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. The interactions between biogenic volatile organic compounds(BVOCs), like isoprene and monoterpenes, and anthropogenic emissions ofnitrogen and sulfur oxides lead to high concentrations of secondary organicaerosol (SOA) in the southeastern United States. To improve our understandingof SOA formation, we study the diurnal evolution of SOA in a land–atmospherecoupling context based on comprehensive surface and upper air observationsfrom a characteristic day during the 2013 Southern Oxidant and Aerosol Study(SOAS) campaign. We use a mixed layer model (MXLCH-SOA) that is updated withnew chemical pathways and an interactive land surface scheme that describesboth biogeochemical and biogeophysical couplings between the land surface andthe atmospheric boundary layer (ABL) to gain insight into the drivers of thedaytime evolution of biogenic SOA. MXLCH-SOA reproduces observed BVOC and surface heat fluxes, gas-phasechemistry, and ABL dynamics well, with the exception of isoprene andmonoterpene mixing ratios measured close to the land surface. This is likelydue to the fact that these species do not have uniform profiles throughoutthe atmospheric surface layer due to their fast reaction with OH andincomplete mixing near the surface. The flat daytime evolution of the SOAconcentration is caused by the dampening of the increase due to locallyformed SOA by entrainment of SOA-depleted air from the residual layer. SOAformation from isoprene through the intermediate species isoprene epoxydiols(IEPOXs) and isoprene hydroxyhydroperoxides (ISOPOOHs) is in good agreementwith the observations, with a mean isoprene SOA yield of 1.8 %. However, SOA from monoterpenes, oxidised by OH and O3 , dominatesthe locally produced SOA (69 %), with a mean monoterpene SOA yield of10.7 %. Isoprene SOA is produced primarily through OH oxidation viaISOPOOH and IEPOX (31 %). Entrainment of aged SOA from the residual layerlikely contributes to the observed more oxidised oxygenated organic aerosol(MO-OOA) factor. A sensitivity analysis of the coupled land surface–boundary layer–SOAformation system to changing temperatures reveals that SOA concentrations arebuffered under increasing temperatures: a rise in BVOC emissions is offsetby decreases in OH concentrations and the efficiency with which SVOCspartition into the aerosol phase.

Biogenic emissions and land–atmosphere interactions as drivers of the daytime evolution of secondary organic aerosol in the southeastern US