June 8, 2021 The IMPRESS 2 project has launched a guidance document describing the typical uncertainty of dTDLAS measurements (Task 1.4). The PTB task was to test HCl monitoring optical spectrometer for the future underpinning of technique development. PTB validates the dTDLAS method in absorption cells and investigate the influence of heterogeneous effects on amount fraction measurements. This document focuses on describing the typical uncertainty of dTDLAS measurements. The document is also available at our Documents and publications page.