Clouds and wet removal as causes of variability in the trace-gas composition of the marine troposphere

Abstract

We describe a modeling study of the effects of clouds and wet removal on the chemistry of the remote marine troposphere. Using a time-dependent model with parameterized vertical transport to calculate trace-gas concentrations, we find that large variations in key species (e.g., HNO₃, H₂CO, and H₂O₂) result from simulations of sporadic rainfall, changes in cloud cover, and external inputs such as surface NO sources. Depending on the frequency and intensity of an event, the effects of these perturbations may persist for several days, thereby invalidating assumptions of photochemical equilibrium in the interpretation of measurements. Long-term integrations with fixed boundary conditions and regularly occurring cloud and rain episodes demonstrate a strong sensitivity of the mean concentration of longer-lived soluble gases to precipitation frequency but also confirm the validity of using properly chosen parameterizations of wet removal in steady state calculations. The marine atmosphere is represented in our model by selecting boundary conditions such as oceanic albedo and lower background NOx and hydrocarbons than observed over continents. The numerical model includes fairly complete gas-phase photochemistry, multiple scattering optics, and a simple parameterization of a marine boundary layer. Although a simple one-dimensional vertical transport is assumed, we demonstrate clearly that rainfall and cloud-cover changes contribute to species variability. Sensitivities to exchange rates of gases with the sea surface are also discussed.