Sabine Eckhardt

Long‐range transport of Saharan dust to northern Europe: The 11–16 October 2001 outbreak observed with EARLINET

The spread of mineral particles over southwestern, western, and central Europe resulting from a strong Saharan dust outbreak in October 2001 was observed at 10 stations of the European Aerosol Research Lidar Network (EARLINET). For the first time, an optically dense desert dust plume over Europe was characterized coherently with high vertical resolution on a continental scale. The main layer was located above the boundary layer (above 1-km height above sea level (asl)) up to 3–5-km height, and traces of dust particles reached heights of 7–8 km. The particle optical depth typically ranged from 0.1 to 0.5 above 1-km height asl at the wavelength of 532 nm, and maximum values close to 0.8 were found over northern Germany. The lidar observations are in qualitative agreement with values of optical depth derived from Total Ozone Mapping Spectrometer (TOMS) data. Ten-day backward trajectories clearly indicated the Sahara as the source region of the particles and revealed that the dust layer observed, e.g., over Belsk, Poland, crossed the EARLINET site Aberystwyth, UK, and southern Scandinavia 24–48 hours before. Lidar-derived particle depolarization ratios, backscatter- and extinction-related Angstrom exponents, and extinction-to-backscatter ratios mainly ranged from 15 to 25%, −0.5 to 0.5, and 40–80 sr, respectively, within the lofted dust plumes. A few atmospheric model calculations are presented showing the dust concentration over Europe. The simulations were found to be consistent with the network observations.

A replacement for simple back trajectory calculations in the interpretation of atmospheric trace substance measurements

Trajectory calculations are often used for the interpretation of atmospheric trace substances measurements. However, two important effects are normally neglected or not considered systematically: first, measurements of trace substances sample finite volumes of air, whereas a trajectory tracks the path of an infinitesimally small particle; second, turbulence and convection. Advection by the deformative synoptic-scale atmospheric flow is responsible for the fact that a compact measurement volume is in fact turned into filamentary structures at earlier times, which a single trajectory cannot represent. Turbulence and convection add to this by causing a growth of the volume (backward in time) where processes such as emissions can affect measured concentrations. In this paper, we show that both effects are substantial and may be the largest sources of error when using trajectory calculations to establish source–receptor relationships. We use backward simulations with a Lagrangian particle dispersion model (LPDM) and cluster analysis of the particle positions to derive more representative single ‘‘trajectories’’ (transport paths) and trajectory ensembles. This reduces errors caused by filamentation and backward growth of the measurement volume to a few percent as compared to using a single, mean-wind trajectory and also yields estimates of the spread of the region of influence. Thus, we recommend to replace simple back trajectory calculations for interpretation of atmospheric trace substances measurements in the future by backward simulations with LPDMs, possibly followed by the clustering of particle positions as introduced in this paper. r 2002 Elsevier Science Ltd. All rights reserved.

Saharan dust over a central European EARLINET‐AERONET site: Combined observations with Raman lidar and Sun photometer

and Sun photometer observations showed excellent agreement. Particle depolarization ratios of up to 25% were derived from lidar observations at 532 nm. Scattering phase functions retrieved from Sun photometer observations indicated particles of nonspherical shape. This shape caused unusually large particle extinction-to-backscatter (lidar) ratios at 532 nm in the range from 50 to 80 sr. There were substantial deviations of the lidar ratio at 532 nm derived from both measurement methods. They are explained by the effect of particle shape. INDEX TERMS: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 0360 Atmospheric Composition and Structure: Transmission and scattering of radiation; 1630 Global Change: Impact phenomena; 1640 Global Change: Remote sensing; KEYWORDS: microphysical properties, optical properties, Raman lidar, Saharan dust, Sun photometer Citation: Muller, D., I. Mattis, U. Wandinger, A. Ansmann, D. Althausen, O. Dubovik, S. Eckhardt, and A. Stohl, Saharan dust over a central European EARLINET-AERONET site: Combined observations with Raman lidar and Sun photometer, J. Geophys. Res., 108(D12), 4345, doi:10.1029/2002JD002918, 2003.

A springtime comparison of tropospheric ozone and transport pathways on the east and west coasts of the United States

[1] We have conducted a study to determine the influence of Asian pollution plumes on free tropospheric ozone above the west coast of the United States during spring. We also explored the additional impact of North American emissions on east coast free tropospheric ozone. Long-term ozone monitoring sites in the United States are few, but we obtained ozonesonde profiles from Trinidad Head on the west coast, Huntsville, Alabama, in the southeast, and Wallops Island, Virginia, on the east coast. Additional east coast ozone profiles were measured by the MOZAIC commercial aircraft at Boston, New York City, and Philadelphia. Kilometer-averaged ozone was compared between Trinidad Head and the three east coast sites (MOZAIC, Wallops Island, and Huntsville). Only in the 0–1 km layer did the MOZAIC site have a statistically significant greater amount of ozone than Trinidad Head. Likewise only the 0–1 and 1–2 km layers had greater ozone at Wallops Island and Huntsville in comparison to Trinidad Head. While Wallops Island did show greater ozone than Trinidad Head at 6–9 km, this excess ozone was attributed to a dry air mass sampling bias. A particle dispersion model was used to determine the surface source regions for each case, and the amount of anthropogenic NOx tracer that would have been emitted into each air mass. Transport times were limited to 20 days to focus on the impact of direct transport of pollution plumes from the atmospheric boundary layer. As expected, the amount of NOx tracer emitted into the east coast profiles was much greater in the lower and mid troposphere than at the west coast. At various altitudes at both coasts there existed a significant positive correlation between ozone and the NOx tracer, but the explained variance was generally less than 30%. On the east coast, Wallops Island had the weakest relationship between ozone and the NOx tracer, while Huntsville had the strongest. During spring, differences in photochemistry and transport pathways in the lowest 2 km of the troposphere results in an extra 5–14 ppbv of ozone on the east coast in comparison to Trinidad Head. However, despite differing amounts of NOx tracer from Asia and North America in the free troposphere, we found no significant difference in free tropospheric ozone between the east and west coasts of the United States during spring.

Arctic haze over Central Europe

An extraordinary aerosol situation over Leipzig, Germany in April 2002 was investigated with a comprehensive set of ground-based volumetric and columnar aerosol data, combined with aerosol profiles from lidar, meteorological data from radiosondes and air mass trajectory calculations. Air masses were identified to stem from the Arctic, partly influenced by the greater Moscow region. An evaluation of ground-based measurements of aerosol size distributions during these periods showed that the number concentrations below about 70 nm in diameter were below respective long-term average data, while number, surface and volume concentrations of the particles larger than about 70 nm in diameter were higher than the long-term averages. The lidar aerosol profiles showed that the imported aerosol particles were present up to about 3 km altitude. The particle optical depth was up to 0.45 at 550 nm wavelength. With a one-dimensional spectral radiative transfer model top of the atmosphere (TOA) radiative forcing of the aerosol layer was estimated for a period with detailed vertical information. Solar aerosol radiative forcing values between −23 and −38 W m−2 were calculated, which are comparable to values that have been reported in heavily polluted continental plumes outside the respective source regions. The present report adds weight to previous findings of aerosol import to Europe, pointing to the need for attributing the three-dimensional aerosol burden to natural and anthropogenic sources as well as to aerosol imports from adjacent or distant source regions. In the present case, the transport situation is further complicated by forward trajectories, indicating that some of the observed Arctic haze may have originated in Central Europe. This aerosol was transported to the European Arctic before being re-imported in the modified and augmented form to its initial source region.