Delivery of halogenated very short-lived substances from the West Indian Ocean to the stratosphere during Asian summer monsoon

Abstract

<p><strong>Abstract.</strong> Halogenated very short-lived substances (VSLS) are naturally produced in the ocean and emitted to the atmosphere. When transported to the stratosphere, these compounds can have a significant influence on the ozone layer and climate. During a research cruise on RV <i>Sonne</i> in the subtropical and tropical West Indian Ocean in July and August 2014, we measured the VSLS, methyl iodide (CH<sub>3</sub>I) and for the first time bromoform (CHBr<sub>3</sub>) and dibromomethane (CH<sub>2</sub>Br<sub>2</sub>), in surface seawater and the marine atmosphere to derive their emission strengths. Using the Lagrangian transport model Flexpart with ERA-Interim meteorological fields, we calculated the direct contribution of observed VSLS emissions to the stratospheric halogen burden during Asian summer monsoon. Furthermore, we compare the in situ calculations with the interannual variability of transport from a larger area of the West Indian Ocean surface to the stratosphere for July 2000–2015. We found that the West Indian Ocean is a strong source region for CHBr<sub>3</sub> (910 pmol m<sup>−2</sup> h<sup>−1</sup>), very strong for CH<sub>2</sub>Br<sub>2</sub> (930 pmol m<sup>−2</sup> h<sup>−1</sup>), and average for CH<sub>3</sub>I (460 pmol m<sup>−2</sup> h<sup>−1</sup>). The atmospheric transport from the tropical West Indian Ocean surface to the stratosphere experiences two main pathways. On very short timescales, especially relevant for the shortest-lived compound CH<sub>3</sub>I (3.5 days lifetime), convection above the Indian Ocean lifts oceanic air masses and VSLS towards the tropopause. On a longer timescale, the Asian summer monsoon circulation transports oceanic VSLS towards India and Bay of Bengal, where they are lifted with the monsoon convection and reach stratospheric levels in the southeastern part of the Asian monsoon anticyclone. This transport pathway is more important for the longer-lived brominated compounds (17 and 150 days lifetime for CHBr<sub>3</sub> and CH<sub>2</sub>Br<sub>2</sub>). The entrainment of CHBr<sub>3</sub> and CH<sub>3</sub>I from the West Indian Ocean to the stratosphere during Asian summer monsoon is less than from previous cruises in the tropical West Pacific Ocean during boreal autumn/early winter, but higher than from the tropical Atlantic during boreal summer. In contrast, the projected CH<sub>2</sub>Br<sub>2</sub> entrainment was very high because of the high emissions during the West Indian Ocean cruise. The 16-year July time series shows highest interannual variability for the short-lived CH<sub>3</sub>I and lowest for the long-lived CH<sub>2</sub>Br<sub>2</sub>. During this time period, a small increase of VSLS entrainment from the West Indian Ocean through the Asian monsoon to the stratosphere is found. Overall, this study confirms that the subtropical and tropical West Indian Ocean is an important source region of halogenated VSLS, especially CH<sub>2</sub>Br<sub>2</sub>, to the troposphere and stratosphere during the Asian summer monsoon.</p>

Ocean Biogeochemistry Control on the Marine Emissions of Brominated Very Short‐Lived Ozone‐Depleting Substances: A Machine‐Learning Approach

“…The new inventory in this work also shows high oceanic CH 2 Br 2 fluxes over the Indian Ocean and South China Sea (0.6–0.9 × 10 7 molecules·cm −2 ·s −1 ), very close to that reported over the South China Sea (0.7 ± 0.6 × 10 7 molecules·cm −2 ·s −1 ; Fuhlbrügge et al, ), but is a factor of ~2 lower than that reported over the Indian Ocean (1.6 ± 3.3 × 10 7 molecules·cm −2 ·s −1 ; Fiehn et al, ). The Ordóñez‐2012 inventory shows less than 0.5 × 10 7 molecules·cm −2 ·s −1 in the entire region, lower than that reported in Fuhlbrügge et al () or Fiehn et al (). More surface seawater observations in this region will certainly improve the performance of the machine‐learning emulator, as well as the bottom‐up oceanic emission inventory for CHBr 3 and CH 2 Br 2 .…”

“…The Ordóñez‐2012 inventory also has fixed values in the subtropical, midlatitude, and high‐latitude oceans. We noted that the new bottom‐up oceanic emission inventory of CHBr 3 shows high sea‐to‐air flux over the Indian Ocean, South China Sea, and Java Sea (2–3 × 10 7 molecules·cm −2 ·s −1 ), which is comparable to the oceanic CHBr 3 flux reported over the South China Sea (2.5 ± 2.9 × 10 7 molecules·cm −2 ·s −1 ; Fuhlbrügge et al, ), but toward the upper end of that over the Indian Ocean (1.5 ± 2.0 × 10 7 molecules·cm −2 ·s −1 ; Fiehn et al, ). The Ordóñez‐2012 inventory, however, shows approximately 1.5–2.0 × 10 7 molecules·cm −2 ·s −1 , in this region, comparable to that reported in Fiehn et al (), but is toward the lower end of that in Fuhlbrügge et al ().…”

“…We noted that the new bottom‐up oceanic emission inventory of CHBr 3 shows high sea‐to‐air flux over the Indian Ocean, South China Sea, and Java Sea (2–3 × 10 7 molecules·cm −2 ·s −1 ), which is comparable to the oceanic CHBr 3 flux reported over the South China Sea (2.5 ± 2.9 × 10 7 molecules·cm −2 ·s −1 ; Fuhlbrügge et al, ), but toward the upper end of that over the Indian Ocean (1.5 ± 2.0 × 10 7 molecules·cm −2 ·s −1 ; Fiehn et al, ). The Ordóñez‐2012 inventory, however, shows approximately 1.5–2.0 × 10 7 molecules·cm −2 ·s −1 , in this region, comparable to that reported in Fiehn et al (), but is toward the lower end of that in Fuhlbrügge et al (). The reason that the new inventory in this work shows high CHBr 3 flux is that one cruise study in the Bay of Bengal (6.87–11.96°N, 88.07–88.26°E) reported high CHBr 3 levels in the surface noncoastal waters (46 ± 10 pmol/L; Yamamoto et al, ); as a result the machine‐learning algorithm predicted high surface seawater concentrations in that region as well (Figure ).…”

Differences in tropical high clouds among reanalyses: origins and radiative impacts

“…These coupled interactions transport energy away from convective regions, which tend to be anchored over the warmest SSTs, into subsidence-dominated regions where SSTs are usually cooler. Associated tracer transports have extensive influences on humidity, ozone, and other constituents in the upper troposphere (Folkins et al, 2002;Jiang et al, 2007;Fiehn et al, 2017;Pan et al, 2017), while momentum transport, latent heat release, and radiative effects modulate circulation patterns in both the troposphere and stratosphere (LeMone et al, 1984;Carr and Bretherton, 2001; Lane and Moncrieff, 2008;Geller et al, 2016;Kim et al, 2017). Changes in precipitation are governed to leading order by the balance of changes in radiative cooling and condensational heating in the atmosphere (O'Gorman et al, 2011), both of which are intimately connected with the distribution and properties of high clouds.…”

Spatio‐temporal stability analysis applied to monsoon anticyclone flow

“…Then from the upper troposphere air may be taken up into the tropical stratosphere via the large‐scale upwelling circulation, or alternatively be transported laterally into the extratropical lower stratosphere through disturbances to the subtropical jet that depend on the time variation of the anticyclone itself. Thus the lower and upper parts of the monsoon combine to form a potentially rapid pathway for chemical species and aerosols originating from anthropogenic emissions at the surface in the south and east Asian regions to reach the stratosphere, with implications for climate and for ozone, since the chemical species include short‐lived halogen species (Park et al ., ; Randel et al, ; Bourassa et al, ; Fiehn et al, ).…”