Ground-based lidars have been used since the early 1970s to quantitatively monitor the stratospheric aerosol layer. In some cases they have measured plumes from volcanic eruptions in remote locations on earth before the eruptions were identified. They provide a relatively inexpensive method to regularly measure the stratospheric layer from a given site with high altitude resolution (tens or hundreds of meters) and time resolution (tens of minutes). The fundamental quantity measured is backscattered light. Multiple laser wavelengths can be used to infer particle size and polarization effects can used to infer non-spherical particle shape.
Long-term monitoring of the stratospheric layer by lidars has been very valuable in tracking the dispersion and decay of eruption plumes. The same is true for smoke injected into the stratosphere by large forest fires. Each event is a potential natural experiment to compare the measured aerosol with model calculations of transport and evolution of the aerosol. The long-term records have also been used to characterize transport effects such as the quasibiennial oscillation (QBO) and tropospheric-stratospheric exchange.
Besides long-term monitoring, ground-based lidar measurements can complement the SSiRC objectives by providing aerosol profiles before, during, and after campaigns. Balloon and aircraft measurements allow far more detailed measurements of aerosols than the lidar, but coverage in time provided by the lidar can often be very useful in interpreting the cloud and aerosol conditions present during the flights.
There are many lidars in operation around the world with most in the northern hemisphere. Many participate in networks, some of which are listed below:
WMO the Global Atmosphere Watch Aerosol Lidar Observation Network (GALION) at global scale (http://www.wmo.int/gaw/galion). GALION is based on the cooperation between existing lidar networks:
The American Lidar Network (ALINE), Latin America;
Asian Dust and Aerosol Lidar Observation Network (AD-Net), East Asia;
CIS-LINET, Commonwealth of Independent States (Belarus, Russia and Kyrgyz Republic) LIdar NETwork;
The Canadian Operational Research Aerosol Lidar Network (CORALNet), Canada;
The European Aerosol Research LIdar NETwork (EARLINET), Europe; 27 lidar stations distributed over Europe which provides information about horizontal, vertical, and temporal distribution of aerosols on a continental scale since 2000 (mostly tropospheric data but also some upper tropospheric and stratospheric profiles)
Network for the Detection of Atmospheric Composition Change (NDACC); lidar stations at Garmisch-Partenkirchen (Germany), Mauna Loa (USA), Boulder (USA), Lauder (NZ), Table Mountain Facility (USA), OHP (France), NyAlesund (Norway, Germany) and many other sites..
REALM/CREST, Eastern North America (global);
The global MicroPulse Lidar NETwork (MPLNET).
Of these, the Network for the Detection of Atmospheric Composition Change (NDACC), formerly the Network for the Detection of Stratospheric Change, has historically made the most contribution to stratospheric aerosol research. The lidar programs (aerosol, ozone, and temperature) have participated in calibration and validation activities specifically to promote reliable and accurate measurements. The data is readily available in Ames format files from primary and complementary sites around the world. The instruments tend to be funded for regular monitoring, but might need additional funds to support an intensive campaign.
The other networks tend to be aimed more towards tropospheric aerosols, but many would easily retrieve eruption plumes in the stratosphere. The funding and schedule of observations vary widely for the various lidar groups.
The NASA CALIOP space-based lidar is currently being used to compare NDACC lidars at locations around the world. This will lead to more consistant datasets that can be used to quantitatively compare aerosol loading in the stratosphere.
Suitability of lidar systems for stratosphere:
Most lidar programs are directed towards tropospheric and boundary layer aerosols. Measurement of stratospheric aerosols use the same techniques and analysis, but the much lower background levels require careful characterization of the lidar system. The most common issues are saturation of the detectors at high count rates and signal-induced noise. The lidar signal must have a linear response over many orders of magnitude. A side-by-side comparison of lidars is ideal to quantify these instrument issues, but many stratospheric aerosol lidars are not easily portable. A promising possibility is to compare individual ground-based lidars with a space-based lidar such as the currently operating CALIOP.
Common analysis methods:
The aerosol retrieval from the signal requires a molecular density profile with altitude. This often comes from a model, radiosonde or combination of sources. It is also possible for the lidar to measure this profile directly using the weaker Raman-shifted light. An altitude range, usually above the stratospheric layer, is assumed to be free of aerosol and that the signal is due to pure molecular-scattered light. Variations in the molecular profiles used, and in the altitude ranges used for normalization, can result in different retrieved aerosol profiles. Comparison with a common instrument, such CALIOP, would also sort out possible differences in the aerosol analysis.
Conversion of lidar backscatter to extinction:
The fundamental measurement of the lidar is back-scattered light, but this is often not the quantity that is desired. Aerosol extinction or total scatter is needed for radiative transfer calculations. The ratio of extinction to back-scattered light, often called the lidar ratio, usually has to be assumed for converting the back-scattered light to extinction. The lidar ratio has been calculated as well as inferred from measurements, but uncertainty in this ratio generally introduces the largest error in lidar extinction. A direct measurement of this ratio is possible with a lidar by carefully measuring extinction in a Raman signal, but this requires higher aerosol loading than usually found in the stratosphere. Measurements of this ratio and its natural variations could be aided with balloon backscatter sondes. In-situ measurements of the full aerosol phase function would be very useful in characterizing the light scattered from aerosols, but the instrument (polar nephelometer) is not currently available.
Coverage of tropics and southern hemisphere
A very important region of the earth, in the study of stratospheric aerosols, is the western pacific. Lidar measurements in this area have mainly been made during campaigns, but more regular observations in both the troposphere and stratosphere would be very useful. Better coverage of the southern hemisphere would complement the many operating lidars in the northern hemisphere.