John E. Barnes

Increase in background stratospheric aerosol observed with lidar at Mauna Loa Observatory and Boulder, Colorado

The stratospheric aerosol layer has been monitored with lidars at Mauna Loa Observatory in Hawaii and Boulder in Colorado since 1975 and 2000, respectively. Following the Pinatubo volcanic eruption in June 1991, the global stratosphere has not been perturbed by a major volcanic eruption providing an unprecedented opportunity to study the background aerosol. Since about 2000, an increase of 4-7% per year in the aerosol backscatter in the altitude range 20-30 km has been detected at both Mauna Loa and Boulder. This increase is superimposed on a seasonal cycle with a winter maximum that is modulated by the quasi-biennial oscillation (QBO) in tropical winds. Of the three major causes for a stratospheric aerosol increase: volcanic emissions to the stratosphere, increased tropical upwelling, and an increase in anthropogenic sulfur gas emissions in the troposphere, it appears that a large increase in coal burning since 2002, mainly in China, is the likely source of sulfur dioxide that ultimately ends up as the sulfate aerosol responsible for the increased backscatter from the stratospheric aerosol layer. The results are consistent with 0.6-0.8% of tropospheric sulfur entering the stratosphere.

Total volcanic stratospheric aerosol optical depths and implications for global climate change

Understanding the cooling effect of recent volcanoes is of particular interest in the context of the post-2000 slowing of the rate of global warming. Satellite observations of aerosol optical depth above 15 km have demonstrated that small-magnitude volcanic eruptions substantially perturb incoming solar radiation. Here we use lidar, Aerosol Robotic Network, and balloon-borne observations to provide evidence that currently available satellite databases neglect substantial amounts of volcanic aerosol between the tropopause and 15 km at middle to high latitudes and therefore underestimate total radiative forcing resulting from the recent eruptions. Incorporating these estimates into a simple climate model, we determine the global volcanic aerosol forcing since 2000 to be −0.19 ± 0.09 Wm−2. This translates into an estimated global cooling of 0.05 to 0.12°C. We conclude that recent volcanic events are responsible for more post-2000 cooling than is implied by satellite databases that neglect volcanic aerosol effects below 15 km.

Lidar measurements of stratospheric aerosol over Mauna Loa Observatory

Dual-wavelength aerosol lidar backscatter measurements at Mauna Loa Observatory are used to monitor and characterize the 15–30 km stratospheric aerosol layer. The decay of aerosol loading following the El Chichon, Mexico (17°N) and Pinatubo, Philippine Islands (15°N) volcanic eruptions of 1982 and 1991, respectively, depends on the phase of the quasibiennial oscillation (QBO) in tropical stratospheric winds. Averaged over a 3-year period, these effects are removed and an exponential decay with a characteristic (e−1) decay time of about 1 year is observed for both eruptions. By the end of 1996, about 5 ½ years after the Pinatubo eruption, stratospheric aerosol levels at Mauna Loa had decayed to pre-eruption levels, approximately matching the lowest aerosol levels seen here in the past 17 years (about 6 × 10−5 sr−1 at 694 nm integrated between 15.8 and 33 km). However, this background stratospheric aerosol level at Mauna Loa may depend on the QBO, being slightly lower during the westerly phase. Analyses of aerosol backscatter, backscatter wavelength dependence, and trajectories provide evidence for a minor injection of aerosol from the Rabaul eruption in Papua, New Guinea (4°S) in September of 1994.

Stratospheric Aerosol--Observations, Processes, and Impact on Climate

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 nonsulfate matter 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.

The Behavior of the Snow White Chilled-Mirror Hygrometer in Extremely Dry Conditions

The Snow White hygrometer, made by Meteolabor AG, Switzerland, is a new chilled-mirror instrument using a thermoelectric Peltier cooler to measure atmospheric water vapor. Its performance under dry conditions is evaluated in simultaneous measurements using the NOAA/CMDL frost-point hygrometer at Boulder, Colorado; San Cristobal, Galapagos Islands, Ecuador; Watukosek, Indonesia; and Mauna Loa Observatory, Hawaii. The Snow White exhibits a lower detection limit of about 3%-6% relative humidity, depending on the sensor configuration. This detection limit is determined by the temperature depression attainable by the thermoelectric cooler. In some cases, loss of frost-point control within layers with relative humidity below this detection limit caused inaccurate measurements above these dry layers, where the relative humidity was within the detection range of the instrument. The sensor does not operate in the stratosphere because of the large frost-point depression and the large potential for outgassing of water from the instrument box and the sensor housing. The instrument has some capabilities in the tropical tropopause region; however, the results are somewhat mixed.

Variability in the stratospheric background aerosol over Mauna Loa Observatory

The stratospheric aerosol layer above Mauna Loa Observatory (MLO), Hawaii, has been at low background levels for the past 5 years. This is the first time that an extended non-volcanic background aerosol period has been observed since modern measurements began in the early 1960s. Lidar backscatter at 532 nm shows a distinct maximum in winter and minimum in summer. The five annual cycles have included three easterly phases and two westerly phases of the quasibiennial oscillation (QBO). Differences in aerosol backscatter versus altitude profiles are seen for different QBO phases. There is also a switching of about 25% in the magnitude of the aerosol backscatter on a weekly time scale with varying particle size derived from multiwavelength data. Assumption of a tropical particle source at background suggests that the differing particle regimes are tropical and midlatitude.

Stratospheric AOD after the 2011 eruption of Nabro volcano measured by lidars over the Northern Hemisphere

Nabro volcano (13.37°N, 41.70°E) in Eritrea erupted on 13 June 2011 generating a layer of sulfate aerosols that persisted in the stratosphere for months. For the first time we report on ground-based lidar observations of the same event from every continent in the Northern Hemisphere, taking advantage of the synergy between global lidar networks such as EARLINET, MPLNET and NDACC with independent lidar groups and satellite CALIPSO to track the evolution of the stratospheric aerosol layer in various parts of the globe. The globally averaged aerosol optical depth (AOD) due to the stratospheric volcanic aerosol layers was of the order of 0.018 ± 0.009 at 532 nm, ranging from 0.003 to 0.04. Compared to the total column AOD from the available collocated AERONET stations, the stratospheric contribution varied from 2% to 23% at 532 nm.

Atmospheric aerosol profiling with a bistatic imaging lidar system

Atmospheric aerosols have been profiled using a simple, imaging, bistatic lidar system. A vertical laser beam is imaged onto a charge-coupled-device camera from the ground to the zenith with a wide-angle lens (CLidar). The altitudes are derived geometrically from the position of the camera and laser with submeter resolution near the ground. The system requires no overlap correction needed in monostatic lidar systems and needs a much smaller dynamic range. Nighttime measurements of both molecular and aerosol scattering were made at Mauna Loa Observatory. The CLidar aerosol total scatter compares very well with a nephelometer measuring at 10 m above the ground. The results build on earlier work that compared purely molecular scattered light to theory, and detail instrument improvements.

Boundary layer scattering measurements with a charge-coupled device camera lidar

A CCD-based bistatic lidar (CLidar) system has been developed and constructed to measure scattering in the atmospheric boundary layer. The system uses a CCD camera, wide-angle optics, and a laser. Imaging a vertical laser beam from the side allows high-altitude resolution in the boundary layer all the way to the ground. The dynamic range needed for the molecular signal is several orders of magnitude in the standard monostatic method, but only approximately 1 order of magnitude with the CLidar method. Other advantages of the Clidar method include low cost and simplicity. Observations at Mauna Loa Observatory, Hawaii, show excellent agreement with the modeled molecular-scattering signal. The scattering depends on angle (altitude) and the polarization plane of the laser.

Spatial and temporal variability of the stratospheric aerosol cloud produced by the 1991 Mount Pinatubo eruption

Received 25 April 2003; revised 7 July 2003; accepted 22 July 2003; published 17 October 2003. (1) As a critical quality control step toward producing a stratospheric data assimilation system for volcanic aerosols, we conducted a comparison between Stratosphere Aerosol and Gas Experiment (SAGE) II aerosol extinction profiles and aerosol backscatter measured by five lidars, both in the tropics and midlatitudes, for the two-year period following the 1991 Mt. Pinatubo eruption. The period we studied is the most challenging for the SAGE II retrieval because the aerosol cloud caused so much extinction of the solar signal that in the tropics few retrievals were possible in the core of the cloud. We compared extinction at two wavelengths at the same time that we tested two sets of conversions coefficients. We used both Thomason and Jagers extinction-to-backscatter conversion coefficients for converting lidar backscatter profiles at 0.532 mm or 0.694 mm wavelengths to the SAGE II extinction wavelengths of 0.525 mm and 1.020 m mo r the nearby ones of 0.532 mm and 1.064 mm respectively. The lidars were located at Mauna Loa, Hawaii (19.5� N, 155.6� W), Camaguey, Cuba (21.4� N, 77.9� W), Hefei, China (31.9� N, 117.2� W), Hampton Virginia (37.1� N, 76.3� W), and Haute Provence, France (43.9� N, 5.7� W). For the six months following the eruption the aerosol cloud was much more heterogeneous than later. Using two alternative approaches, we evaluated the aerosol extinction variability of the tropical core of the Pinatubo stratospheric aerosol cloud at the timescale of 1-2 days, and found it was quite large. Aerosol variability played the major role in producing the observed differences between SAGE II and the lidars. There was in general a good agreement between SAGE II extinction measurements and lidar derived extinction, and we conclude that all five lidar sets we compared can be used in a future data assimilation of stratospheric aerosols. This is the most comprehensive comparison yet of lidar data with satellite data for the Pinatubo period. INDEX TERMS: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 0394 Atmospheric Composition and Structure: Instruments and techniques; 0370 Atmospheric Composition and Structure: Volcanic effects (8409); 3360 Meteorology and Atmospheric Dynamics: Remote sensing; KEYWORDS: volcano, lidar, satellite