Atmospheric trace gases play critical roles in the atmosphere affecting air quality, human and ecosystem health as well as directly and indirectly influencing climate. Some trace gases are regulated air pollutants, some are greenhouse gases while others act as precursors to secondary pollutants such as ozone and particulate matter. Monitoring long term spatial and temporal changes in trace gas concentrations is essential to establish the scientific links and feedbacks between atmospheric composition, air quality and climate and to ensure legislative compliance. Ambient concentrations and stable isotope ratios of many trace gases are routinely observed within networks of monitoring sites and on mobile measurement platforms around the globe. Ensuring the quality and comparability of all these datasets is critical to improve reliability and reduce uncertainty in our understanding of the Earths system. This session invites contributions that seek to address the fundamental metrology needed to underpin long term ambient monitoring and to ensure coherent and comparable measurements from year to year and within, and between different monitoring networks and measurement platforms.