High resolution databases on atmospheric concentrations of pesticides are necessary in order to perform epidemiological studies but there is currently no modeling method to provide high resolution mapping of pesticides concentrations over a whole region. In this study, we propose an online downscaling method for CHIMERE to perform simulations at a sub-kilometer resolution. The main idea of this downscaling approach is to redistribute or interpolate some information simulated on the coarse grid to simulate the transport over a finer subgrid. The performance of the downscaling is analyzed by comparing the CHIMERE nested simulation results at 0.02° and CHIMERE simulation results downscaled from 0.1° to 0.02°. By applying this method to S-metolachlor, we diagnosed an error generated by the downscaling of a few percents on both background and hotspot concentrations. The method was used to simulate concentrations over France at a resolution of 0.004° with a limited increase of the computational time. Based on these simulations, we estimated that around 3 000 inhabitants were exposed to concentrations of S-metolachlor above 10 ng/m from April 15th to May 15th 2014.