1E: Stratospheric Contributions to the Tropical Midtropospheric Ozone

Tropospheric ozone (O3) plays an important role in both atmospheric chemistry and chemistry-climate interactions. Understanding the controlling mechanisms of tropospheric O3 requires integrated studies of both observations and modeling. During the field campaign Convective Transport of Active Species in the Tropics (CONTRAST, January–February 2014), the in situ observations from the NSF/NCAR Gulfstream V (GV) research aircraft revealed a large region over the tropical western Pacific where the midtroposphere had a layered structure with high ozone and low water vapor. Analysis revealed a bimodal ozone distribution in the region (Pan et al., 2015). The processes that produced the dry and ozone rich layer are a topic of research, especially the relative roles of the transport from the stratosphere and the ozone production in the troposphere following biomass burning.

As part of collaborative research between NCAR ACOM and the Juelich Research Center (Germany), a model investigation integrated with data analysis is conducted to understand the roles and quantify the processes revealed by the observations. Using the Chemical Lagrangian Model of the Stratosphere (CLaMS), developed at Juelich, and the NCAR CAM-Chem (Community Atmospheric Model with chemistry, version 4) simulations, the contribution of transported stratospheric air to the observed ozone rich layers is quantified. Using the modeled stratospheric tracer and 3-D back trajectories, this work identified that 60% of the observed ozone-rich layer air masses contain significant stratospheric influence. Figure 1 shows the result of model simulation from both CLaMS and CAM-chem. This result indicates that the isentropic transport is an effective process for stratospheric air to mix into the tropical midtroposphere.

This work also examined possible contribution to the high ozone layer from ozone production related to biomass burning emissions. Clear chemical signature of this process is found in ∼8% of the ozone-rich air masses in the tropical midtroposphere, identified by positive correlations among O3, HCN, and CO. This chemical signature is highlighted in Figure 2.

Overall, this analysis provides the first quantitative diagnosis of the contribution from the stratosphere-to-troposphere transport in the CONTRAST airborne observations, highlights the importance of large-scale transport and mixing that couples the stratosphere and troposphere. This work has been published in JGR-Atmosphere (Tao et al., 2018).

Figure 1. Stratospheric contributions.
Figure 1. Left panel shows the percent of stratospheric air in the tropical mid troposphere (320 K potential temperature surface) from the CLaMS-2D model simulation. Right panel shows the  mixing ratio of stratospheric O3 from NCAR CAM-chem (Community Atmospheric Model with chemistry, version 4) simulation for 29 January 2014. GV flight track for research flight RF07 on the same day is shown as the thick gray lines. Additional information for the dynamics is shown for 3 PVU isoline (solid yellow contours), which identify the boundary of stratosphere and troposphere on this potential temperature surface, and the jet stream location (red contours, horizontal wind 40 m/s).  (from Tao et al. 2018) Click for larger image.
Figure 2. Stratospheric Contributions
Figure 2. The chemical signature of biomass burning emission-facilitated ozone production is shown by positive correlations between the two pairs of species HCN-CO and O3-CO, as well as the elevated CO.  In this figure, there is a clear distinction between samples with CO less or greater then 105 ppbv. The high CO group shows positive correlations (the 10-s averaged observations during RF10 and RF11 (gray crosses)), and the lower CO group do not show a clear relationship between tracers (the 60-s averaged observations marked by circles, both blue and red).  See Tao et al., (2018) for more details. Click for larger image.

References:

Pan, L.L., S. B. Honomichl, W. J. Randel, E. C. Apel, E. L. Atlas, S. P. Beaton, J. F. Bresch, R. Hornbrook, D. E. Kinnison, J-F Lamarque, A. Saiz-Lopez, R. J. Salawitch, and A. J. Weinheimer, Bimodal distribution of free tropospheric ozone over the tropical western Pacific revealed by airborne observations, Geophys. Res. Lett., 42, 7844–7851, doi: 10.1002/2015GL065562.

Tao, M., Pan, L. L., Konopka, P., Honomichl, S. B., Kinnison, D. E., & Apel, E. C. (2018). A Lagrangian model diagnosis of stratospheric contributions to tropical midtropospheric air. Journal of Geophysical Research: Atmospheres, 123, 9764–9785. https://doi.org/10.1029/2018JD028696.