Surface Water Formation on the Natural Surface under Super-saturation: from Local Water Balance to Air Pollutant Deposition

TL;DR

This study investigates how super-saturation leads to surface water formation like dew and frost on natural surfaces, and how this process also helps in removing air pollutants through particle scavenging.

Contribution

It provides a parameterization of super-saturation conditions on natural surfaces based on field observations, linking water formation to air pollutant deposition mechanisms.

Findings

Super-saturation ratio approaches 1, driven mainly by thermophoresis deposition.

Surface water formation occurs on both ground and vegetation due to moisture sources.

SWF acts as a weak air pollutant cleaner, removing particles like SO4 and NO3.

Abstract

Heterogeneous nucleation and subsequent growth of surface water occur on the natural substrate when the water vapor concentration reached the point of super-saturation. This study focuses on the parameterization of super-saturation on the canopy-air interface by field observations monitoring surface water formation (SWF) such as dew and frost in the evergreen shrub at an urban cite during autumn and winter in 2015-2017. Here we show that both the interfacial and vertical temperature differences ranged from 1 to 3 K and were necessary but not sufficient for super-saturated condensation on the natural surface. Excessive supplies of moisture must exist, continuously contribute to the growth of the condensed water embryos, originate from both the local and the external sources such as evapotranspiration and atmospheric advection driven by the reduced air pressure, cause SWF not only on the ground soil but also on the vegetation canopy at 1-2 m height. The super-saturation ratio is mainly determined by the coefficient of thermophoresis deposition, which approaches to 1. SWF on the natural surface is not only an indicator but also a weak cleaner of air pollution. The downward thermophoresis deposition of fine particle and droplets favors SWF and the scavenging of air pollutants. The removal efficiency of the deposition flux during SWF event for [SO42-+NO3-] is estimated by ~0.3 mmol (per [Ca2+] meq)/m2(per leaf area).