A wide field-of-view imaging DOAS instrument for two-dimensional trace gas mapping from aircraft

Abstract

Abstract. The Airborne imaging differential optical absorption spectroscopy (DOAS) instrument for Measurements of Atmospheric Pollution (AirMAP) has been developed for the purpose of trace gas measurements and pollution mapping. The instrument has been characterized and successfully operated from aircraft. Nitrogen dioxide (NO 2 ) columns were retrieved from the AirMAP observations. A major benefit of the push-broom imaging instrument is the spatially continuous, gap-free measurement sequence independent of flight altitude, a valuable characteristic for mapping purposes. This is made possible by the use of a charge coupled device (CCD) frame-transfer detector. A broad field of view across track of around 48 • is achieved with wide-angle entrance optics. This leads to a swath width of about the same size as the flight altitude. The use of fibre coupled light intake optics with sorted light fibres allows flexible instrument positioning within the aircraft and retains the very good imaging capabilities. The measurements yield ground spatial resolutions below 100 m depending on flight altitude. The number of viewing directions is chosen from a maximum of 35 individual viewing directions (lines of sight, LOS) represented by 35 individual fibres. The selection is adapted to each situation by averaging according to signal-to-noise or spatial resolution requirements. Observations at 30 m spatial resolution are obtained when flying at 1000 m altitude and making use of all 35 viewing directions. This makes the instrument a suitable tool for mapping trace gas point sources and small-scale variability. The position and aircraft attitude are taken into account for accurate spatial mapping using the Attitude and Heading Reference System of the aircraft. A first demonstration mission using AirMAP was undertaken in June 2011. AirMAP was operated on the AWI Polar-5 aircraft in the framework of the AIRMETH-2011 campaign. During a flight above a medium-sized coal-fired power plant in north-west Germany, AirMAP clearly detected the emission plume downwind from the exhaust stack, with NO 2 vertical columns around 2 × 10 16 molecules cm −2 in the plume centre. NO x emissions estimated from the AirMAP observations are consistent with reports in the European Pollutant Release and Transfer Register. Strong spatial gradients and variability in NO 2 amounts across and along flight direction are observed, and small-scale enhancements of NO 2 above a motorway are detected.

High-resolution airborne imaging DOAS measurements of NO<sub>2</sub> above Bucharest during AROMAT

“…A detailed description of the instrumental setup, its performance, the viewing geometry and the georeferencing is given in Schönhardt et al (2015). Thus, the instrument is only briefly described here.…”

“…Installed on aircraft, these systems enable the creation of maps of the horizontal trace-gas distribution. The iDOAS measurements of anthropogenic point source emissions of NO 2 were performed by Heue et al (2008) and by Schönhardt et al (2015) above the South African Highveld plateau and above a German power plant, respectively. General et al (2014) performed measurements of volcanic emissions of BrO, OClO and SO 2 at Mt.…”

Investigating differences in DOAS retrieval codes using MAD-CAT campaign data

“…In addition, airborne and car-based measurements (e.g. Schönhardt et al, 2015;Shaiganfar et al, 2011), have been taken to close the gap between ground-based observations providing continuous temporal, but poor spatial resolution and satellite measurements which offer global observations, but only up to one measurement per day above each location. Using ground-based multi-axis (MAX)-DOAS measurements at different elevation angles, a more accurate vertical column (VC) can be retrieved with information on the vertical distribution of NO 2 and other trace gases in the troposphere (e.g.…”

Polar aircraft Polar5 and Polar6 operated by the Alfred Wegener Institute

“…NO is produced by the photolysis of NO 2 . Source of NO x can either be natural processes as lightning, natural biomass burning events, soil emissions or anthropogenic activities as fossil fuel combustion by power plants, industry and tra c (Schönhardt et al, 2015).…”

“…NO 2 was successfully observed on small spatial scales. For details on the method and further reading it is referred to Schönhardt et al (2015). Measurements of methane (CH 4 ) emission over permafrost regions were conducted within the framework of AWI and GFZ joint AIRMETH campaigns in 2012 and 2013.…”