Biogenic emissions and land–atmosphere interactions as drivers of the daytime evolution of secondary organic aerosol in the southeastern US
2018
Abstract. The interactions between biogenic volatile organic compounds
(BVOCs), like isoprene and monoterpenes, and anthropogenic emissions of
nitrogen and sulfur oxides lead to high concentrations of secondary organic
aerosol (SOA) in the southeastern United States. To improve our understanding
of SOA formation, we study the diurnal evolution of SOA in a land–atmosphere
coupling context based on comprehensive surface and upper air observations
from a characteristic day during the 2013 Southern Oxidant and Aerosol Study
(SOAS) campaign. We use a mixed layer model (MXLCH-SOA) that is updated with
new chemical pathways and an interactive land surface scheme that describes
both biogeochemical and biogeophysical couplings between the land surface and
the atmospheric boundary layer (ABL) to gain insight into the drivers of the
daytime evolution of biogenic SOA. MXLCH-SOA reproduces observed BVOC and surface heat fluxes, gas-phase
chemistry, and ABL dynamics well, with the exception of isoprene and
monoterpene mixing ratios measured close to the land surface. This is likely
due to the fact that these species do not have uniform profiles throughout
the atmospheric surface layer due to their fast reaction with OH and
incomplete mixing near the surface. The flat daytime evolution of the SOA
concentration is caused by the dampening of the increase due to locally
formed SOA by entrainment of SOA-depleted air from the residual layer. SOA
formation from isoprene through the intermediate species isoprene epoxydiols
(IEPOXs) and isoprene hydroxyhydroperoxides (ISOPOOHs) is in good agreement
with the observations, with a mean isoprene SOA yield of 1.8 %. However, SOA from monoterpenes, oxidised by OH and O 3 , dominates
the locally produced SOA (69 %), with a mean monoterpene SOA yield of
10.7 %. Isoprene SOA is produced primarily through OH oxidation via
ISOPOOH and IEPOX (31 %). Entrainment of aged SOA from the residual layer
likely contributes to the observed more oxidised oxygenated organic aerosol
(MO-OOA) factor. A sensitivity analysis of the coupled land surface–boundary layer–SOA
formation system to changing temperatures reveals that SOA concentrations are
buffered under increasing temperatures: a rise in BVOC emissions is offset
by decreases in OH concentrations and the efficiency with which SVOCs
partition into the aerosol phase.
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