Hyperspectral camera observations for the determination of aerosol profiles and cloud properties

Abstract. Spectroscopic 2D imaging observations with high temporal resolution are important for the investigation of different atmospheric research topics, e.g. the monitoring of emission plumes from point sources or the radiative effects of rapidly changing cloud fields. Traditional imaging DOAS applications cover one spatial dimension using 2D CCD detectors, while scanning the other dimension by sequentially changing the viewing direction. Several minutes are usually needed to achieve a complete 2D image by such techniques. An alternative with much higher temporal resolution is provided by hyperspectral cameras, which can achieve complete 2D images within a few tens of seconds. The limitations of such observations are sometimes the low spectral resolution (typically a few nanometers) and the rather small field of view (usually covering about 10⁰). Nevertheless, hyperspectral cameras were already successfully used from ground based or satellite observations to derive high-resolution spatial patterns of atmospheric trace gases.

In this study we use a hyperspectral camera covering the visible and near IR spectral range (400 – 1000 nm) with a spectral resolution of about 4.7 nm. We show that the absorptions of O₄ can be well determined from the measured spectra. We use the derived O₄ absorptions together with the simultaneously measured calibrated radiances to determine atmospheric profiles of the aerosol extinction and cloud properties. Here we exploit one important advantage of the 2D imaging capabilities: the unambiguous distinction between clear and cloudy parts within the recorded image. Such a clear distinction is often impossible from simple 1D elevation scans of typical MAX-DOAS observations.

We derive cloud properties (altitude, horizontal distance and optical depth of scattered clouds) from the 2D images, and aerosol extinction profiles from the spectra of the cloud-free parts of the images. We also investigated potential effects of scattered clouds on the radiation fields between the cloud patches and found that the O₄ absorptions and radiances are not significantly affected by the nearby clouds. This is an important finding for the retrieval of aerosol profiles from MAX-DOAS measurements in partly cloudy sky conditions.