4. Microphysical lab studies and process modeling

4.1. Background, motivation and state of the science

Under background (non-volcanic) conditions the stratospheric aerosol later is controlled by the transport of sulfur containing species (mainly OCS and SO2). However, there is some debate regarding the relative contributions of different sulfur containing source gases to the stratospheric aerosol layer (WMO, 2010). The lifetime of sulfur containing source gases, and hence the fraction of them that contributes to the stratospheric sulfur budget, is largely determined by tropospheric OH concentrations in the region where the ascent of air into the stratosphere occurs. Tremendous progress of our understanding of the transport processes in the TTL region was achieved over the last years (e.g. Fueglistaler et al., 2009) and implications for stratospheric water vapor and halogen budgets are topics of active research. In contrast, the implications for our understanding of the stratospheric sulfur budget and aerosol layer have not been addressed so far. For example, recent results show that the tropospheric OH column has a pronounced "OH hole"-like minimum in the region where most of transport into the stratosphere occurs, which results in much longer gas phase SO2 lifetimes and hence a potentially larger impact of tropospheric SO2 emissions on the stratospheric sulfur budget. Limited knowledge of the seasonal variability in OCS, SO2 and OH in the TTL prevents an assessment of the relevant chemical processes in the TTL. Furthermore, these results highlight that our quantitative understanding of the impact of anthropogenic activity in South East Asia or of moderate volcanic activity in the West Pacific region is very limited. Kremser et al. (2009) and Stenke et al. (2008) showed that Eulerian CTMs and CCMs have large problems with a realistic representation of transport processes at the tropical tropopause due to excessive vertical diffusion. Lagrangian models result in a much more realistic representation of transport timescales (Kremser et al., 2009) in that region and the water vapor budget in the stratosphere (Stenke et al., 2008). This implies that Lagrangian approaches have to be used for a robust and realistic representation of the processes in the TTL that determine the stratospheric sulfur and aerosol budget.

Besides the poor representation of transport processes in the TTL in CCMs, also processes such as volcanic injections or pyro-cumulonimbus (pyro-Cb) injections of biomass burning products into the stratosphere , and tropical deep convection are not (or not well) represented in CCMs. The extent to which volcanic gas and aerosol emissions influence stratospheric composition (and, thus, climate ; Robock, 2000) depends critically on the total mass of eruption products and the altitude at which they are effectively released into the atmosphere, neither of which is well known. Volcanoes exhibit a broad range of eruptive styles and variability (Woods, 1995), thus making theoretical attempts at predicting these source parameters challenging. Empirical relationships between the mass flux of tephra and the eruption column heights (Sparks et al., 1997; Mastin et al., 2009) are loose and depend on the ambient conditions of the atmosphere (Tupper et al., 2009), and small-scale mixing processes may affect the amount of material injected into the stratosphere (Luderer et al., 2007). Furthermore, the amount of SO2 emitted and the size distribution of volcanic ash is very variable between eruptions and do not only depend on the overall magnitude of an eruption, as expressed for instance by the total tephra mass flux. Pyro-cumulonimbus clouds (Fromm et al., 2010; Damoah et al., 2006) are in many ways comparable to volcanic eruptions, since they can also inject material into the stratospheric overworld (Jost et al., 2004). In fact, stratospheric aerosol layers due to biomass burning plumes have often wrongly been attributed to volcanic eruptions in the past (Fromm et al., 2010).

Earth System models are currently not capable of directly simulating volcanic eruption or pyro-Cb clouds. Therefore, work is urgently needed to better understand the injection of volcanic and biomass burning material into the stratosphere. This work should proceed via two avenues:

1) Detailed small-scale modeling to understand the relative roles of different processes (e.g., mixing, radiative heating, etc.).

2) Use of observation data to characterize the mass fluxes as a function of altitude.

Regarding avenue 1, the model ATHAM has been developed specifically to simulate the relevant processes in volcanic eruption or pyro-Cb columns (Oberhuber et al., 1998; Trentmann et al., 2006; Luderer et al., 2007; Herzog and Graf, 2010). Regarding avenue 2, an inverse modeling method has recently been developed to determine the effective emissions of SO2 and volcanic ash as a function of time and height, based on satellite observations of total column SO2 and ash loadings (Eckhardt et al., 2008; Kristiansen et al., 2010; Stohl et al., 2011). This method can be used to provide detailed eruption source parameters as input for climate models for historic eruptions in the satellite era.

4.2. Role in SSiRC and contribution to the overall goals of SSiRC

Based on the data provided by the observational working groups the microphysical lab studies and process modeling working group will improve the understanding of the processes that govern the sulfur transport from the base of the TTL to the stratosphere and the transport of small sulfate aerosol to the stratosphere. This working group will identify weaknesses in the representation of processes relevant for stratospheric sulfur and aerosol in global climate models through process-oriented evaluation of climate models. The knowledge gained will improve the representation of the key processes in global models of the atmospheric sulfur cycle and lead to more robust simulations of the impacts of changes in stratospheric sulfur aerosol levels. Furthermore, this working group aims to contribute to the development of novel parameterizations of the interplay of the various processes transporting sulfur to the stratosphere and redistributing sulfur and aerosol within the stratosphere to be implemented in climate models.

4.3. Activities

4.3.1. Develop realistic chemical, microphysical and sedimentation schemes for implementation of a stratospheric aerosol layer in detailed Lagrangian Chemistry Transport Models (CTMs).

The fully Lagrangian chemistry transport model (CTM) ATLAS with all chemical and physical processes relevant to the transport of sulfur to the stratosphere and its conversion to sulfate aerosol particles will be upgraded to :

1) Include the gas phase breakdown of OCS and DMS, the gas phase conversion of SO2 into SO3 and the photolysis of H2SO4, OCS and SO2 into the chemical module of ATLAS.

2) Include the heterogeneous processes for SO2, its uptake into droplets and on ice and its liquid phase / ice phase conversion into sulfate.

3) Implement a microphysical scheme for aerosol nucleation, growth and evaporation and sedimentation.

These schemes will be integrated into ATLAS, resulting in AerATLAS, a version of ATLAS with free running stratospheric aerosol layer and atmospheric sulfur chemistry. At this point AerATLAS will be tested and refined based on short test intergrations.

4.3.2. Tune free parameters based on in-situ and satellite observations of aerosol properties and sulfur containing species. Validate the schemes based on these observations and test hypotheses concerning the sources and processing of atmospheric sulfur compounds sustaining the stratospheric aerosol layer.

AerATLAS will be used in the full global CTM mode to simulate the evolution of the stratospheric aerosol layer for background conditions (i.e. periods without significant volcanic activity), for a Pinatubo type volcanic eruption and for a scenario with SO2 release into the tropical upper troposphere by moderate volcanic activity (Vernier et al, 2011). The strength of these volcanic sources as well as the emission scenarios for SO2 and OCS will be varied and the model results will be compared to observations Ż satellite data products and, where available, in-situ data. Based on these studies uncertain process parameterisations as well as the assumptions made with respect to the different sources will be refined and our understanding of the various processes that sustain the stratospheric aerosol layer and their relative importance will improve. Based on the improved understanding and with targeted model runs for future emission scenarios and potential changes of the aerosol layer under climate change and the potential impact of variability of volcanic activity will be addressed.

Development of a global inventory of emissions of sulfur containing species and SO2 emission source catalogue

Develop a global emission sources catalogue of sulfur containing compounds based on observations (working group 1 & 3). A śprototype∆ including SO2 measurements from the Ozone Monitoring Instrument (OMI) satellite already exists (Fioletov et al., 2011). The next step now is to make the catalogue available which will take up to 3 months.

4.3.4. Volcanic Ash Strategic Initiative Team (VAST)

The European Space Agency has recently funded the VASTproject (started in 2012), which will improve satellite products and improve inversion methods to derive ash and SO2 vertical emission information from the satellite data. The project is targeted towards improving operational procedures used for volcanic ash warning for aviation, but it will deliver emission information for a number of volcanic eruptions, which will be of interest for SSiRC. Extension of these activities to cover a selection of large eruptions that had a significant effect on stratospheric aerosol loadings would be desirable and could provide valuable input to hindcasting climate effects of volcanic eruptions.

4.3.5. Passive tracer studies

Data from the PERTRAS experiment (WP1) will be used to evaluate model simulations of deep convection as well as convection parameterizations used in global climate models. This data in combination with measurements of the concentrations of soluble species will be used to infer information about their vertical transport.

4.3.6. Laboratory studies of aersol process in an aerosol chamber

The CLOUD project at CERN performs process studies on various gas phase and aerosol phase chemical and microphysical processes that feed directly into the implementation objectives of SSiRC. The CLOUD facility provides a very clean aerosol chamber that can be operated over the full stratospheric and tropospheric temperature range (+40 to -80°C). Binary sulfuric acid-water aerosol nucleation (as well as various ternary an multi-component systems), aerosol growth and evaporation of aerosol particles, gas phase reactions and heterogeneous chemistry of relevance for the stratospheric sulfur cycle and aerosol layer. Studies on cirrus cloud activation and trace gas uptake are planned. A special focus of the CLOUD research activities involves the quantification of influences from Galactic Cosmic Rays on aerosol nucleation processes.

Recently, new pathways for the production of gaseous sulfuric acid have been proposed, besides the well-known reaction of SO2 with OH. Especially the reaction with stabilized Criegee intermediates linked to chemistry of alkenes from biogenic sources seems to be an important process in the continental boundary layer. It is unclear if this process also plays any role in the upper troposphere. Similarly, other pathways, for example, ionizing radiation could enable the production of gaseous sulfuric acid from SO2. These reaction processes are explored at various laboratories (U Helsinki, MPI-Chemistry Mainz, FZ Jülich, CLOUD at CERN, etc.) in order to characterize and quantify these processes. Complementing the in-situ measurements, process model ing will be conducted to assess the relative importance of these processes for sulfur conversion in the TTL and LS.

4.3.7. Nature, Origin and Climate impact of the Asian Tropopause Aerosol Layer

For more than a decade, unprecedented economic growth in South Asia has led to the release of large amounts of pollutants up to the upper troposphere and lower stratosphere (UTLS) (e.g. CO, HCN) (Randel et al., 2010). Here, we propose to investigate the occurrence of aerosols in the UTLS from polluted outflow of deep convective systems associated with the Southeast Asian Monsoon that can affect stratospheric chemistry and global climate. In particular, we will investigate the nature, origin and climate impact of the Asian Tropopause Layer (ATAL) (Vernier et al., 2011) by combining NASA satellite data sets and modeling tools to address the following 5 Science objectives :

1) Continue and improve the detection, classification and spatio-temporal characterization of ATAL using observations from the CALIOP lidar.

2) Investigate ATAL bulk aerosol properties (size distribution modes) and trend over 25 years with a combination of satellites including CALIOP, SAGE II, HALOE, GOMOS and others.

3) Determine ATALs composition with aircraft in-situ measurements and global aerosol modeling tools (GOCART, GEOS-Chem).

4) Investigate ATALs origin with a combination of trajectory analysis, observations of deep convection from Geostationary satellites and microphysical models.

5) Assess radiative and climate impacts of ATAL using radiative transfer models and global climate model (GEOS-5 GCM)

4.4. Gaps in current research plan