Comparison of Scanning LiDAR with Other Remote Sensing Measurements and Transport Model Predictions for a Saharan Dust Case

The evolution and the properties of a Saharan dust plume were studied near the city of Karlsruhe in southwest Germany (8.4298°E, 49.0953°N) from 7 to 9 April 2018, combining a scanning LiDAR (90°, 30°), a vertically pointing LiDAR (90°), a sun photometer, and the transport model ICON-ART. Based on this Saharan dust case, we discuss the advantages of a scanning aerosol LiDAR and validate a method to determine LiDAR ratios independently. The LiDAR measurements at 355 nm showed that the dust particles had backscatter coefficients of 0.86 ± 0.14 Mm−1 sr−1, extinction coefficients of 40 ± 0.8 Mm−1, a LiDAR ratio of 46 ± 5 sr, and a linear particle depolarisation ratio of 0.27 ± 0.023. These values are in good agreement with those obtained in previous studies of Saharan dust plumes in Western Europe. Compared to the remote sensing measurements, the transport model predicted the plume arrival time, its layer height, and its structure quite well. The comparison of dust plume backscatter values from the ICON-ART model and observations for two days showed a correlation with a slope of 0.9 ± 0.1 at 355 nm. This work will be useful for future studies to characterise aerosol particles employing scanning LiDARs.

A perspective on the development of gas-phase chemical mechanisms for Eulerian air quality models

An essential component of a three-dimensional air quality model is its gas-phase mechanism. We present an overview of the necessary atmospheric chemistry and a discussion of the types of mechanisms with some specific examples such as the Master Chemical Mechanism, the Carbon Bond, SAPRC and the Regional Atmospheric Chemistry Mechanism (RACM). The first versions of the Carbon Bond and SAPRC mechanisms were developed through a hierarchy of chemical species approach that relied heavily on chemical environmental chamber data. Now a new approach has been proposed where the first step is to develop a highly detailed explicit mechanism such as the Master Chemical Mechanism and the second step is to test the detailed explicit mechanism against laboratory and field data. Finally, the detailed mechanism is condensed for use in a three-dimensional air quality model. Here it is argued that the development of highly detailed explicit mechanisms is very valuable for research, but we suggest that combining the hierarchy of chemical species and the detailed explicit mechanism approaches would be better than either alone.Implication: Many gas-phase mechanisms are available for urban, regional and global air quality modeling. A "hierarchy of chemical species approach," relying heavily on smog-chamber data was used for the development of the early series of mechanisms. Now the development of large, explicit master mechanisms that may be condensed is a significant, trend. However, a continuing problem with air quality mechanism development is due to the high complexity of atmospheric chemistry and the current availability of laboratory measurements. This problem requires a balance between completeness and speculation so that models maintain their utility for policymakers.

The Uptake of SO2 on α-Fe2O3 and Mineral Dust Surfaces in the Temperature Range 250 K to 600 K

The uptake of SO2 on α-Fe2O3 and Saharan dust has been studied in the temperature range 250 K to 600 K using a Knudsen cell reactor. SO2 adsorbs readily and irreversibly on both mineral oxides with a mean initial uptake coefficient of γ ini = (5.9±0.3) · 10−2 on dry surfaces, independent of the surface temperature. In the presence of adsorbed water the initial uptake coefficients at T = 300 K are slightly higher with values of γ ini = (8.4±0.2) · 10−2 for Fe2O3 and γ ini = (7.3±0.4) · 10−2 for mineral dust. The uptake of SO2 is time-dependent and influenced by diffusion into the bulk of the sample at longer timescales. The adsorption capacity has been determined to be (3.0±0.4) · 1018 molecules g−1 for Fe2O3 and (2.6±1) · 1019 molecules g−1 for mineral dust. Sulphite has been identified as the primary reaction product on both surfaces which is readily oxidized to sulphate under atmospheric conditions. Oxidation of sulphite to sulphate only takes place at elevated temperature. From these results a mechanism for the uptake of SO2 onto mineral oxides is inferred, which has been used to compare the experimental concentration-time-profiles with simulated ones.