Single-Column Modelling of Isoprene and Monoterpene Evolution Following Convective Events in Amazonia

The Amazon region frequently experiences intense and extensive mesoscale convective systems (MCSs). Downdraft regions of MCSs transport dry and cold air to the surface, creating cold pools whose vertical extent can reach 2-4 km. Air turbulence abruptly increases throughout the atmospheric boundary layer (ABL) immediately before and after the passage of gust fronts associated with these systems. Storm downdrafts reduce the equivalent potential temperature (qE) at the surface, leading to the development of stable air layers that can last for several consecutive hours during the daytime.

Downdrafts of MCSs transport ozone (O₃)-rich air from the mid-troposphere to the Earth's surface and can increase ground-level O3 by as much as 40 parts per billion. Multiple MCSs concomitantly occur in the Amazon basin and can enrich the regional ABL with O₃ for several consecutive hours. Given that the Amazon rainforest represents the most expansive and contiguous region of the world with the largest and the most diverse emissions of biogenic volatile organic compounds (BVOCs), such regional enhancements of O₃ in the ABL can influence several atmospheric chemical cycles. Following occurrences of MCSs, environmental conditions are suitable to promote BVOC emissions from the rainforest (Gerken et al. 2016).

After the passage of MCSs, the prevailing subsidence and associated elevated O₃ mixing ratios provide the necessary conditions to accelerate the oxidation of rainforest-emitted isoprene, monoterpenes, and sesquiterpenes. Following the MCSs, the chemical processes entailing BVOCs can take place in relatively shallow stable boundary layers whose destabilization can take several hours ensuing the passage of storms.

In the present work, we determine the processes governing temporal and vertically resolved patterns of isoprene and monoterpenes as a function of the destabilization rates of stable ABL that forms in response to downdrafts of MCSs in the central Amazon. In doing that, we estimate rates of isoprene and monoterpenes detrainment out of the stable ABL due to turbulent transport and chemical reactions occurring under the influences of the enhanced O₃ levels associated with storm downdrafts. Given the simultaneous daytime emissions of isoprene and monoterpenes in the rainforest, we also determine the feedbacks generated between ozonolysis of monoterpenes and chemical destruction of isoprene via its reaction with the hydroxyl radical (OH).

The research objectives are achieved through the implementation of a one-dimensional ABL model that resolves both chemical reactions and turbulent transport of mass, energy, and momentum. The model solves prognostic equations for the mean horizontal wind components, temperature, and turbulent kinetic energy (TKE), used in the formulation of eddy diffusivities. A total of 26 chemical reactions are represented. Simulations are performed using observed initial conditions of meteorology and chemistry, observed in 2014 during GO-Amazon field study. The post-convective environment is simulated imposing a cold layer with different thickness, and the temporal evolutions of the profiles of the VOCs are determined as a function of both this thickness and the large-scale geostrophic winds.