Studies of the horizontal inhomogeneities in NO.sub.2 concentrations above a shipping lane using ground-based multi-axis differential optical absorption spectroscopy measurements and validation with airborne imaging DOAS measurements

This study describes a novel application of an "onion-peeling" approach to multi-axis differential optical absorption spectroscopy (MAX-DOAS) measurements of shipping emissions aiming at investigating the strong horizontal inhomogeneities in NO.sub.2 over a shipping lane. To monitor ship emissions on the main shipping route towards the port of Hamburg, a two-channel (UV and visible) MAX-DOAS instrument was deployed on the island Neuwerk in the German Bight, 6-7 km south of the main shipping lane. Utilizing the fact that the effective light path length in the atmosphere depends systematically on wavelength, simultaneous measurements and DOAS retrievals in the UV and visible spectral ranges are used to probe air masses at different horizontal distances to the instrument to estimate two-dimensional pollutant distributions. Two case studies have been selected to demonstrate the ability to derive the approximate plume positions in the observed area. A situation with northerly wind shows high NO.sub.2 concentrations close to the measurement site and low values in the north of the shipping lane. The opposite situation with southerly wind, unfavorable for the on-site in situ instrumentation, demonstrates the ability to detect enhanced NO.sub.2 concentrations several kilometers away from the instrument. Using a Gaussian plume model, in-plume NO.sub.2 volume mixing ratios can be derived from the MAX-DOAS measurements.