How can I convert the TOA reflectance values of L1C Sentinel2A products to BOA (Bottom Of Atmosphere) reflectance values?
Is there an equation?
You'll need to run ESA's Sen2Cor algorithm to process Sentinel-2 Level-1C data to Level-2A.
This will give you atmospherically corrected BOA data. If you then divide the values by the quantification value (10000) you'll end up with BOA reflectance data.
Download and install Sen2Cor: http://step.esa.int/main/third-party-plugins-2/sen2cor/
STEP help forum (useful info for Sen2cor): http://forum.step.esa.int/c/s2tbx
Thank you for your interest in this question.
Because it has attracted low-quality or spam answers that had to be removed, posting an answer now requires 10 reputation on this site (the association bonus does not count).
Would you like to answer one of these unanswered questions instead?