1. Troposphere-to-Stratosphere Transport of VSLS
The impact of very-short-lived bromocarbons (VSLB) on stratospheric composition: Present and future Qing Liang1 and Johan Schmidt NASA Goddard Space Flight Center/USRA GESTAR, Greenbelt, MD USA 2Department of Chemistry, Copenhagen University
Introduction
Troposphere-to-Stratosphere Transport of VSLS $5
Convective transport of oceanic emissions of brominated very-short-lived substances (VSLS) from the surface to the stratosphere exerts significant impacts on the stratospheric bromine budget and ozone depletion. Climate change is expected to influence the frequency and intensity of deep convection, thus impact the contribution of VSLS to stratospheric inorganic bromine (BryVSLS) abundance.
How does climate change impact of the convective transport of VSLS into the stratosphere and the wet scavenging of soluble inorganic bromine from VSLS?
Majority of TST of VSLB happens in the tropical deep convection region. with maximum centers in the (i) Indian Ocean, (ii) Western Pacific, and (iii) off the coast of Latin America.
Figure 1. seasonal mean distribution of simulated CHBr3 (unit ppt Br) at the 355K potential temperature layer (just below tropopause) in DJF, MAM, JJA and SON. The black boxes outline the three regions where most active troposphere-to-stratosphere transport occurs. (From Liang et al., 2014)
Model description and simulations: GEOS-5 CCM
Full stratospheric chemistry simulation with five major brominated VSLS (CHBr3, CH2Br2, CHBr2Cl, CHBrCl2, CH2BrCl), scavenging of HBr, HOBr and BrONO2 in large-scale and convective rainout/washout (Liang et al., 2010, 2014).
Simulations:1°x1° resolution and 72 layers
A present-day simulation: A 35-year standard simulation ( ).
Two future Timeslice simulations
A15-year Timeslice Run in 2100 with present-day climatological rain
A 15-year Timeslice Run in 2100 with real future climate precipitation
Tropical convective transport of VSLB in the 2100s’ appear similar to present-day conditions, but features slightly weaker transport into UT/LS over Latin America.
Figure 2. Longitude-height cross-section of modeled total VSLS organic bromine (color contours) and streamlines (black lines) in the deep tropics.
Convective transport and scavenging of organic and inorganic Br: Present vs. Future
Figure 3. The tropical vertical profiles of the contribution of organic and inorganic bromine from VSLS: (left) present day simulation, (middle) 2100 future simulation with present-day climatological rain, (right) 2100 future simulation with real precipitation.
Figure 5. Longitudinal distribution of total organic VSLS bromine (blue shadings for present-day model results) and BryVSLS (gray shadings for present-day model results) in the tropical lower troposphere (lower panel) and UT/LS (upper panel). Model total VSLS organic and inorganic bromine from the 2100 simulation are shown in thick green and dark green lines, respectively. The observed total organic VSL bromine (compiled from all previous NASA and NSF aircraft missions) are shown in red.
GEOS-5 model simulations suggest that:
Convective transport of VSLS bromine increases in future climate
Wet scavenging of inorganic bromine increases in future climate for two reasons:
Changes in atmospheric composition (increases in OH and HO2) will change the HOBr/HBr vs. BrONO2 partition
Increases in large-scale and convective precipitation.
Figure 4. A schematic of inorganic halogen chemistry (adapted from Carpenter and Reimann et al., 2014)
Climate can impact convective transport of VSLS in several ways:
Convective transport of organic bromine increases with future climate (~ +0.3 ppt Br increase) and transport outflow reaches higher altitudes
The increase in OH and HO2 will alter the partition of BryVSLS, with more BryVSLS in the presence of HBr/HOBr instead of BrONO2. This leads to an increase in wet scavenging of BryVSLS in troposphere (~ -0.5 ppt Br).
Convective scavenging of BryVSLS increases with future climate (~ -0.7 ppt Br)
Regional convective transport (western Pacific vs. N. America) are likely different in future climate
Model vs. observations: the implications
Aircraft observations show that the Gulf of Mexico show particularly high levels of VSL-Br, ~ 13 ppt at the surface and ~8-10 ppt at 200 hPa, compared to average of ~ 5 ppt throughout the tropics. How convective lofting over the subtropical/tropical America vary with climate change is of great importance in understanding the impact of convective transport of VSLS into the stratosphere.
Summary
The majority TST of VSLB occurs in tropical deep convection regions: Indian Ocean, Western Pacific, off the coast of Latin America
Convective lofting of VSLS increases in future climate. On the other hand, wet scavenging of inorganic product gases also increases in future climate. A net decrease of ~1 ppt Br.
The relative importance of convective transport into the UT/LS will likely be different in future climate. The tropical America is particularly of interest due to high VSL bromine abundance.
References:
Carpenter, L. J., and S. Reimann (Lead Authors), J. B. Burkholder, C. Clerbaux, B. D. Hall, R. Hossaini, J. C. Laube, and S. A. Yvon-Lewis, Ozone-depleting substances (ODSs) and other gases of interest to the Montreal Protocol, Chapter 1 in Scientific Assessment of Ozone Depletion: 2014, Global Ozone Research and Monitoring Project – Report No. 55, World Meteorological Organization, Geneva, Switzerland, 2014.
Liang Q., R. S. Stolarski, S. R. Kawa, J. E. Nielsen, J. M. Rodriguez, D. R. Blake, E. L. Atlas, and L. E. Ott, Finding the missing stratospheric Bry: A global modeling study of CHBr3 and CH2Br2, Atmos. Chem. and Phys., 10, , 2010.
Liang, Q., E. Atlas, D. R. Blake, M. Dorf, K. Pfeilsticker, S. Schauffler, Convective transport of very-short-lived bromocarbons to the stratosphere, Atmos. Chem. Phys., 14, , 2014.