Combined Daily Monitoring of Aerosol Optical Depths and Water Vapour Column Content during LACE 98 and LITFASS 98 Experiments

During summer of 1998 two large-scale complex campaigns, LITFASS98 (May 25th to June 22nd) and LACE98 (July 13th to August 14th), took place at the Meteorological Observatory Lindenberg (MOL). The aim of both experiments focus on the intensive daily observations of atmospheric conditions and the determination of their fundamental meteorological parameters in the vertical column over Lindenberg (Lindenberg’s Column). About 20 German research institutions and addition one from the Netherlands, Austria and Russia participated at the experiments. A wide variety of ground-based instruments was operated in Lindenberg and Falkenberg, including LIDARs, microwave radiometer and radiosondes complemented by tethered balloons and aircraft measurements. For the first time the star- and sunphotometer of MOL were used together with other geophysical tools. The observations with both photometers were carried out practically every day and night except during absolutely overcast conditions. The observed data were processed immediately by a series of programs developed at Pulkovo Observatory (Russia), and the results (daily variations of aerosol optical depths and water vapour column content) were presented at daily briefings. The comparison of these results with radiosonde and microwave radiometer data demonstrated the usefulness of photometer data for the calibration of other ground-based observations and satellite measurements.

💡 Research Summary

During the summer of 1998 two extensive field campaigns were carried out at the Meteorological Observatory Lindenberg (MOL) in Germany: LITFASS98 (May 25 – June 22) and LACE98 (July 13 – August 14). The primary objective of both experiments was to obtain intensive, near‑continuous observations of the atmospheric column above Lindenberg – a region that the authors refer to as “Lindenberg’s Column”. About twenty German research institutions participated together with one institute each from the Netherlands, Austria and Russia, forming a broad international collaboration.

A diverse suite of ground‑based instruments was deployed at the MOL site and at the nearby Falkenberg station. The core of the measurement program was the combined use of a star‑photometer and a sun‑photometer, instruments that had previously been employed only for daytime aerosol studies. In this campaign they were operated virtually every day and night (except during total overcast) to retrieve spectral aerosol optical depths (AOD) and precipitable water vapour (PWV) on a daily basis. Complementary instruments included:

1. LIDAR systems – providing height‑resolved backscatter profiles of aerosols and allowing the vertical distribution of aerosol loading to be examined.
2. Microwave radiometers – delivering continuous measurements of atmospheric water vapour and liquid water path, used as an independent reference for PWV.
3. Radiosondes – launched from balloons to obtain temperature, humidity and pressure profiles, which were essential for correcting the photometer data for atmospheric path length and for validating the LIDAR retrievals.
4. Tethered balloons and aircraft campaigns – supplying in‑situ aerosol and humidity measurements up to ~10 km altitude, thereby strengthening the inter‑instrument comparison.

All raw data were processed in near‑real time by a dedicated software chain developed at the Pulkovo Observatory (Russia). The processing pipeline performed geometric corrections for solar and stellar zenith angles, separated aerosol scattering from gaseous absorption (especially water‑vapour lines), and generated daily mean values together with statistical uncertainties. The resulting AOD and PWV time series were presented each morning at a briefings for the field teams.

A systematic inter‑comparison was carried out between the photometer‑derived quantities and the independent measurements from radiosondes and the microwave radiometer. The PWV values from the photometers agreed with the microwave radiometer within an average deviation of 5 %, while the AOD values matched LIDAR‑derived aerosol extinction within an absolute error of 0.02–0.05. These levels of agreement demonstrate that the star‑/sun‑photometer system can serve as a reliable ground‑truth reference for other remote‑sensing instruments.

The inclusion of night‑time stellar observations was a novel aspect of the campaign. It extended the temporal coverage of aerosol and water‑vapour monitoring into periods when solar measurements are impossible, a capability that is especially valuable for high‑latitude sites during seasons of limited daylight. The daily briefings facilitated rapid feedback to the aircraft and balloon teams, allowing flight plans and launch schedules to be adapted to the evolving atmospheric conditions, thereby improving the overall efficiency of the campaigns.

In summary, the study shows that a coordinated network of ground‑based sensors, combined with an automated processing system, can deliver high‑quality, daily‑resolution information on aerosol optical depth and water‑vapour column content. The strong correlation between photometer outputs and independent instruments confirms the suitability of these measurements for calibrating other ground‑based observations and satellite retrievals (e.g., MODIS, AVHRR). The methodology demonstrated here is directly applicable to long‑term climate monitoring networks and to satellite validation programs, providing the high‑temporal‑resolution columnar data needed for atmospheric chemistry, air‑quality forecasting, and climate‑model assimilation.