Airborne solar spectroscopic measurements of nitrogen dioxide column density beneath the boundary layer

Airborne atmospheric measurement of nitrogen dioxide (NO₂) column density was performed using the solar spectroscopic method. The measurement was carried out at different altitudes, from 460 to 700 meters above sea level during a flight in Hong Kong (22.2°N, 114.1°E), the People's Republic of China (PRC). In the territory, the boundary layer height in a sunny day is about 1000 meters. Below the boundary layer, most of the NO₂ exists. The airborne solar spectroscopic measurement gives the NO₂ vertical profile below the boundary layer. In particular, the solar spectroscopic measurement requires a solar tracking system to collect the direct sunlight. However, in an acceptable small percentage of error in tracking the sun position, it is possible to collect the direct sunlight manually. In this paper, to reduce the complexity of the experimental setup, the sunlight is collected by a portable miniature CCD spectrometer. In the retrieval of NO ₂ column density, the airborne solar spectrum is normalized to a reference solar spectrum, which is taken at a high altitude (11,230 meters) during another flight in Xinjiang (42.208°N, 83.949°E) province, PRC. The column density retrieval is achieved from the normalized solar spectrum using the differential optical absorption spectroscopy. A ground-based off-axis control experiment is also performed to estimate the error in the slant column density from the airborne measurement.