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Analysis of the Effects of Midlatitude Deep Convection on the Composition and Chemistry of the Upper Troposphere/Lower Stratosphere Using Airborne Measurements of VOCs and other Trace Gases

  • Author(s): Schroeder, Jason
  • Advisor(s): Blake, Donald R
  • et al.

Measurements of trace gases were taken onboard the NASA DC-8 during the Deep Convective Clouds and Chemistry (DC3) field project with the goal of understanding the role that midlatitude deep convection plays in altering the vertical distribution of atmospherically-relevant species. Measurements of VOCs were obtained via UC Irvine’s whole air sampler (WAS) instrument, while measurements of CH4, O3, NOx, N2O, water vapor, CO, and meteorological variables were performed by a variety of other instruments operated by collaborators onboard the DC-8.

Using known VOC atmospheric lifetimes and measured VOC mixing ratios in the PBL, a tracer for rapid vertical lofting of air from the planetary boundary layer (PBL) to the upper troposphere/lower stratosphere region (UT/LS) by convection was created. In this study, it was found that light hydrocarbons associated with oil and natural gas (O&NG) and vehicular sources were widespread throughout the PBL of the DC3 study regions. In the UT/LS, enhanced levels of these light hydrocarbons were strongly correlated with water vapor, indicating a convective source. On the other hand, decreases in the measured mixing ratios of CFCs, HCFCs, and other long-lived halocarbons (LLHCs) in the UT were used as tracers for stratosphere-to-troposphere transport (STT). These two sets of tracers were used to divide the DC3 WAS merge into many subsets of data corresponding to: the PBL, convective outflow in the UT, convective outflow in

the LS (i.e. overshooting tops), STT-influenced air in the troposphere, background UT air, and background LS air.

Using these derived subsets of data, interactions and mixing between stratospheric intrusions and tropospheric air masses was investigated. 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 tracers of anthropogenically-influenced air, it is shown that this type of mixing was present both at low altitudes and in the 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, it was concluded 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, it is shown that the instantaneous production of OH in these mixed stratospheric-polluted air masses was 11 ± 8 (± 1σ) times higher than that of stratospheric air, and 4.2 ± 1.8 times higher than that of background upper tropospheric air, which could result in a quick, high magnitude pulse of O3 production and reduced lifetimes of OH-controlled species in the UT.

These derived subsets of data were used to investigate the effects of deep convection on mixing ratios of organic chlorine and organic bromine in the UT and LS over the DC3 study region. In the LS, it was found that mixing ratios of organic chlorine in overshooting tops were higher than mixing ratios of organic chlorine in the background LS by an average of 217 ± 179 pptv (6.3 ± 5.2% enhancement), while the total organic bromine mixing ratio in overshooting tops was higher than that of the background LS by an average of 2.8 ± 3.2 pptv (17.4 ± 19.9% enhancement). In both cases, short-lived halocarbons made up a large portion of this enhancement. In the UT, convection was found to play a much more complicated role on the organic halogen content of the region. Model back trajectories and analysis of the chemical composition of the background UT revealed that long-range transport of outflow from East Asia and from the central Pacific affected the background UT of the DC3 study region to varying degrees on different days. When the background UT was affected by East Asian outflow, mixing ratios of organic chlorine in convective outflow were lower than those of the local background UT by up to 150 ± 115 pptv (3.7 ± 2.9% enhancement). On the other hand, when the background UT was affected by clean outflow from the central Pacific, mixing ratios of organic chlorine in convective outflow were higher than the local background UT by up to 115 ± 98 pptv (3.2 ± 2.6% enhancement). Mixing ratios of organic bromine in the background UT were unaffected by these long-range transport processes. However, mixing ratios of organic bromine in convective outflow were highly variable and were affected by the transport of brominated very short-lived halocarbons (VSLH) from the Gulf of Mexico to the surface of the DC3 study region. When organic bromine enhancements in convective outflow were calculated on a flight-by-flight basis, organic bromine mixing ratios in convective outflow were higher than those in the background UT by an average of 1.7 ± 1.6 pptv (8.5 ± 8.1%). Based on these results, it is speculated that

deep convection may play an indirect role in climate change by introducing pulses of short-lived halocarbons into the UT/LS, which in turn result in relatively quick (on the order of a few months) pulses of O3 loss in the region.

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