The potential role of methanesulfonic acid (MSA) in aerosol formation and growth and the associated radiative forcings
Authors: Hodshire Anna L. , Campuzano-Jost Pedro , Kodros John K. , Croft Betty , Nault Benjamin A. , Schroder Jason C. , Jimenez Jose L. , Pierce Jeffrey R. 
Published: March 2019

Abstract. Atmospheric
marine aerosol particles impact Earth's albedo and climate. These particles
can be primary or secondary and come from a variety of sources, including sea
salt, dissolved organic matter, volatile organic compounds, and
sulfur-containing compounds. Dimethylsulfide (DMS) marine emissions
contribute greatly to the global biogenic sulfur budget, and its oxidation
products can contribute to aerosol mass, specifically as sulfuric acid and
methanesulfonic acid (MSA). Further, sulfuric acid is a known nucleating
compound, and MSA may be able to participate in nucleation when bases are
available. As DMS emissions, and thus MSA and sulfuric acid from DMS
oxidation, may have changed since pre-industrial times and may change in a
warming climate, it is important to characterize and constrain the climate
impacts of both species. Currently, global models that simulate aerosol size
distributions include contributions of sulfate and sulfuric acid from DMS
oxidation, but to our knowledge, global models typically neglect the impact
of MSA on size distributions. In this study, we use the GEOS-Chem-TOMAS (GC-TOMAS) global aerosol
microphysics model to determine the impact on aerosol size distributions and
subsequent aerosol radiative effects from including MSA in the size-resolved
portion of the model. The effective equilibrium vapor pressure of MSA is
currently uncertain, and we use the Extended Aerosol Inorganics Model (E-AIM)
to build a parameterization for GC-TOMAS of MSA's effective volatility as a
function of temperature, relative humidity, and available gas-phase bases,
allowing MSA to condense as an ideally nonvolatile or semivolatile species or
too volatile to condense. We also present two limiting cases for MSA's
volatility, assuming that MSA is always ideally nonvolatile (irreversible
condensation) or that MSA is always ideally semivolatile (quasi-equilibrium
condensation but still irreversible condensation). We further present
simulations in which MSA participates in binary and ternary nucleation with
the same efficacy as sulfuric acid whenever MSA is treated as ideally
nonvolatile. When using the volatility parameterization described above (both
with and without nucleation), including MSA in the model changes the global
annual averages at 900 hPa of submicron aerosol mass by 1.2 %, N3
(number concentration of particles greater than 3 nm in diameter) by
−3.9 % (non-nucleating) or 112.5 % (nucleating), N80 by 0.8 %
(non-nucleating) or 2.1 % (nucleating), the cloud-albedo aerosol indirect
effect (AIE) by −8.6 mW m−2 (non-nucleating) or −26 mW m−2
(nucleating), and the direct radiative effect (DRE) by −15 mW m−2
(non-nucleating) or −14 mW m−2 (nucleating). The sulfate and
sulfuric acid from DMS oxidation produces 4–6 times more submicron mass than
MSA does, leading to an ∼10 times stronger cooling effect in the DRE.
But the changes in N80 are comparable between the contributions from MSA and
from DMS-derived sulfate/sulfuric acid, leading to comparable changes in the
cloud-albedo AIE. Model–measurement comparisons with the Heintzenberg et al. (2000) dataset
over the Southern Ocean indicate that the default model has a missing source
or sources of ultrafine particles: the cases in which MSA participates in
nucleation (thus increasing ultrafine number) most closely match the
Heintzenberg distributions, but we cannot conclude nucleation from MSA is the
correct reason for improvement. Model–measurement comparisons with
particle-phase MSA observed with a customized Aerodyne high-resolution
time-of-flight aerosol mass spectrometer (AMS) from the ATom campaign show
that cases with the MSA volatility parameterizations (both with and without
nucleation) tend to fit the measurements the best (as this is the first use
of MSA measurements from ATom, we provide a detailed description of these
measurements and their calibration). However, no one model sensitivity case
shows the best model–measurement agreement for both Heintzenberg and the
ATom campaigns. As there are uncertainties in both MSA's behavior (nucleation
and condensation) and the DMS emissions inventory, further studies on both
fronts are needed to better constrain MSA's past, current, and future impacts
upon the global aerosol size distribution and radiative forcing.