the information you refer to in the text is focused around converting to radiance. These two things should not be confused.
2. It does not make physical sense to atmospherically correct pansharpend data using standard methods due to the addition of the panchromatic signal to the multispectral data. However, that doesn't mean that you can't ignore those problems and do it anyway. If you choose to atmospherically correct pansharpend data you should keep in mind that the pansharpening will have changed the DN-values slightly and as such, you will have introduced a small source of errors into your reflectance errors.
All in all, my own experience shows that the errors caused by atmospherically correcting pansharpend data are minor and if the objects that you are mapping truely require the improved resolution the errors introduced are small enough to justify it.
On a related note, don't forego the chance to compare the results (a) atmospheric correction followed by pansharpening and (b) pansharpening followed by atmospheric correction. This will help you understand which errors your methodology introduces.
If your data is pansharpend by DigitalGlobe, you will need to use the formulas from the User Guide with the variable values from the supplied .IMD-file to convert to TOA Radiance. In this case, you may end up with larger than necessary errors, as you have no control of the pansharpening process. This is one of the reasons why the bundle containing separate multispectral and panchromatic bands should be purchased by technical users, rather than already pansharpend data.
Related to all of this, consider checking with your data distributor if you can acquire atmospherically corrected data from them. DigitalGlobe have recently started offering their own atmospheric correction through some resellers at a slight increase in price.