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 nonsulfatematter 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.
Carbonyl sulfide (OCS) and carbon disulfide (CS2) are
volatile sulfur gases that are naturally formed in seawater and exchanged
with the atmosphere. OCS is the most abundant sulfur gas in the ...atmosphere,
and CS2 is its most important precursor. They have attracted increased interest due
to their direct (OCS) or indirect (CS2 via oxidation to OCS)
contribution to the stratospheric sulfate aerosol layer. Furthermore, OCS
serves as a proxy to constrain terrestrial CO2 uptake by vegetation.
Oceanic emissions of both gases contribute a major part to their atmospheric
concentration. Here we present a database of previously published and
unpublished (mainly shipborne) measurements in seawater and the marine
boundary layer for both gases, available at https://doi.org/10.1594/PANGAEA.905430 (Lennartz et
al., 2019). The database contains original measurements as well as data
digitalized from figures in publications from 42 measurement campaigns, i.e.,
cruises or time series stations, ranging from 1982 to 2019. OCS data cover
all ocean basins except for the Arctic Ocean, as well as all months of the
year, while the CS2 dataset shows large gaps in spatial and temporal
coverage. Concentrations are consistent across different sampling and
analysis techniques for OCS. The database is intended to support the
identification of global spatial and temporal patterns and to facilitate the
evaluation of model simulations.
Chemistry Climate Models (CCMs) are essential tools for characterizing and predicting the role of atmospheric composition and chemistry in Earth's climate system. This study demonstrates the use of ...airborne in situ observations to diagnose the representation of chemical composition and transport by CCMs. Process‐based diagnostics using dynamical and chemical coordinates are presented which minimize the spatial and temporal sampling differences between airborne in situ measurements and CCM grid points. The chosen process is the chemical impact of the Asian summer monsoon (ASM), where deep convection serves as a rapid transport pathway for surface emissions to reach the upper troposphere and lower stratosphere (UTLS). We examine two CCM configurations for their representation of the ASM UTLS using a set of airborne observations from south Asia. The diagnostics reveal good model performance at representing tropospheric tracer distribution throughout the troposphere and lower stratosphere, and excellent representation of chemical aging in the lower stratosphere when chemical loss is dominated by photolysis. Identified model limitations include the use of zonally averaged mole fraction boundary conditions for species with sufficiently short tropospheric lifetimes, which may obscure enhanced regional emissions sources. Overall, the diagnostics underscore the skill of current‐generation models at representing pollution transport from the boundary layer to the stratosphere via the ASM mechanism, and demonstrate the strength of airborne in situ observations toward characterizing this representation.
Plain Language Summary
The chemical composition of Earth's atmosphere is important to understand for future climate prediction. This study establishes an approach for evaluating the representation of chemical composition in global climate models, and demonstrates the capabilities of the approach using a set of observations collected by research aircraft. We specifically target an evaluation of the Asian summer monsoon, a process with a well‐documented transport pathway for chemical species near the surface to reach the upper atmosphere. In doing so, we identify specific areas where focused model improvement is needed.
Key Points
Process‐based diagnostics for model evaluation using airborne in situ observations are presented
The Asian summer monsoon is explored for its role in impacting global composition and climate
The diagnostics use dynamical and chemical coordinates to identify model strengths and limitations
PDF
