R. Fitzenberger

Lower stratospheric organic and inorganic bromine budget for the Arctic winter 1998/99

In the Arctic winter 1998/99, two balloon payloads were launched in a co-ordinated study of stratospheric bromine. Vertical profiles (9-28 km) of all known major organic Br species (CH 3 Br, C 2 H 5 Br, CH 2 BrCl, CHBrCl 2 , CH 2 Br 2 , CHBr 2 Cl, CHBr 3 , H1301, H1211, H2402, and H1202) were measured, and total organic Br (henceforth called Br org y ) originating from these organic precursors was inferred as a function of altitude. This was compared with total inorganic reactive Br (henceforth called Br in y ) derived from spectroscopic BrO observations, after accounting for modeled stratospheric Br y partitioning. Within the studied altitude range the two profiles differed by less than the estimated accumulated uncertainties. This good agreement suggests that the lower stratospheric budget and chemistry of Br is well understood for the specified conditions. For early 1999 our data suggest 2 Br in y mixing ratio of 1.5 ppt in air just above the local Arctic tropopause (∼ 9.5 km), whilst at 25 km in air of 5.6 yr mean age it was estimated to be 18.4(+1.8/-1.5) ppt from organic precursor measurements, and (21.5±3.0) ppt from BrO measurements and photochemical modelling, respectively. This suggests a Br in y influx of 3.1(-2.9/+3.5) ppt from the troposphere to the stratosphere.

First profile measurements of tropospheric BrO

Tropospheric BrO profiles (about 0.6+0.2 ppt, and 2.0-t-0.8 ppt at profile maximum) were mea- sured for the first time. Our measurements add new information to recent speculations - based on indirect evidence- of BrO possibly being ubiquitous in the free troposphere (Harder.et al., 1998; Friefi et al., 1999; Van Roozendael et al., 1999; Pundt et al., 2000). Our study relies on a detailed comparison of BrO slant column densities (BrO-SCD) measured in the tropos- phere from the LPMA/DOAS (Laboratoire de Physi- que Mol6culaire et Applications and Differential Op- tical Absorption Spectroscopy) gondola by direct Sun absorption, and BrO-SCD values subsequently measu- red in the lowermost stratosphere during balloon ascent. The difference in total atmospheric BrO-SCDs measu- red in the troposphere and lowermost stratosphere- af- ter a suitable correction for the change in BrO due to photochemistry and the observation geometry- is then attributed to tropospheric BrO.

Stratospheric BrO profiles measured at different latitudes and seasons: Atmospheric observations

Stratospheric BrO profiles were measured at different latitudes and in different seasons in 1996/97 during three flights of the LPMA/DOAS balloon gondola (LPMA/Laboratoire Physique Moleculaire et Application and DOAS/Differential Optical Absorption Spectrometry). Using direct sunlight DOAS spectrometry the following BrO mixing ratios were measured; (1) 9 to 14 ppt in the height range from 20 to 30 km (at solar zenith angles, SZA < 88°) during ascent, (2) about (14±2) ppt for altitudes above the balloon float altitude at 30.6 km, 30.0 km, and 39.8 km, and (3) 5 to 10 ppt in the 20–30 km region during sunset. The lower BrO concentrations during sunset than those observed prior at daytime indicate a conversion of BrO into nighttime reservoir species (BrONO2, HOBr, and BrCl). The overall agreement of our UV spectroscopic BrO profiles with recent measurements using the chemical conversion/resonance fluorescence technique is good. Our BrO profiles are also in reasonable agreement with the present stratospheric Bry burden and chemistry. Conversily collocated ground-based and satellite column measurements, however, show significantly more total atmospheric BrO (50–100%) than the integrated stratospheric BrO balloon profiles can account for. This indicates a global tropospheric BrO background, estimated at 1–2 ppt.

First atmospheric profile measurements of UV/visible O4 absorption band intensities: Implications for the spectroscopy, and the formation enthalpy of the O2-O2 dimer

The first atmospheric profiles of the ultraviolet/visible (UV/vis) absorption bands of the collision complex O2-O2, or O4 in brief, are reported. The O4 absorption profiles are inferred from direct Sun spectra observed from the LPMA/DOAS (Laboratoire Physique Moleculaire et Application/Differential Optical Absorption Spectroscopy) balloon gondola. Seven O4 absorption bands - centered at ∼360.7, 380.2, 446.7, 477.1, 532.2, 577.2, and 630.0 nm - are investigated for atmospheric pressures (p) ranging from ∼500 hPa to ∼40 hPa and temperatures (T) ranging from 203 K to 250 K. For the encountered atmospheric conditions, it is found that, (a) the band shapes do not change with T and p and (b) the peak collision pair absorption intensities (αi) concurrently increase with decreasing T (by about 11 % over a ΔT=50 K). That result is in agreement with previous laboratory O4 studies mostly conducted at high O2 partial pressures (up to several hundred bars). Furthermore, by reasonably assuming that the O4 absorption cross sections are T-independent, the inferred T-dependence of αi(T) suggests a thermally averaged enthalpy change = −(1207±83) J/Mol involved in the formation of O4. Our inferred ΔH is in reasonable agreement with the orientation and spin averaged O4 well depth De(O4) (= -(1130±80) J/Mol) measured in a recent O2-O2 collision experiment, when accounting for the rovibrational energy change during O4 formation (186 J/Mol).

Comparison of measured and modeled stratospheric BrO: Implications for the total amount of stratospheric bromine

Stratospheric BrO was spectroscopically monitored by its UV absorption in direct sunlight on 8 balloon flights that were conducted at middle and high latitudes in 1996 through 2000 [e.g. Ferlemann et al., 1998, 2000]. For conditions where detailed photochemical model calculations [Chipper field, 1999] correctly predict other measured chemical (e.g., NO 2 , O 3 ) and dynamical (e.g. N 2 O, F12, et cetera) tracers, the total stratospheric bromine (organic and inorganic) amount (Bry) is inferred. An excellent agreement between measured and modeled stratospheric BrO is found, assuming JPL-97 kinetics and Bry=20 ppt in the photochemical model. As the BrO absorption cross section (σ BrO ) is the only external parameter used in the measurement, our finding tightly constrains the amount of total inorganic bromine (Br in y ). 20±2.5 ppt above 25 km in 1996/97, as well as the photochemistry of stratospheric bromine.

Stratospheric BrO profiles measured at different latitudes and seasons: Instrument description, spectral analysis and profile retrieval

Measurements of stratospheric BrO profiles are reported using a novel DOAS instrument (Differential Optical Absorption Spectrometry) operated on the LPMA/DOAS balloon gondola (LPMA/Laboratoire de Physique Moleculaire et Applications) during three flights (Leon, Nov. 23, 1996, Kiruna, Feb. 14, 1997, and Gap, June 20, 1997). BrO was detected by its vibrational bands (4–0 at 354.7nm; 5–0 at 348.8nm) of the A(²π)←X(²π) transition in direct sunlight spectra from balloon ascent, descent and during solar occultation at maximum (float) altitude. We show that our accuracy is about ±18% (1 σ) during balloon ascent (solar zenith angles, SZA < 88°), and about ±25% for solar occultation measurements. For altitudes above the balloon float our observations indicate average BrO mixing ratios of (14.4±2.5)ppt, (15.6±2.8)ppt, and (15.3±2.8)ppt above 30.6km, 30.0km, and 39.8km, respectively.

Comparison of measured and modeled stratospheric UV/Visible actinic fluxes at large solar zenith angles

Measured and modeled stratospheric filter sensitivity weighted ultraviolet/visible (UV/vis) actinic fluxes—approximating the NO2 photolysis rate coefficients (jNO2)— are compared. The measurements were performed with two calibrated 2π-actinometers assembled on the azimuth angle-controlled LPMA/DOAS (Laboratoire de Physique Moleculaire et Applications/Differential Optical Absorption Spectroscopy) gondola during a series of balloon flights. Since the actinometers spectral sensitivity curve did not exactly match the actinic spectrum of NO2 and the skylights spectrum shape changes with atmospheric height and solar illumination, only proxies (proxy-)jNO2 rather than true jNO2 values were monitored during balloon ascents (0–30 km) for solar zenith angle (SZA) 75° < SZA < 86°, and at balloon float altitude during solar occultation (86° < SZA < 95°). The measured direct and diffuse total proxy-jNO2 values compare excellently with radiative transfer (RT) modeling. That finding allows us to rule out uncertainties in computing UV/vis actinic fluxes as a significant factor in the still insufficient modeling of stratospheric NO2 at large SZAs.

Spectroscopic and Thermochemical Information on the O2−O2 Collisional Complex Inferred From Atmospheric Uv/Visible O4 Absorption Band Profile Measurements

Atmospheric profiles of the ultraviolet/visible (UV/vis) absorption bands of the collision complex O2−O2, or O4 in brief, are reported. The O4 absorption profiles are inferred from direct Sun spectra observed from the LPMA/DOAS (Laboratoire de Physique Moleculaire et Applications/Differential Optical Absorption Spectroscopy) balloon gondola. Seven O4 absorption bands — centered at ~ 360.7, 380.2, 446.7, 477.1, 532.2, 577.2, and 630.0 nm — are investigated for atmospheric pressures (p) ranging from ~ 500 hPa to ~ 40hPa and temperatures (T) ranging from 203 K to 250 K. For the encountered atmospheric conditions, it is found that, (a) the band shapes do not change with T and p and (b) the peak collision pair absorption intensities (αi) concurrently increase with decreasing T (by about 11% over a ΔT = 50K). That result is in agreement with previous laboratory O4 studies mostly conducted at high O2 partial pressures (up to several hundred bars). Furthermore, by reasonably assuming that the O4 absorption cross sections are T-independent, the inferred T-dependence of αi (T) suggests a thermally averaged enthalpy change = − (1207 ± 83) J/Mol involved in the formation of O4. Our inferred ΔH is in reasonable agreement with the orientation and spin averaged O4 well depth De(O4)(= − (1130 ± 80) J/Mol) measured in a recent O2−O2 collision experiment, when accounting for the rovibrational energy change during O4 formation (189 J/Mol).