Science Highlight

Lightning NOx-impacted convection during the DC3 campaign

Apel, E. C., R. S. Hornbrook, A. J. Hills, M. C. BarthA. WeinheimerC. CantrellS. A. RutledgeB. BasarabJ. CrawfordG. DiskinC. R. HomeyerT. CamposF. FlockeA. FriedD. R. BlakeW. BruneI. PollackJ. PeischlT. RyersonP. O. WennbergJ. D. CrounseA. WisthalerT. MikovinyG. HueyB. HeikesD. O'Sullivan, and D. D. Riemer (2015), Upper tropospheric ozone production from lightning NOx-impacted convection: Smoke ingestion case study from the DC3 campaign. J. Geophys. Res. Atmos., 120, 2505–2523. doi: 10.1002/2014JD022121.

The DC3 field campaign, co-led by investigators from NCAR, Penn State University and Colorado State University, took place is spring/summer 2012. The campaign, involving both instrumented aircraft flights and ground-based measurements, investigated the impact of deep, mid-latitude continental convective clouds on upper tropospheric (UT) composition and chemistry. A primary motivation of DC3 was to determine the role of these convective events on the concentration of ozone in the UT, where ozone acts as a greenhouse gas. The aircraft (NSF/NCAR G-V, NASA DC8 and DLR Falcon) were based in Salina, Kansas, while networks of ground-based instrumentation were deployed in Colorado, Oklahoma and Alabama in support of the project.

One of the goals of DC3 was to characterize the processing of chemical constituents by thunderstorms. The 22 June 2012 case study in northeastern Colorado provided the opportunity to contrast the composition of the anvil outflow from a storm ingesting a wildfire smoke plume to that without fresh smoke emissions. Figure 1 shows satellite imagery of the two storms and the smoke plume as well as the flight tracks of the GV (white trace)and DC-8 (black trace) aircraft. Two tracers of smoke emissions were measured by TOGA, acrolein, a unique short-lived tracer, and HCN, a long-lived tracer; both tracers were shown to have very high values in the outflow of the northern storm (purple oval area) compared to the southern storm. Nitric oxide (NO) mixing ratios were much higher in the southern storm outflow because of the greater lightning activity of the southern storm.

Fig 1

Figure 1. Flight tracks from the 22 June 2012 DC3 Smoke Ingestion Case. Top left panel shows the DC-8 (black) and GV (white) flight tracks overlaid on the visible satellite imagery (the smoke plume originates at the green dot.) Two thunderstorms are evident, the northern storm in western Nebraska ingesting the smoke plume, and the southern storm in northeast Colorado. Top right panel shows mixing ratios from a biomass-burning marker of fresh emissions, acrolein, with high mixing ratios measured from the northern storm. Lower left panel show the HCN mixing ratios with elevated levels in both storm outflows. Lower right panel shows NO mixing ratio with higher values from the southern storm, which had more lightning occurring during this time period. The regions outlined in purple is the outflow area of the northern storm, in gold is the outflow area of the southern storm, and in blue the background upper troposphere.

Box modeling predicts substantial downwind ozone production in the upper troposphere for both the fresh biomass burning (BB)-impacted northern storm (NS) and more aged BB-impacted southern storm (SS). The SS was predicted to produce more ozone over two days (14 ppbV) than the NS (11 ppbV) despite having lower VOC OH reactivity. Sensitivity tests showed that this is principally due to more NOx being present in the SS outflow from lightning. In the absence of lightning NOx, the SS is predicted to produce only 2 ppbV of ozone over two days but with lightning NOx, a much higher ozone production efficiency leads to the predicted 14 ppbV ozone.

 CONTRAST photo

Convective storm anvil during DC3.

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