Troposphere – stratosphere integrated BrO, NO2, HCHO profile retrievals over the central Pacific Ocean

Abstract:

Even small amounts of halogen radicals (sub parts per trillion levels of bromine and iodine) are relevant for the lifetime of greenhouse gases (ozone and methane), oxidative capacity, mercury oxidation, and iodine participates in new particle formation. In order to test our understanding of the relevance of halogens for atmospheric composition better means to quantify such low concentrations routinely, accurately, and cost effectively are needed. Previous measurements of halogen radicals have used research aircraft, which are expensive to operate, and only provide snapshots in time. Mountain top observations provide an alternative means to access the remote free troposphere more routinely to measure halogen radicals systematically.

Measurements of active radical species such as active bromine (BrOx ≡ Br + BrO) and active nitrogen (NOx ≡ NO + NO2) in the remote free troposphere (FT) are very limited. Recent aircraft measurements have confirmed the inorganic bromine injection to the stratosphere (WMO Ozone Report 2018); but differences remain between in-situ and remote-sensing measurements of BrO in the FT that require further investigation. Since February 2017 the University of Colorado has operated Multi-AXis Differential Optical Absorption Spectroscopy (MAX-DOAS) instruments at two remote tropical marine mountaintops, Mauna Loa Observatory (MLO) at 19.5°N, 155.6°W, 3.4 km AMSL, and Maïdo Observatory (Maïdo) at 21.1°S, 55.4°E, 2.2 km AMSL. This presentation describes an integrated high-Degree-of-Freedom (DoF) retrieval of BrO, HCHO, and NO2 from DOAS data that leverages zenith sky observations at twilight to retrieve the stratosphere (~2 DoF) and MAX-DOAS observations during daylight to retrieve the tropospheric profile (2 - 2.5 DoF), including significant information in the upper FT (≳ 0.5 DoF). We focus on two case study days over MLO, when a stratospheric intrusion significantly perturbed BrO and NO2 concentrations in the upper FT, and leverage aircraft data from the Convective Transport of Active Species in the Tropics (CONTRAST) campaign that sampled a similar event. The implications and prospects of the developed methods to larger data sets and other chemical species (IO and CHOCHO) are discussed, and the impact of the observed halogens for tropospheric oxidation capacity is assessed.