Method for determining a concentration of a gas component in a process gas, comprising the steps of - generating a transmitted light beam with a tunable laser light source , wherein a tunable wavelength range ([λ1, λ2]) contains an absorption line of the gas component to be measured, - guiding the transmitted light beam through a measuring volume in which the process gas to be measured with the process gas component to be measured is present and through an ambient air volume in which ambient air with a non-interesting proportion of the gas component is present, - receiving the light beam guided through the measuring volume and the ambient air volume and generating reception signals with a receiver , - generating measured values from the received signals, the measured values forming an absorption spectrum and determining the concentration of the gas component from the absorption spectrum with an evaluation unit , - wherein a measuring curve is numerically fitted to the measured values by determining first and second fitting parameters, - wherein the first adjustment parameters comprise the line position and line width, and additionally line area and/or line height of the absorption line of the gas component in the process, and the second adjustment parameters comprise the line position and line width, and additionally line area and/or line height of the absorption line of the gas component in ambient air, and - wherein the pressure and temperature of the process gas are recorded to calculate initial values for the first adjustment parameters and - wherein pressure and temperature of the ambient air are detected for calculating initial values for the second adjustment parameters line position and/or line width and - the numerical adjustment is carried out with these initial values and the concentration is determined from the adjustment parameters determined in this way. - generating a transmitted light beam with a tunable laser light source , wherein a tunable wavelength range ([λ1, λ2]) contains an absorption line of the gas component to be measured, - guiding the transmitted light beam through a measuring volume in which the process gas to be measured with the process gas component to be measured is present and through an ambient air volume in which ambient air with a non-interesting proportion of the gas component is present, - receiving the light beam guided through the measuring volume and the ambient air volume and generating reception signals with a receiver , - generating measured values from the received signals, the measured values forming an absorption spectrum and determining the concentration of the gas component from the absorption spectrum with an evaluation unit , - wherein a measuring curve is numerically fitted to the measured values by determining first and second fitting parameters, - wherein the first adjustment parameters comprise the line position and line width, and additionally line area and/or line height of the absorption line of the gas component in the process, and the second adjustment parameters comprise the line position and line width, and additionally line area and/or line height of the absorption line of the gas component in ambient air, and - wherein the pressure and temperature of the process gas are recorded to calculate initial values for the first adjustment parameters and - wherein pressure and temperature of the ambient air are detected for calculating initial values for the second adjustment parameters line position and/or line width and - the numerical adjustment is carried out with these initial values and the concentration is determined from the adjustment parameters determined in this way. <EOS>