Troposphere – stratosphere integrated BrO profile retrieval over the central Pacific Ocean

Abstract. Bromine is a reactive trace element in the atmosphere, that destroys ozone, oxidizes mercury, modifies oxidative capacity and affects the lifetime of climate-active gases (e.g., methane). About 75 % of tropospheric ozone and methane is destroyed in the tropics, primarily in the lower free troposphere, where bromine monoxide (BrO) radical measurements are generally scarce. The few available aircraft observations find BrO is variable, and measurements in different compartments of the atmosphere are not easily reconciled. While zenith-sky DOAS measurements provide long-term records of the stratospheric O₃ and NO₂ abundances, autonomous MAX-DOAS placed at remote mountaintop observatories (MT-DOAS) provides cost effective and maximally sensitive access to probe the lower free troposphere, a climate-relevant yet understudied region of the atmosphere.

Here we describe and evaluate an innovative full-atmosphere BrO and formaldehyde (HCHO) profile retrieval algorithm using MT-DOAS measurements at Mauna Loa Observatory (19.536° N; 155.577° W; 3401 m asl) during two case study days, characterized by the absence (26 Apr 2017, base case) and presence of a Rossby Wave breaking double tropopause (29 Apr 2017, RW-DT case) above Big Island, Hawaii. The full atmosphere retrieval is based on time-dependent optimal estimation, and simultaneously inverts 190+ individual BrO (and formaldehyde, HCHO) SCDs (slant column densities, SCD = dSCD + SCD_Ref) from solar stray light spectra measured in the zenith and off-axis geometries at high and low solar zenith angle (92º > SZA > 30º) to derive BrO concentration profiles with 7.5 degrees of freedom (DoF) from 1.9 to 35 km altitude. Stratospheric BrO vertical columns are near identical on both days (VCD = (1.5 ± 0.2) ×10¹³ molec cm^-2), and the stratospheric BrO profile peaks at a lower altitude during the Rossby wave breaking event (1.6 – 2.0 DoFs). Tropospheric BrO VCDs increase from (0.70 ± 0.14) ×10¹³ molec cm^-2 (base case) to (1.00 ± 0.14) ×10¹³ molec cm^-2 (RW-DT), owing to a tropospheric BrO profile re-distribution characterized by a three-fold increase in BrO located in the upper troposphere (1.7 – 1.9 DoF). BrO is found to be more variable in the lower free troposphere (0.2 pptv < BrO < 0.9 pptv) and characterized in three altitude layers (near, above, below) MLO with added time resolution (~3.8 DoF). The BrO mixing ratio at MLO increases from (0.23 ± 0.03) pptv (base case) to (0.46 ± 0.03) pptv BrO (RW-DT); while the maximum of (0.9 ± 0.1) pptv BrO is observed above MLO in the lower free troposphere in absence of the double tropopause.

We validate the retrieval using aircraft BrO profiles and in-situ HCHO measurements aboard the NSF/NCAR GV aircraft above MLO (11 Jan 2014) that establish BrO peaks around 2.4 pptv above 13 km in the UTLS during a similar RW-DT event (0.83 ×10¹³ molec cm^-2 tropospheric BrO VCD above 2 km). The tropospheric BrO profile measured from MT-DOAS (RW-DT case) and the aircraft agree well (after averaging kernel smoothing). Furthermore, these tropospheric BrO profiles over the Central Pacific are found to closely resemble those over the Eastern Pacific Ocean (2–14 km); and contrast with the Western Pacific Ocean, where a C-shaped tropospheric BrO profile shape had been observed.