Simulating secondary organic aerosol in a regional air quality model using the statistical oxidation model – Part 3: Assessing the influence of semi-volatile and intermediate-volatility organic compounds and NOx
Authors: Akherati Ali , Cappa Christopher D. , Kleeman Michael J. , Docherty Kenneth S. , Jimenez Jose L. , Griffith Stephen M. , Dusanter Sebastien , Stevens Philip S. , Jathar Shantanu H. 
Published: April 2019

Abstract. Semi-volatile and intermediate-volatility
organic compounds (SVOCs and IVOCs) from anthropogenic sources are likely to
be important precursors of secondary organic aerosol (SOA) in urban airsheds,
yet their treatment in most models is based on limited and obsolete data or
completely missing. Additionally, gas-phase oxidation of organic precursors
to form SOA is influenced by the presence of nitric oxide (NO), but this
influence is poorly constrained in chemical transport models. In this work,
we updated the organic aerosol model in the UCD/CIT (University of California
at Davis/California Institute of Technology) chemical
transport model to include (i) a semi-volatile and reactive treatment of
primary organic aerosol (POA), (ii) emissions and SOA formation from IVOCs,
(iii) the NOx influence on SOA formation, and (iv) SOA
parameterizations for SVOCs and IVOCs that are corrected for vapor wall loss
artifacts during chamber experiments. All updates were implemented in the
statistical oxidation model (SOM) that simulates the oxidation chemistry,
thermodynamics, and gas–particle partitioning of organic aerosol (OA). Model
treatment of POA, SVOCs, and IVOCs was based on an interpretation of a
comprehensive set of source measurements available up to the year 2016 and
resolved broadly by source type. The NOx influence on SOA
formation was calculated offline based on measured and modeled
VOC:NOx ratios. Finally, the SOA formation from all organic
precursors (including SVOCs and IVOCs) was modeled based on recently derived
parameterizations that accounted for vapor wall loss artifacts in chamber
experiments. The updated model was used to simulate a 2-week summer episode
over southern California at a model resolution of 8 km. When combustion-related POA was treated as semi-volatile, modeled POA mass
concentrations were reduced by 15 %–40 % in the urban areas in southern
California but were still too high when compared against “hydrocarbon-like
organic aerosol” factor measurements made at Riverside, CA, during the Study
of Organic Aerosols at Riverside (SOAR-1) campaign of 2005. Treating all POA
(except that from marine sources) to be semi-volatile, similar to diesel
exhaust POA, resulted in a larger reduction in POA mass concentrations and
allowed for a better model–measurement comparison at Riverside, but this
scenario is unlikely to be realistic since this assumes that POA from
sources such as road and construction dust are semi-volatile too. Model
predictions suggested that both SVOCs (evaporated POA vapors) and IVOCs did
not contribute as much as other anthropogenic precursors (e.g., alkanes,
aromatics) to SOA mass concentrations in the urban areas (< 5 %
and < 15 % of the total SOA respectively) as the timescales for
SOA production appeared to be shorter than the timescales for transport out
of the urban airshed. Comparisons of modeled IVOC concentrations with
measurements of anthropogenic SOA precursors in southern California seemed
to imply that IVOC emissions were underpredicted in our updated model by a
factor of 2. Correcting for the vapor wall loss artifact in chamber
experiments enhanced SOA mass concentrations although the enhancement was
precursor-dependent as well as NOx-dependent. Accounting for the influence of
NOx using the VOC:NOx ratios resulted in better predictions of OA
mass concentrations in rural/remote environments but still underpredicted OA
mass concentrations in urban environments. The updated model's performance
against measurements combined with the results from the sensitivity
simulations suggests that the OA mass concentrations in southern California
are constrained within a factor of 2. Finally, simulations performed for
the year 2035 showed that, despite reductions in VOC and NOx emissions
in the future, SOA mass concentrations may be higher than in the year 2005,
primarily from increased hydroxyl radical (OH) concentrations due to lower
ambient NO2 concentrations.