Aircraft measurements of bromine monoxide, iodine monoxide, and glyoxal profiles in the tropics: comparison with ship-based and in situ measurements.

Tropospheric chemistry of halogens and organic carbon over tropical oceans modifies ozone and atmospheric aerosols, yet atmospheric models remain largely untested for lack of vertically resolved measurements of bromine monoxide (BrO), iodine monoxide (IO), and small oxygenated hydrocarbons like glyoxal (CHOCHO) in the tropical troposphere. BrO, IO, glyoxal, nitrogen dioxide (NO₂), water vapor (H₂O) and O₂-O₂ collision complexes (O₄) were measured by the CU Airborne Multi AXis Differential Optical Absorption Spectroscopy (CU AMAX-DOAS) instrument, in situ aerosol size distributions by an Ultra High Sensitivity Aerosol Spectrometer (UHSAS), and in situ H₂O by Vertical-Cavity Surface-Emitting Laser hygrometer (VCSEL). Data are presented from two research flights (RF12, RF17) aboard the NSF/NCAR GV aircraft over the tropical Eastern Pacific Ocean (tEPO) as part of the "Tropical Ocean tRoposphere Exchange of Reactive halogens and Oxygenated hydrocarbons" (TORERO) project. We assess the accuracy of O₄ slant column density (SCD) measurements in the presence and absence of aerosols, and find O₄-inferred aerosol extinction profiles at 477 nm agree within 5% with Mie calculations of extinction profiles constrained by UHSAS. CU AMAX-DOAS provides a flexible choice of geometry which we exploit to minimize the SCD in the reference spectrum (SCD_REF, maximize signal-to-noise), and to test the robustness of BrO, IO, and glyoxal differential SCDs. The RF12 case study was conducted in pristine marine and free tropospheric air. The RF17 case study was conducted above the NOAA RV Ka'imimoana (TORERO cruise, KA-12-01), and provides independent validation data from ship-based in situ Cavity Enhanced- and MAX-DOAS. Inside the marine boundary layer (MBL) no BrO was detected (smaller than 0.5 pptv), and 0.2-0.55 pptv IO and 32-36 pptv glyoxal were observed. The near surface con centrations agree within 20% (IO) and 10% (glyoxal) between ship and aircraft. The BrO concentration strongly increased with altitude to 3.0 pptv at 14.5 km (RF12, 9.1 to 8.6° N; 101.2 to 97.4° W). At 14.5km 5-10 pptv NO₂ agree with model predictions, and demonstrate good control over separating tropospheric from stratospheric absorbers, (NO₂ and BrO). Our profile retrievals have 12-20 degrees of freedom (DoF), and up to 500m vertical resolution. The tropospheric BrO VCD was 1.5x10¹³ moleccm^-2 (RF12), and at least 0.5x10¹³ moleccm^-2 (RF17, 0-10 km, lower limit). Tropospheric IO VCDs correspond to 2.1x1012 moleccm^-2 (RF12) and 2.5x1012 moleccm^-2 (RF17), and glyoxal VCDs of 2.6_1014 moleccm^-2 (RF12) and 2.7x1014 moleccm^-2 5 (RF17). Surprisingly, essentially all BrO, and the dominant IO and glyoxal VCD fraction was located above 2km (IO: 58±5%, 0.1-0.2 pptv; glyoxal: 52±5%, 3-20 pptv). To our knowledge there are no previous vertically resolved measurements of BrO and glyoxal from aircraft in the tropical free troposphere. [ABSTRACT FROM AUTHOR]