Tropospheric and Stratospheric BrO Profiles from Mountaintops in the Remote Marine Atmosphere

Bromine Monoxide (BrO) has important impacts on the stratospheric ozone layer, and the oxidative capacity in the troposphere. Bromine oxidizes gas phase mercury, modifies HO_x and NO_x, and catalytically destroys ozone, which increases the lifetime of other greenhouse gases such as methane. However, BrO measurements in the remote free troposphere are extremely scarce. Recent aircraft observations provide tropospheric BrO profiles in the tropical troposphere (Volkamer et al., 2015, doi: 10.5194/amt-8-212-2015; Wang et al., 2015, doi: 10.1073/pnas.1505142112; Koenig et al., 2017, doi: 10.1073/acp-2017-572) that can serve as a priori information to interpret Differential Optical Absorption Spectroscopy (DOAS) measurements of scattered sunlight from mountaintop observatories in the free troposphere over remote oceans to fill the lack of data to evaluate the importance of bromine chemistry in the remote troposphere.

Since February 2017 the University of Colorado is conducting long-term MAX-DOAS measurements of free tropospheric and lower stratospheric BrO from two remote tropical marine mountaintops at 1) Mauna Loa Observatory (MLO) at 19.5°N, 155.6°W, 3.4 km AMSL, and 2) Maïdo Observatory (Maïdo) at 21.1°S, 55.4°E, 2.2 km AMSL. We measure strong absorbers: O₄ (at 360 and 477 nm), NO₂ (at 360 and 430 nm), and O₃ (at 330 and 485 nm); halogen radicals: BrO and IO; OVOC: HCHO and CHOCHO, and water vapor. In addition, BIRA operates a second MAX-DOAS instrument at sea-level in Le Port on Réunion 20.9°S, 55.3°E, less than 20 km from Maïdo viewing in a parallel direction. These mountaintop observatories provide unique long term access to the remote tropical free troposhere, removed from anthropogenic emissions. The access to the free troposphere is unobstructred by boundary layer aerosols, thus simplifying radiative transfer, and maximizing the sensitivity of the technique to detect BrO. Downward viewing geometries further provide information on absorber profiles below the instrumment.

This presentation leverages recent advances in 1) the molecular spectroscopy of O₃, and 2) the treatment of strong O₃ absorptions to develop consistent DOAS settings for both zenith sky (ZS-) and MAX-DOAS spectra; uses 3) a parameterization of stratospheric O₃, and NO₂ VCDs (developed for satellite retirevals); and 4) leverages a priori information from aircraft observations of tropospheric BrO profiles during the TORERO and CONTRAST field campaigns to develop a single 5 DoF (degree of freedom) total atmosphere BrO profile retrieval (0-25km altitude). Aircraft profiles of BrO up to 15 km altitude are used to assess the sensitivity in the tropospheric BrO towards the a priori profile shape. Further, two research flighs during the CONTRAST field campaign, namely RF01 on January 11, 2014 (which probed a stratospheric intrusion over Hawaii), and RF17 on February 28, 2014 (which observed more ordinary tropospheric conditions) provide test cases to evaluate the accuracy of the profile retrieval with independent aircraft observations.