Impact of Aerosol-Cloud Interactions on Tropospheric Ozone

Aerosol-cloud interactions may impact tropospheric ozone (O3) directly through modifications to photolysis rates and washout of precursors and indirectly through changes in climate variables such as temperature. Most previous studies of past and future O3 changes have neglected the influence of aerosol-cloud interactions. We apply the NASA Goddard Institute for Space Studies model for Physical Understanding of Composition- Climate INteractions and Impacts (G-PUCCINI) to quantify the impact of aerosol-cloud interactions on O3 radiative forcing and surface air quality for the 2030 A1B future atmosphere relative to 1995. Aerosols simulated include sulfate, organic carbon, black carbon, sea-salt and dust. The aerosol-cloud interactions simulated for liquid-phase stratus and cumulus clouds include (1) the first aerosol indirect effect: increased (decreased) cloud reflectivity due to an increase (decrease) in aerosols and cloud droplet number concentrations and reduced (increased) droplet sizes: and (2) the second aerosol indirect effect: change in cloud cover, cloud liquid water path and precipitation due to smaller droplet sizes that inhibit precipitation processes. For 2030 relative to 1995, the aerosol indirect effect is -0.69 Wm-2 mainly due to increases in cloud liquid water path at the expense of suppressed precipitation leading to increases in cloud optical depth. The global annual mean O3 radiative forcing across this time period is +0.126 Wm-2, 10% less than for experiments that do not consider aerosol-cloud interactions. Our results imply that aerosol-cloud interactions have limited the O3 forcing since the preindustrial. When aerosol-cloud interactions are included, simulated surface O3 at 2030 is decreased across the United States and South Asia and increased across Europe, around 1-2 pbbv on the annual mean. We use the model to explore the effect of the observed global decrease in reflective aerosol since 1990 on tropospheric O3 despite the counteracting influence of air quality control legislation on O3 precursors in the United States and Europe, and the simultaneous decline in the methane growth rate across the same time period.