Metrological challenges for measurements of key climatological observables, Part 4: Atmospheric relative humidity

Water in its three ambient phases plays the central thermodynamic role in the terrestrial climate system. Clouds control Earth's radiation balance, atmospheric water vapour is the strongest "greenhouse" gas, and non-equilibrium relative humidity at the air-sea interface drives evaporation and latent heat export from the ocean. In this paper, we examine the climatologically relevant atmospheric relative humidity, noting fundamental deficiencies in the definition of this key observable. The metrological history of this quantity is reviewed, problems with its current definition and measurement practice are analysed, and options for future improvements are discussed in conjunction with the recent seawater standard TEOS-10. It is concluded that the International Bureau of Weights and Measures, (BIPM), in cooperation with the International Association for the Properties of Water and Steam, IAPWS, along with other international organisations and institutions, can make significant contributions by developing and recommending state-of-the-art solutions for this long standing metrological problem, such as are suggested here.

Tropospheric H(2) budget and the response of its soil uptake under the changing environment

Molecular hydrogen (H(2)) is an indirect greenhouse gas present at the trace level in the atmosphere. So far, the sum of its sources and sinks is close to equilibrium, but its large-scale utilization as an alternative energy carrier would alter its atmospheric burden. The magnitude of the emissions associated with a future H(2)-based economy is difficult to predict and remains a matter of debate. Previous attempts to predict the impact that a future H(2)-based economy would exert on tropospheric chemistry were realized by considering a steady rate of microbial-mediated soil uptake, which is currently responsible of ~80% of the tropospheric H(2) losses. Although soil uptake, also known as dry deposition is the most important sink for tropospheric H(2), microorganisms involved in the activity remain elusive. Given that microbial-mediated H(2) soil uptake is influenced by several environmental factors, global change should exert a significant effect on the activity and then, assuming a steady H(2) soil uptake rate for the future may be mistaken. Here, we present an overview of tropospheric H(2) sources and sinks with an emphasis on microbial-mediated soil uptake process. Future researches are proposed to investigate the influence that global change would exert on H(2) dry deposition and to identify microorganisms involved H(2) soil uptake activity.

Lightweight diode laser spectrometer CHILD (Compact High-altitude iN-situ Laser Diode) for balloonborne measurements of water vapor and methane

A new lightweight near-infrared tunable diode laser spectrometer CHILD (Compact High-altitude In-situ Laser Diode spectrometer) was developed for flights to the stratosphere as an additional in situ sensor on existing balloonborne payloads. Free-air absorption measurements in the near infrared are made with an open-path Herriott cell with new design features. It offers two individual absorption path lengths optimized for CH4 with 74 m (136 pass) and H2O with 36 m (66 pass). New electronic features include a real-time gain control loop that provides an autocalibration function. In flight-ready configuration the instrument mass is approximately 20 kg, including batteries. It successfully measured stratospheric CH4 and H2O profiles on high-altitude balloons on four balloon campaigns (Environmental Satellite validation) between October 2001 and June 2003. On these first flights, in situ spectra were recorded from ground level to 32,000-m altitude with a sensitivity of 0.1 ppm [(parts per million), ground] to 0.4 ppm (32,000 m) for methane and 0.15-0.5 ppm for water.

Extreme deuterium enrichment in stratospheric hydrogen and the global atmospheric budget of H2

Molecular hydrogen (H2) is the second most abundant trace gas in the atmosphere after methane (CH4). In the troposphere, the D/H ratio of H2 is enriched by 120 per thousand relative to the world's oceans. This cannot be explained by the sources of H2 for which the D/H ratio has been measured to date (for example, fossil fuels and biomass burning). But the isotopic composition of H2 from its single largest source--the photochemical oxidation of methane--has yet to be determined. Here we show that the D/H ratio of stratospheric H2 develops enrichments greater than 440 per thousand, the most extreme D/H enrichment observed in a terrestrial material. We estimate the D/H ratio of H2 produced from CH4 in the stratosphere, where production is isolated from the influences of non-photochemical sources and sinks, showing that the chain of reactions producing H2 from CH4 concentrates D in the product H2. This enrichment, which we estimate is similar on a global average in the troposphere, contributes substantially to the D/H ratio of tropospheric H2.