Ground-level Ozone Enhancement by Convective Storms

The Science

Ozone is an important greenhouse gas and also plays a critical role in many atmospheric chemical cycles.  In the tropical Amazon rain forest, ozone interacts with organic molecules released by trees to form aerosol particles known as secondary organic aerosol.  Understanding the drivers of the temporal and spatial distribution, sources, and sinks of ozone in the tropical boundary layer is necessary to fully understand these chemical cycles. In the Amazon basin, large-scale convective storms redistribute gases and aerosols throughout the troposphere, often enhancing ozone in the boundary layer.  In this study, scientists use data from the ARM GOAmazon2014/15 field campaign to examine how ground-level enhancement of ozone by storms depends on storm conditions.

The Impact

Ozone enhancements are analyzed as a function of wind shear, low-level jet maximum wind speed, and altitude of jet core. Strong and sudden increases in ozone levels are associated with simultaneous changes in variables such as horizontal wind speed, convective available potential energy, turbulence intensity and vertical velocity skewness. Rapid increases in vertical velocity skewness give support to the hypothesis that the ozone enhancements are directly related to downdrafts. Low-level jets associated with advancing density currents are often present during and after storm downdrafts that transport ozone-enriched air from aloft to the surface. One key conclusion derived from this study is that, in the rainforest, one must consider the influences of both mesoscale convective storms and low-level jets originating in their downdrafts on the vertical transport of ozone to the forest canopy to determine the oxidation of reactive gases such as rainforest-emitted hydrocarbons.

Summary

One objective of the present study is to investigate the atmospheric thermodynamic and dynamic conditions associated with storm-generated ozone enhancements in the Amazon rainforest. A second objective is to determine the magnitude and the frequency of ground-level ozone enhancements related to the appearance of ‘‘non-classical’’ low level jets (LLJ’s) that develop in response to those storms. This study analyzed extensive air chemistry and meteorological datasets obtained in the Amazon region, near Manacapuru, Amazonas, Brazil, during 2014/2015. These data provided better and improved statistical descriptions of the relationship between convective storm downdrafts, as evidenced in physical variables related to the dynamics and thermodynamics of the atmosphere, low-level jets caused by density currents, and ozone enhancement events. Convective systems, which are pervasive features in the Amazon region during the wet season, have the ability to transport ozone from the middle troposphere to the atmospheric boundary layer. Although it has been conjectured that the type of convective activity may be related to the strength of the ozone increase, a Richardson number criterion applied to all enhanced ozone events suggests that, on the average, different types of convective complexes have approximately the same ability to transport ozone downwards.

In particular, the observations of the skewness of the vertical velocity provide direct evidence of the role of the storm downdrafts in transporting tropospheric ozone to the surface. Other characteristic signatures of ozone-enhancement events are high values of the horizontal wind speed, high turbulence intensity and large decreases of convective available potential energy. In the majority of the cases analyzed, low-level jets formed almost simultaneously to the ozone increases and the reductions in equivalent potential temperature. Analysis of simultaneous surface data shows these jets to be ‘‘non-classical’’ (i.e., their origin is different from that of fair-weather nocturnal jets), and to be associated with advancing density currents originating in the cold air downdrafts. The presence of these jets may have important consequences to the subsequent exchanges between the atmospheric surface layer and the mixed layer above.

Principal Investigator(s)

Nelson Luis Dias
Department of Environmental Engineering, Federal University of Parana, Brazil
[email protected]

Funding

MC and JDF acknowledge support received from the Office of Biological and Environmental Research of the U.S. Department of Energy (under grant SC0011075) as part of the Atmospheric Radiation Measurement Climate Research Facility deployment as part of the GoAmazon project. CQD-Jr, NLD and MCD acknowledge support from Brazil’s CNPq’s Research Grant 401146/2014-6. JDF acknowledges support from the U.S. National Science Foundation (grant number AGS-1263225).

References

Cléo Q. Dias-Junior, Nelson Luís Dias, José D. Fuentes, and Marcelo Chamecki. 2017. “Convective storms and non-classical low-level jets during high ozone level episodes in the Amazon region: An ARM/GOAMAZON case study.” Atmospheric Environment, 155, 10.1016/j.atmosenv.2017.02.006.