We show that a fire plume injected into the lower stratosphere at high northern latitudes during the Canadian wildfire event in August 2017 partly reached the tropics. The transport to the tropics ...was mediated by the anticyclonic flow of the Asian monsoon circulation. The fire plume reached the Asian monsoon area in late August/early September, when the Asian monsoon anticyclone (AMA) was still in place. While there is no evidence of mixing into the center of the AMA, we show that a substantial part of the fire plume is entrained into the anticyclonic flow at the AMA edge and is transported from the extratropics to the tropics, and possibly the Southern Hemisphere particularly following the north–south flow on the eastern side of the AMA. In the tropics the fire plume is lifted by ∼5 km in 7 months. Inside the AMA we find evidence of the Asian tropopause aerosol layer (ATAL) in August, doubling background aerosol conditions with a calculated top of the atmosphere shortwave radiative forcing of −0.05 W m−2. The regional climate impact of the fire signal in the wider Asian monsoon area in September exceeds the impact of the ATAL by a factor of 2–4 and compares to that of a plume coming from an advected moderate volcanic eruption. The stratospheric, trans-continental transport of this plume to the tropics and the related regional climate impact point to the importance of long-range dynamical interconnections of pollution sources.
Solar radiation management through artificially
increasing the amount of stratospheric sulfate aerosol is being considered
as a possible climate engineering method. To overcome the challenge of
...transporting the necessary amount of sulfur to the stratosphere, Quaglia and
co-workers suggest deliberate emissions of carbonyl sulfide (OCS), a
long-lived precursor of atmospheric sulfate. In their paper, published in
Atmospheric Chemistry and Physics in 2022, they outline two scenarios with OCS emissions either at the
Earth's surface or in the tropical upper troposphere and calculate the
expected radiative forcing using a climate model. In our opinion, the study
(i) neglects a significantly higher surface uptake that will inevitably be
induced by the elevated atmospheric OCS concentrations and (ii)
overestimates the net cooling effect of this OCS geoengineering approach due
to some questionable parameterizations and assumptions in the radiative
forcing calculations. In this commentary, we use state-of-the-art models to
show that at the mean atmospheric OCS mixing ratios of the two emissions
scenarios, the terrestrial biosphere and the oceans are expected to take up
more OCS than is being released to reach these levels. Using chemistry
climate models with a long-standing record for estimating the climate
forcing of OCS and stratospheric aerosols, we also show that the net
radiative forcing of the emission scenarios suggested by Quaglia and
co-workers is smaller than suggested and insufficient to offset any
significant portion of anthropogenically induced climate change. Our
conclusion is that a geoengineering approach using OCS will not work under
any circumstances and should not be considered further.
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.
Carbonyl sulfide (COS), a trace gas in our atmosphere that leads to the formation of aerosols in the stratosphere, is largely taken up by terrestrial ecosystems. Quantifying the biosphere uptake of ...COS could provide a useful quantity to estimate gross primary productivity (GPP). Some COS sources and sinks still contain large uncertainties, and several top-down estimates of the COS budget point to an underestimation of sources, especially in the tropics. We extended the inverse model TM5-4DVAR to assimilate Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) satellite data, in addition to National Oceanic and Atmospheric Administration (NOAA) surface data as used in a previous study. To resolve possible discrepancies among the two observational data sets, a bias correction scheme is necessary and implemented. A set of inversions is presented that explores the influence of the different measurement streams and the settings of the prior fluxes. To evaluate the performance of the inverse system, the HIAPER Pole-to-Pole Observations (HIPPO) aircraft observations and NOAA airborne profiles are used. All inversions reduce the COS biosphere uptake from a prior value of 1053 GgS a−1 to much smaller values, depending on the inversion settings. These large adjustments of the biosphere uptake often turn parts of Amazonia into a COS source. Only inversions that exclusively use MIPAS observations, or strongly reduce the prior errors on the biosphere flux, maintain the Amazon as a COS sink. Inclusion of MIPAS data in the inversion leads to a better separation of land and ocean fluxes. Over the Amazon, these inversions reduce the biosphere uptake from roughly 300 to 100 GgS a−1, indicating a strongly overestimated prior uptake in this region. Although a recent study also reported reduced COS uptake over the Amazon, we emphasise that a careful construction of prior fluxes and their associated errors remains important. For instance, an inversion that gives large freedom to adjust the anthropogenic and ocean fluxes of CS2, an important COS precursor, also closes the budget satisfactorily with much smaller adjustments to the biosphere. We achieved better characterisation of biosphere prior and uncertainty, better characterisation of combined ocean and land fluxes, and better constraint of both by combining surface and satellite observations. We recommend more COS observations to characterise biosphere and ocean fluxes, especially over the data-poor tropics.
Carbonyl sulfide (OCS), the most abundant sulfur gas in
the Earth's atmosphere, is a greenhouse gas, a precursor to stratospheric
sulfate aerosol, and a proxy for terrestrial CO2 uptake. Estimates of
...important OCS sources and sinks still have significant uncertainties and the
global budget is not considered closed. One particularly uncertain source
term, the OCS production during the atmospheric oxidation of dimethyl
sulfide (DMS) emitted by the oceans, is addressed by a series of experiments
in the atmospheric simulation chamber SAPHIR in conditions comparable to the
remote marine atmosphere. DMS oxidation was initiated with OH and/or Cl
radicals and DMS, OCS, and several oxidation products and intermediates were
measured, including hydroperoxymethyl thioformate (HPMTF), which was recently
found to play a key role in DMS oxidation in the marine atmosphere. One
important finding is that the onset of HPMTF and OCS formation occurred
faster than expected from the current chemical mechanisms. In agreement with
other recent studies, OCS yields between 9 % and 12 % were observed in our
experiments. Such yields are substantially higher than the 0.7 % yield
measured in laboratory experiments in the 1990s, which is generally used to
estimate the indirect OCS source from DMS in global budget estimates.
However, we do not expect the higher yields found in our experiments to
directly translate into a substantially higher OCS source from DMS oxidation
in the real atmosphere, where conditions are highly variable, and, as pointed
out in recent work, heterogeneous HPMTF loss is expected to effectively
limit OCS production via this pathway. Together with other experimental
studies, our results will be helpful to further elucidate the DMS oxidation
chemical mechanism and in particular the paths leading to OCS formation.
Every year during the Asian summer monsoon season from about mid-June to early September, a stable anticyclonic circulation system forms over the Himalayas. This Asian summer monsoon (ASM) ...anticyclone has been shown to promote transport of air into the stratosphere from the Asian troposphere, which contains large amounts of anthropogenic pollutants. Essential details of Asian monsoon transport, such as the exact timescales of vertical transport, the role of convection in cross-tropopause exchange, and the main location and level of export from the confined anticyclone to the stratosphere are still not fully resolved. Recent airborne observations from campaigns near the ASM anticyclone edge and centre in 2016 and 2017, respectively, show a steady decrease in carbon monoxide (CO) and increase in ozone (O3) with height starting from tropospheric values of around 100 ppb CO and 30–50 ppb O3 at about 365 K potential temperature. CO mixing ratios reach stratospheric background values below ∼25 ppb at about 420 K and do not show a significant vertical gradient at higher levels, while ozone continues to increase throughout the altitude range of the aircraft measurements. Nitrous oxide (N2O) remains at or only marginally below its 2017 tropospheric mixing ratio of 333 ppb up to about 400 K, which is above the local tropopause. A decline in N2O mixing ratios that indicates a significant contribution of stratospheric air is only visible above this level. Based on our observations, we draw the following picture of vertical transport and confinement in the ASM anticyclone: rapid convective uplift transports air to near 16 km in altitude, corresponding to potential temperatures up to about 370 K. Although this main convective outflow layer extends above the level of zero radiative heating (LZRH), our observations of CO concentration show little to no evidence of convection actually penetrating the tropopause. Rather, further ascent occurs more slowly, consistent with isentropic vertical velocities of 0.7–1.5 K d−1. For the key tracers (CO, O3, and N2O) in our study, none of which are subject to microphysical processes, neither the lapse rate tropopause (LRT) around 380 K nor the cold point tropopause (CPT) around 390 K marks a strong discontinuity in their profiles. Up to about 20 to 35 K above the LRT, isolation of air inside the ASM anticyclone prevents significant in-mixing of stratospheric air (throughout this text, the term in-mixing refers specifically to mixing processes that introduce stratospheric air into the predominantly tropospheric inner anticyclone). The observed changes in CO and O3 likely result from in situ chemical processing. Above about 420 K, mixing processes become more significant and the air inside the anticyclone is exported vertically and horizontally into the surrounding stratosphere.
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
During the StratoClim Geophysica campaign, air with total water mixing ratios up to 200 ppmv and ozone up to 250 ppbv was observed within the Asian summer monsoon anticyclone up to 1.7 km above the ...local cold-point tropopause (CPT). To investigate the temporal evolution of enhanced water vapor being transported into the stratosphere, we conduct forward trajectory simulations using both a microphysical and an idealized freeze-drying model. The models are initialized at the measurement locations and the evolution of water vapor and ice is compared with satellite observations of MLS and CALIPSO. Our results show that these extremely high water vapor values observed above the CPT are very likely to undergo significant further freeze-drying due to experiencing extremely cold temperatures while circulating in the anticyclonic “dehydration carousel”. We also use the Lagrangian dry point (LDP) of the merged back-and-forward trajectories to reconstruct the water vapor fields. The results show that the extremely high water vapor mixed with the stratospheric air has a negligible impact on the overall water vapor budget. The LDP mixing ratios are a better proxy for the large-scale water vapor distributions in the stratosphere during this period.
Carbonyl sulphide (OCS) is the most abundant, long-lived
sulphur gas in the atmosphere and a major supplier of sulphur to the
stratospheric sulphate aerosol layer. The short-lived gas carbon ...disulphide
(CS2) is oxidized to OCS and constitutes a major indirect source to the
atmospheric OCS budget. The atmospheric budget of OCS is not well
constrained due to a large missing source needed to compensate for
substantial evidence that was provided for significantly higher sinks.
Oceanic emissions are associated with major uncertainties. Here we provide a
first, monthly resolved ocean emission inventory of both gases for the
period 2000–2019 (available at
https://doi.org/10.5281/zenodo.4297010) (Lennartz et al.,
2020a). Emissions are calculated with a numerical box model (2.8∘×2.8∘ resolution at the Equator, T42 grid) for the oceanic
surface mixed layer, driven by ERA5 data from ECMWF and chromophoric dissolved organic matter (CDOM) from
Aqua MODIS. We find that interannual variability in OCS emissions is smaller
than seasonal variability and is mainly driven by variations in CDOM, which influences both
photochemical and light-independent production. A comparison with a global
database of more than 2500 measurements reveals overall good agreement.
Emissions of CS2 constitute a larger sulphur source to the atmosphere
than OCS and equally show interannual variability connected to variability
in CDOM. The emission estimate of CS2 is associated with higher
uncertainties as process understanding of the marine cycling of CS2 is
incomplete. We encourage the use of the data provided here as input for
atmospheric modelling studies to further assess the atmospheric OCS budget
and the role of OCS in climate.
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