A gas analysis data handling device for computing a gas flux of a target gas comprising: a processor; a communication interface to receive data from one or more measuring devices; and a memory store for storing data received by the communication interface, the processor comprising a calibration surface defining a multiplication factor as a function of temperature and pressure for correcting for effects of temperature and pressure on gas absorption line measurements and configured to: receive a plurality of data, the plurality of data including: a plurality of gas density measurement data for the target gas measured by means of spectral absorption measurements taken by a light absorption based gas analyzer; a plurality of wind speed measurement data indicative of speed of movement of the target gas; and a plurality of temperature measurement data; a pressure measurement data; determine a raw flux of the target gas based on the gas density measurement data for the target gas and the wind speed measurement data; determine a thermal expansion effect term based on the temperature measurement data; and compute the gas flux of the target gas based on the raw flux and the thermal expansion effect term, at least one of the raw flux or the thermal expansion effect term being adjusted by the multiplication factor, the multiplication factor being defined as an instrument response function of the gas analyser used to obtain the gas density measurement data, the multiplication factor of the instrument response function correcting for temperature and pressure (T, P) broadening of the gas absorption line, the response function multiplication factor relating the measured gas density from the gas analyser used to obtain the gas density measurement data and the actual gas density of the target gas as a function of measured temperature and pressure. a processor; a communication interface to receive data from one or more measuring devices; and a memory store for storing data received by the communication interface, the processor comprising a calibration surface defining a multiplication factor as a function of temperature and pressure for correcting for effects of temperature and pressure on gas absorption line measurements and configured to: receive a plurality of data, the plurality of data including: a plurality of gas density measurement data for the target gas measured by means of spectral absorption measurements taken by a light absorption based gas analyzer; a plurality of wind speed measurement data indicative of speed of movement of the target gas; and a plurality of temperature measurement data; a pressure measurement data; determine a raw flux of the target gas based on the gas density measurement data for the target gas and the wind speed measurement data; determine a thermal expansion effect term based on the temperature measurement data; and compute the gas flux of the target gas based on the raw flux and the thermal expansion effect term, at least one of the raw flux or the thermal expansion effect term being adjusted by the multiplication factor, the multiplication factor being defined as an instrument response function of the gas analyser used to obtain the gas density measurement data, the multiplication factor of the instrument response function correcting for temperature and pressure (T, P) broadening of the gas absorption line, the response function multiplication factor relating the measured gas density from the gas analyser used to obtain the gas density measurement data and the actual gas density of the target gas as a function of measured temperature and pressure. receive a plurality of data, the plurality of data including: a plurality of gas density measurement data for the target gas measured by means of spectral absorption measurements taken by a light absorption based gas analyzer; a plurality of wind speed measurement data indicative of speed of movement of the target gas; and a plurality of temperature measurement data; a pressure measurement data; a plurality of gas density measurement data for the target gas measured by means of spectral absorption measurements taken by a light absorption based gas analyzer; a plurality of wind speed measurement data indicative of speed of movement of the target gas; and a plurality of temperature measurement data; a pressure measurement data; determine a raw flux of the target gas based on the gas density measurement data for the target gas and the wind speed measurement data; determine a thermal expansion effect term based on the temperature measurement data; and compute the gas flux of the target gas based on the raw flux and the thermal expansion effect term, at least one of the raw flux or the thermal expansion effect term being adjusted by the multiplication factor, the multiplication factor being defined as an instrument response function of the gas analyser used to obtain the gas density measurement data, the multiplication factor of the instrument response function correcting for temperature and pressure (T, P) broadening of the gas absorption line, the response function multiplication factor relating the measured gas density from the gas analyser used to obtain the gas density measurement data and the actual gas density of the target gas as a function of measured temperature and pressure.