A large‐eddy simulation of the phase transition of ammonium nitrate in a convective boundary layer

Abstract

[1] Under warm and dry conditions, ammonium nitrate aerosol outgasses to form ammonia and nitric acid in the lower atmospheric boundary layer. In the upper boundary layer, where the temperature is lower and the relative humidity is higher, nitric acid and ammonia condense back to the aerosol phase. Measurements show that aerosol nitrate mixing ratios increase with altitude, confirming this phase transition. Since phase equilibrium is not reached instantaneously, updrafts transport aerosol-poor air from the surface to high altitudes and aerosol-rich air subsides from high altitudes to the surface under turbulent conditions. As a result, the partitioning deviates from equilibrium, so the horizontal and temporal variabilities of the aerosol nitrate mixing ratio are enhanced and a continuous downward aerosol nitrate flux emerges. We postulate that observations of this variability and flux should not be interpreted as instrument noise and deposition of nitrate. In an idealized large-eddy simulation (LES) experiment of a convective boundary layer, we find that the larger variability and flux occurred at about one third of the boundary layer height. Both are largest when the gas-aerosol partitioning time scale is assumed to be about half the time scale of turbulence. Our LES result shows negatively skewed nitrate mixing ratios. Under colder conditions, a smaller fraction of ammonium nitrate aerosol outgasses at the surface, so the absolute effect of nitrate repartitioning becomes smaller. However, dimensionless statistical properties do not change as long as the turbulent properties of the boundary layer remain similar. This indicates that the identified processes are also present under colder conditions. Citation: Aan de Brugh, J. M. J., H. G. Ouwersloot, J. Vila`-Guerau de Arellano, and M. C. Krol (2013), A large-eddy simulation of the phase transition of ammonium nitrate in a convective boundary layer,

Numerical simulation of the interaction between ammonium nitrate aerosol and convective boundary-layer dynamics

“…The dependence of the gas-aerosol nitrate equilibrium on absolute temperature and relative humidity is carefully discussed in Mozurkewich (1993) and Nenes et al (1998). The equilibrium is obtained provided the total concentrations of nitrate and ammonia, (TNO 3 and TNH 4 , respectively) absolute temperature and relative humidity (Mozurkewich, 1993;Aan de Brugh et al, 2013). These fields are dynamically obtained from the LES and serve as input parameters for the ISORROPIA2 calculations.…”

“…Several studies estimated that t eq for aerosol nitrate ranges from a few seconds to several minutes, i.e. t eq is of similar order of magnitude as the turbulent timescale of the CBL (Dassios and Pandis, 1999;Morino et al, 2006;Aan de Brugh et al, 2013). In such circumstances, non-linearities between CBL dynamics and chemistry are expected (Fitzjarrald and Lenschow, 1983;Krol et al, 2000;Vinuesa and Vil a-Guerau de Arellano, 2003).…”

“…Extending previous studies (Vinuesa and Vil a-Guerau de Arellano, 2003; Vil a-Guerau de Arellano et al, 2005;Aan de Brugh et al, 2013;Barbaro et al, 2014) we present here a Large-Eddy Simulation (LES) modeling framework. The novelty of this study lies in the dynamical coupling of the CBL (thermo)dynamics and turbulence with the surface, radiation, chemistry, and aerosols.…”

“…According to previous studies (Harrison and Pio, 1983;Mozurkewich, 1993;Morino et al, 2006;Fountoukis and Nenes, 2007;Aan de Brugh et al, 2013) the physical mechanisms that drive the gas-aerosol nitrate spatial distribution are: (i) the availability of NH 3 and SO 2À 4 , (ii) CBL (thermo)dynamics, (iii) gas-aerosol equilibration timescale (t eq ), and (iv) dry deposition of gas phase nitrate. As discussed in previous work (Morino et al, 2006;Fountoukis and Nenes, 2007;Aan de Brugh et al, 2012), t eq is the effective timescale required for ammonia and gaseous nitric acid to reach equilibrium with the inorganic aerosol particles.…”

“…Aan de Brugh et al (2013) have shown by means of an LES (albeit without accounting for deposition effects and chemistry) that fast vertical mixing in the CBL in combination with a temperaturedependent partitioning of atmospheric nitrate between the gas and aerosol phases lead to interactions between dynamics and aerosol formation. Close to the top of the CBL (cooler) gaseous nitric acid (henceforth called g HNO 3 ) and ammonia (NH 3 ) condense on A NO 3 , thus the gas-aerosol equilibrium shifts towards the aerosol phase.…”