Abstract: The invention concerns an on-board device and a mobile method for capturing and recording the fine particles and/or the density of NOx gases in the air for the purpose of dynamically using the gathered data. The device (1) comprises at least one sensor (3) for sensing particles and/or the density of the NOx gases in the air, capable of counting a number of particles or dust particles present in the air, having a dimension ranging between defined values in a volume of air, and a means (7,19) for displaying and/or recording said number and/or density correlated to a determined measurement location of the sensor, the device further being configured to be installed on and to function on a moving vehicle (2) and to be initialised at certain points on the map, or indeed recalibrated, in regular reference to fixed standardised sensors.