skip to main content

Title: Global seasonal distribution of CH 2 Br 2 and CHBr 3 in the upper troposphere and lower stratosphere

Abstract. Bromine released from the decomposition of short-lived brominated source gases contributes as a sink of ozone in the lower stratosphere.The two major contributors are CH2Br2 and CHBr3.In this study, we investigate the global seasonal distribution of these two substances, based on four High Altitude and Long Range Research Aircraft (HALO) missions, the HIAPER Pole-to-Pole Observations (HIPPO) mission, and the Atmospheric Tomography (ATom) mission.Observations of CH2Br2 in the free and upper troposphere indicate a pronounced seasonality in both hemispheres, with slightly larger mixing ratios in the Northern Hemisphere (NH).Compared to CH2Br2, CHBr3 in these regions shows larger variability and less clear seasonality, presenting larger mixing ratios in winter and autumn in NH midlatitudes to high latitudes.The lowermost stratosphere of SH and NH shows a very similar distribution of CH2Br2 in hemispheric spring with differences well below 0.1 ppt, while the differences in hemispheric autumn are much larger with substantially smaller values in the SH than in the NH.This suggests that transport processes may be different in both hemispheric autumn seasons, which implies that the influx of tropospheric air (“flushing”) into the NH lowermost stratosphere is more efficient than in the SH.The observations of CHBr3 support the suggestion, with a steeper vertical gradient in the upper troposphere and lower stratosphere in SH autumn than in NH autumn.However, the SH database is insufficient to quantify this difference.We further compare the observations to model estimates of TOMCAT (Toulouse Off-line Model of Chemistry And Transport) and CAM-Chem (Community Atmosphere Model with Chemistry, version 4), both using the same emission inventory of Ordóñez et al. (2012).The pronounced tropospheric seasonality of CH2Br2 in the SH is not reproduced by the models,presumably due to erroneous seasonal emissions or atmospheric photochemical decomposition efficiencies.In contrast, model simulations of CHBr3 show a pronounced seasonality in both hemispheres, which is not confirmed by observations.The distributions of both species in the lowermost stratosphere of the Northern and Southern hemispheres are overall well captured by the models with the exception of southern hemispheric autumn,where both models present a bias that maximizes in the lowest 40 K above the tropopause, with considerably lower mixing ratios in the observations.Thus, both models reproduce equivalent flushing in both hemispheres, which is not confirmed by the limited available observations.Our study emphasizes the need for more extensive observations in the SH to fully understand the impact of CH2Br2 and CHBr3 on lowermost-stratospheric ozone loss and to help constrain emissions.

more » « less
Award ID(s):
1853948 0959853
Author(s) / Creator(s):
; ; ; ; ; ; ; ; ; ; ; ; ; ;
Publisher / Repository:
European Geophysical Union
Date Published:
Journal Name:
Atmospheric Chemistry and Physics
Page Range / eLocation ID:
15049 to 15070
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. Abstract

    This study investigates changes in stratosphere‐troposphere exchange (STE) of air masses and ozone concentrations from 1960 to 2099 using multiple model simulations from Chemistry Climate Model Initiative (CCMI) under climate change scenario RCP6.0. We employ a lowermost stratosphere mass budget approach with dynamic isentropic surfaces fitted to the tropical tropopause as the upper boundary of lowermost stratosphere. The multi‐model mean (MMM) trends of air mass STEs are all small over all regions, which are within 0.3 (0.1) % decade−1for 1960–2000 (2000–2099). The MMM trends of ozone STE for 1960–2000 are 0.3%, −2.7%, 3.4%, −0.9%, and −2.7% decade−1over the Northern hemisphere (NH) extratropics, Southern hemisphere (SH) extratropics, tropics, extratropics, and globe, respectively. The corresponding ozone STE trends for 2000–2099 are 3.0%, 4.3%, 0.8%, 3.5%, and 4.7% decade−1. Changes in ozone STEs are dominated by ozone concentration changes, driven by climate‐induced changes and ozone‐depleting substance (ODS) changes. For 1960–2000, small changes in ozone STEs in the NH extratropics are due to a cancellation between effects of climate‐induced changes and ODS increases, while the ODS effect dominates in the SH extratropics, leading to a large ozone STE magnitude decrease. Increased ozone transport from tropical troposphere to stratosphere for 1960–2000 is due to increased tropospheric ozone. A decreased global ozone STE magnitude for 1960–2000 was largely caused by ODS‐induced ozone loss that is partly compensated by climate‐induced ozone changes. For 2000–2099, about two‐thirds of global ozone STE magnitude increases are caused by ozone increases in the extratropical lower stratosphere due to climate‐induced changes. The remaining one‐third is caused by ozone recovery due to the phaseout of ODS.

    more » « less
  2. Abstract

    Stratospheric ozone depletion from halocarbons is partly countered by pollution‐driven increases in tropospheric ozone, with transport connecting the two. While recognizing this connection, the ozone assessment's evaluation of observations and processes have often split the chapters at the tropopause boundary. Using a chemistry‐transport model we find that air‐pollution ozone enhancements in the troposphere spill over into the stratosphere at significant rates, that is, 13%–34% of the excess tropospheric burden appears in the lowermost extra‐tropical stratosphere. As we track the anticipated recovery of the observed ozone depletion, we should recognize that two tenths of that recovery may come from the transport of increasing tropospheric ozone into the stratosphere.

    more » « less
  3. Abstract

    Stratosphere‐Troposphere exchange (STE) of air mass and ozone in ERA5 and Modern Era Retrospective analysis for Research and Application, version 2 (MERRA2) reanalyses from 1980 to 2022 are investigated on their seasonal cycle, annual‐mean climatology, and monthly anomalies smoothed using a 1‐year Lanczos low‐pass filter. We employ a lowermost stratosphere mass budget approach with dynamic isentropic surfaces fitted to tropical tropopause as the upper boundary of lowermost stratosphere. The annual‐mean ozone STEs over the NH extratropics, SH extratropics, tropics, extratropics, and globe in ERA5 are −342, −239, 201, −581, and −380 Tg year−1, respectively, versus −305, −224, 168, −529, −361 Tg year−1from MERRA2. The annual‐mean global ozone STE difference between ERA5 and MERRA2 is dominated by the diabatic heating difference, partly compensated by the ozone concentration difference. There are about 40% (−40%) differences between ERA5 and MERRA2 in global ozone STEs in boreal summer (autumn), mainly due to the difference in seasonal breathing of the lowermost stratosphere ozone mass between reanalyses. The correlation coefficient between ERA5 and MERRA2 global ozone mass STE monthly anomalies is 0.57 and thus ERA5 and MERRA2 can only explain each other's variance by 33%. Multiple linear regression analysis shows that El Niño–Southern Oscillation, quasi‐biennial oscillation, and Brewer‐Dobson circulation explain the variance in the ERA5 (MERRA2) global ozone STE monthly anomalies by 17.3 (5.0), 5.4 (7.2), and 1.0 (3.1)%, respectively. The volcanic aerosol impacts on ozone STEs from ERA5 and MERRA2 have opposite signs and thus are inconclusive. Cautions are therefore needed when using ERA5 and MERRA2 to investigate the STE seasonal cycle and interannual variability.

    more » « less
  4. Abstract. Stratosphere–troposphere exchange (STE) is an important source oftropospheric ozone, affecting all of atmospheric chemistry, climate, and air quality. The study of impacts needs STE fluxes to be resolved by latitude and month, and for this, we rely on global chemistry models, whose results diverge greatly. Overall, we lack guidance from model–measurement metrics that inform us about processes and patterns related to the STE flux of ozone (O3). In this work, we use modeled tracers (N2O and CFCl3), whose distributions and budgets can be constrained by satellite and surfaceobservations, allowing us to follow stratospheric signals across thetropopause. The satellite-derived photochemical loss of N2O on annualand quasi-biennial cycles can be matched by the models. The STE flux ofN2O-depleted air in our chemistry transport model drives surfacevariability that closely matches observed fluctuations on both annual andquasi-biennial cycles, confirming the modeled flux. The observed tracercorrelations between N2O and O3 in the lowermost stratosphereprovide a hemispheric scaling of the N2O STE flux to that ofO3. For N2O and CFCl3, we model greater southern hemisphericSTE fluxes, a result supported by some metrics, but counter to the prevailing theory of wave-driven stratospheric circulation. The STE flux of O3, however, is predominantly northern hemispheric, but evidence shows that this is caused by the Antarctic ozone hole reducing southern hemispheric O3 STE by 14 %. Our best estimate of the current STE O3 flux based on a range of constraints is 400 Tg(O3) yr−1, with a 1σ uncertainty of ±15 % and with a NH : SH ratio ranging from 50:50 to 60:40. We identify a range of observational metrics that can better constrain the modeled STE O3 flux in future assessments. 
    more » « less
  5. Abstract

    Tropopause‐overshooting convection transports air from the lower troposphere to the upper troposphere and lower stratosphere (UTLS) where the resulting chemistry and mixing of trace gases can modify the radiation budget. While recent work has examined output from model simulations as well as aircraft and satellite observations of the impacts of tropopause‐overshooting convection on UTLS composition, the range of potential impacts and their dependence on characteristics of storms and their environments is not known. Here, two 10‐day periods, one representative of springtime convection and one representative of summertime convection, were simulated with the Weather Research and Forecasting (WRF) model with Chemistry to examine the range of UTLS composition impacts from tropopause‐overshooting convection. Overall, springtime convection has a larger impact on UTLS composition than summertime convection, with a net effect of increasing water vapor (H2O) in the lower stratosphere and increasing ozone (O3) in the upper troposphere. Springtime convection frequently increases the domain average H2O mixing ratio in the lowermost stratosphere by over 20% while changes in stratospheric H2O from summertime convection are much lower (∼7%–11% increase), reflecting a dependence of the maximum possible H2O increase on UTLS temperature. Increases in upper troposphere O3mixing ratios span the range 8%–19% from springtime convection and are minimal from summertime convection. Changes in the composition of the UTLS from tropopause‐overshooting convection are largely dependent on the height and temperature of the tropopause, with the largest changes being in environments with relatively low tropopause heights between 11 and 13 km (typical of springtime environments in the United States).

    more » « less