Andreas Herber

Aerosols in polar regions: A historical overview based on optical depth and in situ observations

Large sets of filtered actinometer, filtered pyrheliometer and Sun photometer measurements have been carried out over the past 30 years by various groups at different Arctic and Antarctic sites and ...

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.

Synoptic airborne thickness surveys reveal state of Arctic sea ice cover

[1] While summer Arctic sea-ice extent has decreased over the past three decades, it is subject to large interannual and regional variations. Methodological challenges in measuring ice thickness continue to hamper our understanding of the response of the ice-thickness distribution to recent change, limiting the ability to forecast sea-ice change over the next decade. We present results from a 2400 km long pan-Arctic airborne electromagnetic (EM) ice thickness survey in April 2009, the first-ever large-scale EM thickness dataset obtained by fixed-wing aircraft over key regions of old ice in the Arctic Ocean between Svalbard and Alaska. The data provide detailed insight into ice thickness distributions characteristic for the different regions. Comparison with previous EM surveys shows that modal thicknesses of old ice had changed little since 2007, and remained within the expected range of natural variability.

European pollution outbreaks during ACE 2: Optical particle properties inferred from multiwavelength lidar and star-Sun photometry

(1) On the basis of multiwavelength backscatter and 532-nm extinction profiling with lidar at Sagres (37� N, 9� W), southern Portugal, and optical depth observations with a star photometer at the lidar site and a Sun photometer atop a nearby mountain, several European pollution outbreaks were characterized during the Second Aerosol Characterization Experiment (ACE 2) in the summer of 1997. A sophisticated analysis scheme applied to the lidar-photometer data set is described. The observations are mainly presented in terms of profiles of the 532-nm extinction-to-backscatter ratio (lidar ratio) and of A u ngstrom exponents calculated for the wavelength ranges 400-532 nm and 532-800 nm. The lidar ratio indicates the aerosol type (marine, soil, pollution) whereas the A u ngstrom exponents are sensitive to changes in the particle size distribution (accumulation mode, coarse mode). Results of an extensive correlation analysis considering all determined optical parameters, relative humidity, and measurement height are discussed. Finally, the spectrally resolved optical depth and the column A u ngstrom exponents for the lofted outbreak plumes determined from the lidar profiles are compared with respective values derived from the star and Sun photometer measurements. INDEX TERMS: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 0345 Atmospheric Composition and Structure: Pollution— urban and regional (0305); 0365 Atmospheric Composition and Structure: Troposphere—composition and chemistry; 0368 Atmospheric Composition and Structure: Troposphere—constituent transport and chemistry; 1630 Global Change: Impact phenomena; KEYWORDS: multiwavelength lidar, star photometer, Sun photometer, anthropogenic aerosols, aerosol optical properties

European pollution outbreaks during ACE 2 : Lofted aerosol plumes observed with Raman lidar at the Portuguese coast

During the Second Aerosol Characterization Experiment (ACE 2) in the summer of 1997, four pollution outbreaks from the European continent were monitored at Sagres (37°N, 9°W), Portugal, by applying the Raman lidar technique at a wavelength of 532 nm. The findings are presented in terms of vertical profiles of the volume extinction coefficient of the particles, the backscatter coefficient, the extinction-to-backscatter ratio (lidar ratio), the relative humidity, and the geometrical and optical depths of the marine boundary layer and the continental aerosol layers aloft. The lidar-derived optical depths are compared with results of simultaneously conducted star photometer observations. During all outbreaks, similar aerosol profiles were observed. Above the cold marine boundary layer the warm continental pollution plume extended from about 500 to a maximum height of 3500 m. Strong temperature inversions prohibited mixing of the two layers. The ACE 2 mean optical depths of the marine boundary layer and the aerosol layers aloft were about 0.05 and 0.1, respectively. Maximum values of the optical depth in the pollution plumes were close to 0.25 at 532 nm. On average, the free-tropospheric layers contributed about 60% to the total optical depth at the coast. Volume extinction coefficients were mainly between 30 and 70 Mm−1, and the lidar ratios ranged from 30 to 80 sr in the polluted continental air.

Arctic Study of Tropospheric Aerosol and Radiation (ASTAR) 2000: Arctic haze case study

The ASTAR 2000 (Arctic Study of Tropospheric Aerosol and Radiation) campaign ran from 12 March until 25 April 2000 with extensive flight operations in the vicinity of Svalbard (Norway) from Longyearbyen airport (78.25°N, 15.49°E). It was a joint Japanese (NIPR Tokyo)–German (AWI Bremerhaven/Potsdam) airborne measurement campaign using AWI aircraft POLAR 4 (Dornier 228-101). Simultaneous ground-based measurements were done at the international research site Ny-Ålesund (78.95°N, 11.93°E) in Svalbard, at the German Koldewey station, at the Japanese Rabben station and at the Scandinavian station at Zeppelin Mountain (475 m above sea level). During the campaign 19 profiles of various aerosol properties were measured. In general, the Arctic spring aerosol in the vicinity of Svalbard had significant temporal and vertical variability. A strong haze event occurred between 21 and 25 March in which the optical depth from ground-based observation was 0.18, which was significantly greater than the background value of 0.06. Airborne measurements on 23 March during this haze event showed a high aerosol layer with an extinction coefficient of 0.03 km−1 or more up to 3 km and a scattering coefficient from 0.02 in the same altitude range. From the chemical analyses of airborne measurements, sulfate, soot and sea salt particles were dominant, and there was a high mixing ratio of external soot particles in some layers during the haze event, whereas internal mixing of soot in sulfate was noticeable in some layers for the background condition. We argue that the high aerosol loading is due to direct transport from anthropogenic source regions. In this paper we focus on the course of the haze event in detail through analyses of the airborne and ground-based results.

Volcanic perturbation of the atmosphere in both polar regions: 1991–1994

Long-term measurements by sunphotometers of the spectral dependence of aerosol optical depth are reported for several sites in the Arctic and Antarctic for the period January 1991 through December 1994. In the Antarctic a pronounced increase of atmospheric turbidity was observed at the end of September 1991. The observed wavelength dependence in aerosol optical depth indicated that the increase was due to the presence of fresh and therefore small stratospheric aerosol particles associated with the eruption of Cerro Hudson in August 1991. After the breakdown of the polar vortex in mid-November 1991 we measured a second significant increase of the aerosol optical depth. At this time the 1.0-μm aerosol optical depth was approximately 0.12 or about 10 times background levels. This second incrcase is shown to be the result of the influx of Mount Pinatubo aerosols. A similar perturbation of the aerosol optical depth was observed in the Arctic with the return of sunlight in March 1992. However, the increased loading of the Arctic stratosphere by the Pinatubo aerosols was already evident at high northern latitudes in satellite measurements at the end of 1991. Stratospheric Aerosol and Gas Experiment II stratospheric 1.0-μm optical depth measurements show that meridional transport of Pinatubo aerosol from equatorial to middle and higher latitudes is greatest in the winter/spring hemisphere. This observation explains the observed seasonal trend of aerosol optical depth during the posteruption. A significant decrease of the perturbation by Mount Pinatubo aerosol was observed in both polar regions by the end of 1994. The measured 1.0-μm aerosol optical depths at this time were only 0.04 ; these values exceed the background level by about 0.01-0.02. Therefore the aerosol optical depth values are still slightly higher than during undisturbed conditions. In addition, we show that the occurrence of volcanic aerosols caused changes in the spectral dependence of the aerosol optical depth in the Arctic and the Antarctic. These variations, including the changes in the aerosol size distribution, derived from the aerosol optical depth, are discussed in comparison to undisturbed conditions.

Comparison of trends in the tropospheric and stratospheric aerosol optical depths in the Antarctic

Temporal variations of the aerosol optical depth of the Antarctic troposphere and stratosphere are considered on the basis of long-term Sun photometer and actinometer measurements which have been made at Mirny and Georg Forster stations since 1956 and 1988, respectively. This data is supplemented by measurements of the stratospheric aerosol optical depth by the satellite-borne stratospheric aerosol measurement II instrument. These observations indicate that under undisturbed conditions, the stratospheric aerosol optical depth represents approximately 25% of the total atmospheric aerosol optical depth. The aerosol optical depth in the Antarctic is most notably affected by volcanic eruptions, such as El Chichon in 1982 and Mount Pinatubo and Cerro Hudson in 1991, and by the occurrence of polar stratospheric clouds during Antarctic winter and spring. Apart from these episodic events, no long-term trend in the aerosol optical depth can be discerned from the nearly 40-year record.

In-situ airborne observations of the microphysical properties of the Arctic tropospheric aerosol during late spring and summer

In-situ aerosol data collected in the Arctic troposphere during a three-week period in 2004 were analysed. The measurements took place during late spring, i.e., at the time of the year when the characteristics of the aerosol distribution change from being accumulation-mode dominated to being primarily of the Aitken-mode type, a process that previously has been observed in the boundary layer. To address the question whether this transition is also detectable in the free troposphere of an aircraft-measured data from the ASTAR 2004 campaign were analysed. In this study, we present vertically as well as temporally results from both ground-based and airborne measurements of the total number concentrations of particles larger than 10 and 260 nm. Aircraft-measured size distributions of the aerosol ranging from 20 to 2200 nm have been evaluated with regard to conditions in the boundary layer as well as in the free troposphere. Furthermore an analysis of the volatile fraction of the aerosol population has been performed both for the integrated and size-distributed results. From these investigations we find that the transition takes place in the entire troposphere.

Vertical distribution of the spectral aerosol optical depth in the Arctic from 1993 to 1996

During the period from summer 1993 to spring 1996, 51 profiles of spectral aerosol optical depth were measured with airborne Sun photometers throughout the Arctic. This period was influenced by volcanic aerosols after the Pinatubo eruption and the removal of volcanic material from the stratosphere into the troposphere. By 1996, stratospheric aerosol concentration has decreased to pre-Pinatubo levels. Mean values of aerosol optical depth have changed during the period from 1993 to 1996 from 0.09 to 0.02 at 403 nm and from 0.065 to 0.01 at 1041 nm/ 1057 nm, respectively. Size spectra of stratospheric aerosol also show the influence of volcanic aerosols. A bimodal distribution was found with main radius modes at 0.1 μm to 0.3 μm and 0.75 μm to 0.9 μm. The typical vertical distribution of tropospheric aerosols in the Arctic in this time period and inferred size spectra are presented. The seasonal change in the spectral aerosol optical depth with a minimum in summer and high values in spring is shown, as well as the high extinction and distinct layering of Arctic haze up into the free troposphere. Size spectra of Arctic haze also change with height and show, in addition to particles with radii smaller than 0.1 μm, main radius modes between 0.1 μm and 0.2 μm and between 1 μm and 2 μm. Possible influence of tropospheric aerosols on the radiation balance is also discussed for the spring and summer season. While in summer the influence on the radiation balance of the very small amount of tropo-spheric aerosols is negligible, in spring, haze layers with high extinction can result in a slight warming.