Simulation of organic aerosol formation during the CalNex study: updated mobile emissions and secondary organic aerosol parameterization for intermediate-volatility organic compounds
Authors: Lu Quanyang , Murphy Benjamin N. , Qin Momei , Adams Peter J. , Zhao Yunliang , Pye Havala O. T. , Efstathiou Christos , Allen Chris , Robinson Allen L. 
Published: April 2020

Abstract. We describe simulations using an updated version of the
Community Multiscale Air Quality model version 5.3 (CMAQ v5.3) to
investigate the contribution of intermediate-volatility organic compounds
(IVOCs) to secondary organic aerosol (SOA) formation in southern California
during the CalNex study. We first derive a model-ready parameterization for
SOA formation from IVOC emissions from mobile sources. To account for SOA
formation from both diesel and gasoline sources, the parameterization has
six lumped precursor species that resolve both volatility and molecular
structure (aromatic versus aliphatic). We also implement new mobile-source
emission profiles that quantify all IVOCs based on direct measurements. The
profiles have been released in SPECIATE 5.0. By incorporating both
comprehensive mobile-source emission profiles for semivolatile organic compounds (SVOCs) and IVOCs and
experimentally constrained SOA yields, this CMAQ configuration best
represents the contribution of mobile sources to urban and regional ambient
organic aerosol (OA). In the Los Angeles region, gasoline sources emit 4 times more
non-methane organic gases (NMOGs) than diesel sources, but diesel emits
roughly 3 times more IVOCs on an absolute basis. The revised model predicts
all mobile sources (including on- and off-road gasoline, aircraft, and on-
and off-road diesel) contribute ∼1 µg m−3 to the
daily peak SOA concentration in Pasadena. This represents a ∼70 % increase in predicted daily peak SOA formation compared to the base
version of CMAQ. Therefore, IVOCs in mobile-source emissions contribute
almost as much SOA as traditional precursors such as single-ring aromatics.
However, accounting for these emissions in CMAQ does not reproduce
measurements of either ambient SOA or IVOCs. To investigate the potential
contribution of other IVOC sources, we performed two exploratory simulations
with varying amounts of IVOC emissions from nonmobile sources. To close the
mass balance of primary hydrocarbon IVOCs, IVOCs would need to account for
12 % of NMOG emissions from nonmobile sources (or equivalently 30.7 t d−1 in the Los Angeles–Pasadena region), a value that is well within
the reported range of IVOC content from volatile chemical products. To close
the SOA mass balance and also explain the mildly oxygenated IVOCs in
Pasadena, an additional 14.8 % of nonmobile-source NMOG emissions would
need to be IVOCs (assuming SOA yields from the mobile IVOCs apply to
nonmobile IVOCs). However, an IVOC-to-NMOG ratio of 26.8 % (or
equivalently 68.5 t d−1 in the Los Angeles–Pasadena region) for
nonmobile sources is likely unrealistically high. Our results highlight the
important contribution of IVOCs to SOA production in the Los Angeles region but underscore that other uncertainties must be addressed (multigenerational
aging, aqueous chemistry and vapor wall losses) to close the SOA mass
balance. This research also highlights the effectiveness of regulations to
reduce mobile-source emissions, which have in turn increased the relative
importance of other sources, such as volatile chemical products.