In situ CO concentrations at the sites Zugspitze (47°N, 11°E) and Cape Point (34°S, 18°E) in April and October 1994

Abstract

Continuous measurements of CO have been performed at the coastal site Cape Point (34°S, 18°E) and at the continental mountain site Zugspitze (47°N, 11°E) since 1978 and 1990, respectively. The CO concentrations observed at both stations in 1994 during the periods April 9–18, and September 30 to October 10 were studied in detail with the aim to supplement CO observations from space made by NASA during two shuttle flights (measurement of air pollution from satellites (MAPS) project). All CO values reported were adjusted to the NOAA Climate Monitoring and Diagnostics Laboratory (CMDL) calibration scale. CO mixing ratios at Cape Point comprised episodes of baseline CO levels in maritime air (average: 48.8 ppb (April); 69.4 ppb (September/October)) and of elevated levels related to continental air and regional pollution (average: 89.5 ppb (April); 102.1 ppb (September/‐October)). During the same periods the mean CO mixing ratio at the Zugspitze amounted to 220.1 ppb (range 150 ‐ 358 ppb) and 124.8 ppb (range 78–175 ppb), respectively. Due to the prevailing airflow, CO levels at the Zugspitze were unusually high compared to the long‐term average during the April mission (time of seasonal maximum). In contrast, they were close to the average during the October mission. Baseline concentrations at Cape Point were about 3–4 ppb above the long‐term average for both April (increasing side of seasonal cycle) and October (time of seasonal maximum), which is within the range of normal interannual variability. A preliminary comparison with the MAPS data for the respective geographic regions showed good agreement of the CO mixing ratios for clean air at Cape Point in April and at the Zugspitze in October. In addition, the CO levels observed in continental air at Cape Point in October agreed well with the MAPS data. Obvious differences in CO concentrations for other parts of the records mainly reflected uncomparable atmospheric conditions. In particular, they seemed to be related to either vertical gradients of CO or small‐scale differences in air mass composition as indicated by meteorological analyses.