Understanding measured water rotational temperatures and column densities in the very innermost coma of Comet 73P/Schwassmann-Wachmann 3 B

► A tool to interpret very high-resolution IR profiles is presented. ► Direct Simulation Monte Carlo model was applied to Comet 73P/Schwassmann-Wachmann 3. ► Icy grains are needed to reproduce H2O column densities and rotational temperatures. ► Pure and dirty icy grains are considered. ► Electron-H2O collision rotational heating is not negligible.