Mathias Palm

Stratospheric Aerosol--Observations, Processes, and Impact on Climate

Interest in stratospheric aerosol and its role in climate have increased over the last decade due to the observed increase in stratospheric aerosol since 2000 and the potential for changes in the sulfur cycle induced by climate change. This review provides an overview about the advances in stratospheric aerosol research since the last comprehensive assessment of stratospheric aerosol was published in 2006. A crucial development since 2006 is the substantial improvement in the agreement between in situ and space-based inferences of stratospheric aerosol properties during volcanically quiescent periods. Furthermore, new measurement systems and techniques, both in situ and space based, have been developed for measuring physical aerosol properties with greater accuracy and for characterizing aerosol composition. However, these changes induce challenges to constructing a long-term stratospheric aerosol climatology. Currently, changes in stratospheric aerosol levels less than 20% cannot be confidently quantified. The volcanic signals tend to mask any nonvolcanically driven change, making them difficult to understand. While the role of carbonyl sulfide as a substantial and relatively constant source of stratospheric sulfur has been confirmed by new observations and model simulations, large uncertainties remain with respect to the contribution from anthropogenic sulfur dioxide emissions. New evidence has been provided that stratospheric aerosol can also contain small amounts of nonsulfate matter such as black carbon and organics. Chemistry-climate models have substantially increased in quantity and sophistication. In many models the implementation of stratospheric aerosol processes is coupled to radiation and/or stratospheric chemistry modules to account for relevant feedback processes.

Recent Northern Hemisphere stratospheric HCl increase due to atmospheric circulation changes

The abundance of chlorine in the Earth’s atmosphere increased considerably during the 1970s to 1990s, following large emissions of anthropogenic long-lived chlorine-containing source gases, notably the chlorofluorocarbons. The chemical inertness of chlorofluorocarbons allows their transport and mixing throughout the troposphere on a global scale, before they reach the stratosphere where they release chlorine atoms that cause ozone depletion. The large ozone loss over Antarctica was the key observation that stimulated the definition and signing in 1987 of the Montreal Protocol, an international treaty establishing a schedule to reduce the production of the major chlorine- and bromine-containing halocarbons. Owing to its implementation, the near-surface total chlorine concentration showed a maximum in 1993, followed by a decrease of half a per cent to one per cent per year, in line with expectations. Remote-sensing data have revealed a peak in stratospheric chlorine after 1996, then a decrease of close to one per cent per year, in agreement with the surface observations of the chlorine source gases and model calculations. Here we present ground-based and satellite data that show a recent and significant increase, at the 2σ level, in hydrogen chloride (HCl), the main stratospheric chlorine reservoir, starting around 2007 in the lower stratosphere of the Northern Hemisphere, in contrast with the ongoing monotonic decrease of near-surface source gases. Using model simulations, we attribute this trend anomaly to a slowdown in the Northern Hemisphere atmospheric circulation, occurring over several consecutive years, transporting more aged air to the lower stratosphere, and characterized by a larger relative conversion of source gases to HCl. This short-term dynamical variability will also affect other stratospheric tracers and needs to be accounted for when studying the evolution of the stratospheric ozone layer.

Positive trends in Southern Hemisphere carbonyl sulfide

Transport of carbonyl sulfide (OCS) from the troposphere to the stratosphere contributes sulfur to the stratospheric aerosol layer, which reflects incoming short-wave solar radiation, cooling the climate system. Previous analyses of OCS observations have shown no significant trend, suggesting that OCS is unlikely to be a major contributor to the reported increases in stratospheric aerosol loading and indicating a balanced OCS budget. Here we present analyses of ground-based Fourier transform spectrometer measurements of OCS at three Southern Hemisphere sites spanning 34.45°S to 77.80°S. At all three sites statistically significant positive trends are seen from 2001 to 2014 with an observed overall trend in total column OCS at Wollongong of 0.73 ± 0.03%/yr, at Lauder of 0.43 ± 0.02%/yr, and at Arrival Heights of 0.45 ± 0.05%/yr. These observed trends in OCS imply that the OCS budget is not balanced and could contribute to constraints on current estimates of sources and sinks.

Starting long-term stratospheric observations with RAMAS at Summit, Greenland

The new microwave Radiometer for Atmospheric Measurements at Summit (RAMAS) has started operation at Summit station, Greenland (72/spl deg/N, 38/spl deg/W, 3200 m), and is now being prepared for continuous measurements. RAMAS operates in the frequency band from 265-280 GHz with an instantaneous bandwidth of currently 1 GHz. It observes the emission of thermally induced rotational transitions of a variety of stratospheric trace gases, including O/sub 3/, ClO, N/sub 2/O, HNO/sub 3/, and HCN. Tropospheric water vapor content, a major constraint on ground-based microwave radiometry, is exceptionally low at Summit station. Initial measurements and retrieved profiles are presented, demonstrating the excellent measurement conditions.

Influence of Solar Radiation on the Diurnal and Seasonal Variability of O3 and H2O in the Stratosphere and Lower Mesosphere, Based on Continuous Observations in the Tropics and the High Arctic

During the CAWSES DFG (German Research Association) priority program measurements of stratospheric and mesospheric O3 using ground based millimeterwave radiometry have been established and analyzed. Instruments have been operated at two different locations, at Merida, Venezuela, a high altitude tropic station and at Ny Alesund, Spitsbergen, an Arctic station. Additionally, data obtained from the millimeterwave radiometer based at Kiruna, Sweden, have been used for an analysis of the 5-day planetary wave.

Near-Infrared Lunar Absorption Spectroscopy for the Retrieval of Column Averaged CO2 and CH4

High resolution Fourier-Transform InfraRed (FTIR) absorption spectroscopic measurements are used to retrieve trace gas abundances in the atmosphere at a number of sites throughout the world. Typically, the sun is used as an infrared light source above the atmosphere of the Earth, however, at polar sites, such as our measurement site at Spitsbergen, sunlight is not available during polar night (October–March). Instead, the moon can be used as a substitute infrared light source. In this article we present a proof of concept for the usage of a thermoelectrically cooled InGaAs (Indium-Gallium-Arsenide) near infrared detector to measure the column averaged trace gas abundances by lunar absorption spectroscopy based on trial measurements made in Bremen. These measurements demonstrate the potential of using lunar measurements for retrieval of atmospheric columns of CO2 and CH4. The resulting data can be used to fill a crucial gap in the seasonal cycle at polar sites.

Atmospheric Inverse Modeling via Sparse Reconstruction

Many applications in atmospheric science involve ill-posed inverse problems. A crucial component of many inverse problems is the proper formulation of a priori knowledge about the unknown parameters. In most cases, this knowledge is expressed as a Gaussian prior. This formulation often performs well at capturing smoothed, large-scale processes but is often ill equipped to capture localized structures like large point sources or localized hot spots. Over the last decade, scientists from a diverse array of applied mathematics and engineering fields have developed sparse reconstruction techniques to identify localized structures. In this study, we present a new regularization approach for ill-posed inverse problems in atmospheric science. It is based on Tikhonov regularization with sparsity constraint and allows bounds on the parameters. We enforce sparsity using a dictionary representation system. We analyze its performance in an atmospheric inverse modeling scenario by estimating anthropogenic US methane (CH4) emissions from simulated atmospheric measurements. Different measures indicate that our sparse reconstruction approach is better able to capture large point sources or localized hot spots than other methods commonly used in atmospheric inversions. It captures the overall signal equally well but adds details on the grid scale. This feature can be of value for any inverse problem with point or spatially discrete sources. We show an example for source estimation of synthetic methane emissions from the Barnett shale formation.