3. Satellite remote sensing

3.1. Background, motivation and state of the science

3.1.1. Aerosol

The sources of stratospheric aerosol include OCS and SO2 from a variety of natural and human-derived sources. Major volcanic eruptions can increase the aerosol total loading by several orders of magnitude and this large dynamic range makes measurements of stratospheric aerosol properties challenging. Most climate and chemistry models prescribe aerosol properties and their subsequent radiative effects which are derived from a variety of space-based sources. For instance, a number of on-going SPARC activities including CCMVAL and SDI make use of climatologies of space-based aerosol measurements. The favorable aspects of these data sets include near-global spatial and temporal coverage with primary stratospheric aerosol properties such as extinction coefficient as well as the ability to infer some bulk aerosol properties (e.g., Surface Area Density).

Since 1979 and prior to the early 2000s, virtually all space-based measurements of stratospheric aerosol were based on the solar occultation technique, including members of the SAGE-series and HALOE. Since 2002, limb scatter (OSIRIS, SCIAMACHY), stellar occultation (GOMOS) and backscatter lidar (CALIPSO) techniques have been employed to measure aerosol properties from space. Since 2005, the stratospheric record has entirely been dependent on the new methods as no solar occultation instruments remained operational. As a result, the inference of underlying aerosol properties from any of these data sets alone or in combination remains uncertain and residual uncertainties are not well understood. Beyond the simple change from instrument to instrument, the challenges presented by the change in measurement strategies present a significant challenge to maintaining homogeneity of the aerosol record from which long-term changes including possible human-derived change can be inferred.

3.2.SO2 and other stratospheric aerosol precursors

The history of SO2 observations from space is similar to aerosol measurements with routine observations taking place from 1979 to the present with other short gaps in a more or less continuous data record. The SO2 measurement record is primarily based on UV backscatter measurements including BUV, TOMS, the GOME series, SCIMACHY, and OMI and contributions by infrared emission measurements by from TOVS, AIRS and IASI. Most often these measurements are restricted to column measurements except some case-by-case studies in which vertical profile inferences are made. One exception is recent work by Höpfner et al., in which profile measurements of SO2 for much of the stratosphere can be inferred from MIPAS observations. While these measurements require substantial averaging to reduce measurement noise, the results are nonetheless encouraging. In any case, the scale of direct stratospheric injection of SO2 by volcanoes is not clear particularly for smaller eruptions. Measurements of OCS made by instruments like MIPAS and ACE FTS seem well in hand and no significant issue related to the measurement of OCS is apparent. Like aerosol, the homogeneity of a long-term SO2 data set is dependent on understanding the quality and character of measurements made by different instruments that vary in detail if not in overall technique and which may or may not have adequate overlap to understand differences in the SO2 record. The uncertainty associated with the homogeneity of the SO2 record represents a risk to the understanding the long-term role of SO2 in aerosol formation.

3.3. The role of the Satellite Measurement work package to the overall goals of SSIRC

This work package will facilitate on-going activities related to improving data sets, improving data set to data set homogeneity, improved understanding of the inference of aerosol bulk properties from these measurements, improved SO2 stratospheric injections and generally improve aerosol climatological data sets available as inputs to climate models.

3.4. Activities

3.4.1. Produce updated versions of stratospheric aerosol measurements for on-going experiments

On-going experiments (where measurements continue to be made) include CALIPSO and OMPS. CALIPSO is currently in production of a new version that will mitigate some calibration issues noted in past versions and, for the first time, produce a formal stratospheric backscatter product. OMPS has produced preliminary aerosol products but is dependent on a proposal currently in review to support further improvements and validation. ACE Imager and MAESTRO are currently producing aerosol products but at this time neither is fully usable and further work is required before they are fully accessible to scientific applications. This is occurring as a part of on-going ACE/MAESTRO operations.

3.4.2. Provide new, publically available aerosol products up-coming experiments

Over the next few years, several new instruments will begin operations which aimed in part at stratospheric aerosol and its precursors. The Cloud-Aerosol Transport System (CATS) is a three wavelength (1064, 532, 355 nm) elastic backscatter lidar with HSRL capability at 532 nm. Depolarization measurements will be made at all wavelengths. As a demonstration experiment, the degree to which this data will be available and when this will occur is not clear at this time. Nonetheless, it is a new and potentially powerful method to measure stratospheric aerosol. A SAGE III instrument virtually identical to the SAGE III/Meteor 3M instrument will fly aboard the International Space Station (ISS) beginning in 2014 and provide 9 channels of aerosol extinction coefficient measurements from the UV to the NIR for latitudes between 70°S and 70°N. EarthCARE/Atmospheric Lidar (ATLID) uses high spectral resolution lidar at 355 nm to measure aerosol properties. While this mission is, like CALIPSO, aimed primarily at boundary aerosol properties, a stratospheric aerosol product will be produced. The Tropospheric Monitoring Instrument that will fly aboard the Sentinel 5-precursor in 2015 will measure SO2 at very high horizontal resolution (7.5 km by 7.5 km) and, for the first time, produce a standard SO2 layer product (though I have no idea what resolution).

3.4.3. Produce updated versions of historical data sets and improved evaluations of existing aerosol-related products

The SAGE series of instrument is currently undergoing a major revision in which the data sets, for the first time, will be reanalyzed using common analysis tools and auxiliary data (e.g., MERRA temperature and pressure analysis). The results will be tied by common version numbers to facilitate cross platform usage. SAGE I and SAM II have not gone through a significant revision since the mid-1980s. Another on-going reanalysis project is associated with ESA-SPARC SPIN project which, among other activities, will result in a significant improvement to the publically released OSIRIS version and the first publically released version of SCIAMACHY limb-scatter aerosol extinction coefficient product. Given the spatial/temporal coverage provided by limb-scatter measurements, new avenues of scientific research will be enabled by these new data sets.

3.4.4. Produce improved aerosol extinction coefficient measurements from GOMOS through ESA Aerosol_CCI project.

The GOMOS stellar occultation instrument is undergoing a major revision as a part of the AerGOM project. Initial results suggest a substantial improvement of the aerosol product with multiple wavelengths of aerosol extinction coefficient available for the first time.

3.4.5.Produce long-term data sets for aerosol and SO2 using a diverse instrument set

• Submitted: October 2012, notification: March 2013

Projects have been proposed to improve the homogenization of aerosol and SO2 long-term data sets. One effort proposes to assemble diverse sources of stratospheric aerosol measurements, primarily from satellites, in order to develop a continuous and as consistent as possible stratospheric aerosol data base for the period from 1979 through the end of the proposal period. It also proposes to develop tools to provide for straightforward sustainability of the data set in years beyond direct funding. The climatological data set will include multiple optical and bulk aerosol products and realistic errors estimates for all parameters. Another project proposes to enhance and archive the long-term SO2 Earth System Data Record (ESDR) combining solar backscatter Ultraviolet (BUV), thermal infrared (IR) and limb microwave (MLS) measurements from past and present satellites (1978-2017). The ESDR will include measured SO2 masses as well as estimates of SO2 emissions, duration and height for volcanic eruptions and anthropogenic point sources for use in climate and air quality models. These proposals were submitted in October 2012 and expect funding beginning in the next calendar year. In both cases, the proposed products will be archived and publically available.

3.5. Gaps in current research plan

The HALOE aerosol data (and other products) are in desperate need of reanalysis. The current HALOE aerosol products are dependent on out-of-date auxiliary products (primarily gas species not measured by HALOE but required to estimate aerosol extinction). It is not clear that further developments with this data set are possible. This problem, perhaps contrasted to continued NASA support for the SAGE II data set, highlights the need for the funding agencies to continue even some support to maintain and improve the historical data sets which continue to be the backbone of trend analysis. It is also worth noting that we are unaware of any plans to measure aersol extinction coefficient in the infrared (at wavelengths above 2.8 microns where sulfate absorbs).

The robust estimation of direct SO2 injections into the stratosphere from volcanic eruptions has been demonstrated by Prata et al. (2012). A general application of this process (or similar) to the historical data set would be highly valuable.

A proposal has been submitted to NASA that will facilitate the product or an operational aerosol product. As previously noted, it is not clear that under current funding levels that a usable data product will be produced. OMPS is by far the youngest of instruments currently capable of producing stratospheric data products and the likelihood of platform or instrument failure for the remaining instruments prior to EarthCARE and SAGE III/ISS deployment is significant. As a result the urgency attached to funding for this project is high.