Clouds and sulfate are anticorrelated: A new diagnostic for global sulfur models

[1] We consider the correlation between clouds and sulfate in order to assess the relative importance of cloud aqueous-phase production of sulfate, precipatation scavenging of sulfate, and inhibition of gas-phase sulfate production beneath clouds. Statistical analysis of observed daily cloud cover and sulfate surface concentrations in Europe and North America indicates a significant negative correlation between clouds and sulfate. This implies that clouds remove sulfate via precipitation scavenging and/or inhibit sulfate gas-phase production more than they enhance sulfate concentration through aqueous-phase production. Persistent sulfate/cloud anticorrelations at long timescales (8 - 64 days) apparently result from large-scale dynamical influences on clouds, which in turn impact sulfate. A statistical analysis of output from the general circulation model (GCM) of the Goddard Institute for Space Studies (GISS) shows weak coherence between sulfate and cloud cover. However, there is stronger anticorrelation between the model's sulfate generated by gas-phase oxidation and cloud cover. Sulfate/cloud anticorrelation in the GCM strengthens if we extinguish gas-phase sulfate production beneath clouds, as should happen since the oxidant OH is photochemically generated. However the only way to achieve strong anticorrelation between total sulfate and clouds is by correcting our treatment of aqueous-phase sulfate production. Our model, like many other global tracer models, released dissolved species ( including sulfate) from clouds after each cloud time step rather than making release contingent upon cloud evaporation. After correcting this in the GISS model, more sulfate is rained out, the sulfate burden produced via the aqueous phase decreases to half its former amount, and the total sulfate burden is 25% lower.