Abstract
AbstractAircraft measurements, including non‐methane hydrocarbons (NMHCs), long‐lived halocarbons, carbon monoxide (CO), and ozone (O3) collected on board the NASA DC‐8 during the Deep Convection, Clouds, and Chemistry (DC3) field campaign (May – June 2012), were used to investigate interactions and mixing between stratospheric intrusions and polluted air masses. Stratospherically influenced air masses were detected using a suite of long‐lived halocarbons, including chlorofluorocarbons (CFCs) and HCFCs, as a tracer for stratospheric air. A large number of stratospherically influenced samples were found to have reduced levels of O3 and elevated levels of CO (both relative to background stratospheric air), indicative of mixing with anthropogenically influenced air. Using n‐butane and propane as further tracers of anthropogenically influenced air, we show that this type of mixing was present both at low altitudes and in the upper troposphere (UT). At low altitudes, this mixing resulted in O3 enhancements consistent with those reported at surface sites during deep stratospheric intrusions, while in the UT, two case studies were performed to identify the process by which this mixing occurs. In the first case study, stratospheric air was found to be mixed with aged outflow from a convective storm, while in the second case study, stratospheric air was found to have mixed with outflow from an active storm occurring in the vicinity of a stratospheric intrusion. From these analyses, we conclude that deep convective events may facilitate the mixing between stratospheric air and polluted boundary layer air in the UT. Throughout the entire DC3 study region, this mixing was found to be prevalent: 72% of all samples that involve stratosphere‐troposphere mixing show influence of polluted air. Applying a simple chemical kinetics analysis to these data, we show that during DC3, the instantaneous production of hydroxyl radical (OH) in these mixed stratospheric‐polluted air masses was 11 ± 8 times higher than that of stratospheric air, and 4.2 ± 1.8 times higher than that of background upper tropospheric air.
Highlights
The Earth’s stratosphere is a large reservoir of ozone (O3), which shields life from the Sun’s harmful ultraviolet radiation
Stratosphere-to-troposphere transport (STT) events are known to often occur in the vicinity of convective events, which can result in mixing of stratospheric air with convectively lofted air from the lower troposphere, including polluted air from the planetary boundary layer (PBL) [Cho et al, 2001; Stohl, 2003; Colette and Ancellet, 2006; Homeyer et al, 2011]
We investigate STT in the vicinity of convective storms and the time scale and extent to which mixing between stratospheric air and convective outflow occurs
Summary
The Earth’s stratosphere is a large reservoir of ozone (O3), which shields life from the Sun’s harmful ultraviolet radiation. As stratospheric O3 levels continue to recover due to regulation of chlorofluorocarbons (CFCs) and other O3depleting substances, the impact of STT may become even larger in the coming decades [Zeng et al, 2010] Due to their high levels of O3 and low levels of water vapor and CO, pristine stratospheric intrusions are relatively easy to detect by ground-based lidar and simple in situ surface and airborne-based measurements [Fenn et al, 1999; Bithell et al, 2000; Vaughan et al, 2001; Browell, 2003; Langford et al, 2012; Lin et al, 2012]. Certain anthropogenic halocarbons, including long-lived species like CFCs and their replacement HCFCs, are only photochemically destroyed in the stratosphere and (as a result of the Montreal Protocol) currently have minimal surface sources, even on a global scale [World Meteorological Organization, WMO/United Nations Environment Programme, UNEP, 2007] As a result, these halocarbons are evenly distributed throughout the troposphere, but relatively depleted in the stratosphere—making them ideal tracers for stratospheric air. Potential impacts on convective outflow and upper tropospheric chemistry are investigated
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
