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