Abstract
Abstract. Observations of atmospheric trace gases in the tropical upper troposphere (UT), tropical tropopause layer (TTL), and lower stratosphere (LS) require dedicated measurement platforms and instrumentation. Here we present a new limb-scanning Differential Optical Absorption Spectroscopy (DOAS) instrument developed for NASA's Global Hawk (GH) unmanned aerial system and deployed during the Airborne Tropical TRopopause EXperiment (ATTREX). The mini-DOAS system is designed for automatic operation under unpressurized and unheated conditions at 14–18 km altitude, collecting scattered sunlight in three wavelength windows: UV (301–387 nm), visible (410–525 nm), and near infrared (900–1700 nm). A telescope scanning unit allows selection of a viewing angle around the limb, as well as real-time correction of the aircraft pitch. Due to the high altitude, solar reference spectra are measured using diffusors and direct sunlight. The DOAS approach allows retrieval of slant column densities (SCDs) of O3, O4, NO2, and BrO with relative errors similar to other aircraft DOAS systems. Radiative transfer considerations show that the retrieval of trace gas mixing ratios from the observed SCD based on O4 observations, the most common approach for DOAS measurements, is inadequate for high-altitude observations. This is due to the frequent presence of low-altitude clouds, which shift the sensitivity of the O4 SCD into the lower atmosphere and make it highly dependent on cloud coverage. A newly developed technique that constrains the radiative transfer by comparing in situ and DOAS O3 observations overcomes this issue. Extensive sensitivity calculations show that the novel O3-scaling technique allows the retrieval of BrO and NO2 mixing ratios at high accuracies of 0.5 and 15 ppt, respectively. The BrO and NO2 mixing ratios and vertical profiles observed during ATTREX thus provide new insights into ozone and halogen chemistry in the UT, TTL, and LS.
Highlights
Transport and transformation of tropospheric gases in the tropical upper troposphere (UT), tropical tropopause layer (TTL), and lower stratosphere (LS) play an important role in controlling stratospheric water vapor and ozone, as wellPublished by Copernicus Publications on behalf of the European Geosciences Union.J
During this part of the flight, the mini-Differential Optical Absorption Spectroscopy (DOAS) was operated in the elevation scanning mode, which allows visualization of the precision of the instrument during flight
One aspect of the O3-scaling technique that requires additional scrutiny is the performance of the Tomcat/Slimcat model relative to the observations, as the radiative transfer (RT) calculations that are used for the determination of the SCDref and α factors depend on the modeled vertical trace gas profiles
Summary
Transport and transformation of tropospheric gases in the tropical upper troposphere (UT), tropical tropopause layer (TTL), and lower stratosphere (LS) play an important role in controlling stratospheric water vapor and ozone, as well. A different approach to retrieve trace gas profiles relies on the capability of the aircraft to ascend and descend in the atmosphere, together with observations in the limb, which provide the highest sensitivity at flight altitude (Prados-Roman et al, 2011; Baidar et al, 2013; Volkamer et al, 2015). Baidar et al (2013) and Volkamer et al (2015) describe a somewhat different approach in which measured O4 DSCD profiles are used to derive the aerosol extinction profile In both cases, vertical resolution of the retrieved trace gas profiles is in the range of 0.5–2 km throughout the range of the aircraft ascent or descent maneuver. The scientific results of the ATTREX 2013 deployment are discussed in a companion paper by Werner et al (2017)
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