Role of Wet Scavenging of HOx Precursors in DC3 Oklahoma and Alabama Thunderstorms as Determined Using Aircraft Observations and Results from WRF-Chem Simulations
Friday, 19 December 2014
In deep convective storms wet scavenging of soluble species as well as aqueous and ice chemistry affects the net transport of HOx precursors to the upper troposphere (UT), and thus impacts UT O3 production, air quality and climate. The DC3 (Deep Convective Clouds and Chemistry) field campaign took place in the central US from May-June 2012 and sampled inflow and outflow of convective storms with different dynamical and emission characteristics. This work compares wet scavenging and net transport of HOx precursors and other soluble trace gases in the DC3 May 29 Oklahoma and May 21 Alabama thunderstorms. WRF-Chem simulations at cloud resolving scales (dx=1km) are conducted with two different wet scavenging schemes. The first scheme, based on Neu and Prather (ACP, 2012), removes gases in precipitation and includes ice deposition of HNO3. However, it does not transport species dissolved in hydrometeors, and uses a constant retention fraction for soluble species during hydrometeor freezing. The second scheme, described in Barth et al. (JGR, 2001), tracks solute in individual hydrometeor classes, and includes aqueous chemistry and ice deposition of additional species. A new capability to specify the fraction of each species that is retained in ice upon hydrometeor freezing is added, and sensitivity simulations are compared with observations to determine the best estimate of the retention factor for each species. Simulated vertical distributions of trace gases of varying solubilities within the storm and immediately surrounding the storm are compared with aircraft observations in storm inflow and outflow regions. Scavenging efficiencies are calculated from the model by several flux methods and compared with scavenging efficiencies derived from observations. For the Oklahoma storm, using the Neu and Prather scheme, observed mean vertical profiles of SO2, HNO3, H2O2 and CH3OOH in outflow are better represented in the model when scavenging is included. While H2O2 is two orders of magnitude more soluble than CH3OOH, and twice as much H2O2 than CH3OOH is removed in the simulation with scavenging, its observed net transport is the same as for CH3OOH. In addition, half as much CH2O is transported in the model as observed. Conclusions are presented about the role of different microphysical processes in the removal of soluble species.