Interpreting Measurements of the Global Diurnal Variation of Fine Particulate Matter Using the GEOS-Chem Model

Globally distributed measurements of the diurnal variation of fine particulate matter (PM_2.5) reveal a remarkable overall consistency with similar bimodal patterns and some regional variation, neither of which is well understood. We interpret these observations using the GEOS-Chem global model of atmospheric composition in its high-performance configuration (GCHP) at fine resolution of C180 (∼50 km). The base simulation overestimates the PM_2.5 accumulation overnight, leading to excessive diurnal amplitude and earlier PM_2.5 morning peaks than observations. These biases are reduced by applying sector- and species-wise diurnal scaling factors on anthropogenic emissions, by resolving the aerosol subgrid vertical gradient within the surface model layer, by applying revised wet deposition, and by revising the mixing coefficient in the boundary layer. Budget analyses indicate that the morning peak of PM_2.5 is likely driven by changes in the aerosol subgrid vertical gradient with fumigation after sunrise, that the concentration decrease until late afternoon is driven by boundary layer mixing and thermodynamic partitioning of a semivolatile aerosol to the gas phase, that the concentration increase during evening is driven by enhanced secondary chemical production and persistent primary anthropogenic emissions, and that the consistently high concentration overnight is driven by the balance between emissions, chemical production, and boundary layer mixing and deposition.