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.
Highlights
Trace gas photochemistryin the marine tropospheremay tions depend on our understanding their allowable variabe highlyvariable in both time and space.Spatialvariability, tions
For example, that a short-lived perturbafor example, between remote marine and coastal regions, tion may disrupt the photostationary relations among some may be caused by differences in background meteorology gases for a number of days so that field measurements of and characteristicphysical and chemicalprocesses.Tempo- these speciesare difficult to rationalize without a knowledge ral variability within a given environment may result from of the recent history of the air mass being sampled
Depending on the mean cloud and precipitation frequency of a given environment, the spatial variability of these same compounds may approach an order of magnitude.Even without wet removal an extendedperiod of Chemical distributionsof trace gasesare determined as a function of altitude from a systemof one-dimensionaltransport-kinetics equations where the time rate of change of speciesj is the sum of its chemical reaction rates and flux divergence,representedby eddy diffusion: cloudinessmay causesubstantialchangesin trace gasdistributions
Summary
Trace gas photochemistryin the marine tropospheremay tions depend on our understanding their allowable variabe highlyvariable in both time and space.Spatialvariability, tions. Depending on the mean cloud and precipitation frequency of a given environment, the spatial variability of these same compounds may approach an order of magnitude.Even without wet removal an extendedperiod of Chemical distributionsof trace gasesare determined as a function of altitude from a systemof one-dimensionaltransport-kinetics equations where the time rate of change of speciesj is the sum of its chemical reaction rates and flux divergence,representedby eddy diffusion: cloudinessmay causesubstantialchangesin trace gasdistributions. These resultsare not of theoretical interest because to date measurementsof trace specieshave been so few in where z = altitude, t = time; K(z) = eddy diffusioncoefficient;N(z) = moleculardensity(cm-3) of backgroundair; Copyrig1h9t8b2ytheAmericGaneophysUicnailon.
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