4 Conclusions
Using composition data from multiple campaigns spanning a variety of
environments, we calculated statistics of the
OXL:SO42- mass ratio (median: 0.0217;
R = 0.76; N = 2948), with 95% confidence interval bounds indicating a
relative uncertainty range of ~±30% about the median
(0.0154 – 0.0296). Ground-based size-resolved measurements show overall
agreement with the proposed range, specifically within the submicrometer
mode, suggesting our results are robust within the mixed layer for
PM1. Results from remote marine measurements from AToM
over the Pacific and near-surface Atlantic Oceans also corroborate the
bootstrapped OXL:SO42- range. As
analyzed environments span continental and coastal North America; west,
east, and central Pacific Ocean; and west and central Atlantic Ocean,
the confidence interval of the ratio is proposed to be robust to a wide
range of factors that can impact the formation and removal of both OXL
and SO42-. Furthermore, remarkable
similarity betwen our 95% confidence interval (0.0154 – 0.0296) and
the ratio of yields between SO42- and
aqueous SOA (~0.008 – 0.033) (Ervens et al., 2011)
supports the hypothesis of a generalizable range of
OXL:SO42-.
One exception to the hypothesized
OXL:SO42- range was the occurrence of
gas-phase OXL and/or its precursors partitioning onto dust aloft during
CAMP2Ex. Additionally, BB emissions as a source of
both OXL and SO42- may produce a
strong correlation and greatly elevate their ratio. Thus, we caution
against interpretating a strong OXL and
SO42- correlation as a standalone
signature of aqueous processing when coarse particle types (e.g., dust)
and/or BB emissions are present.
Given its relative uncertainty range (~±30% about the
median) when taken across multiple environments, the 95% confidence
interval of the OXL:SO42- ratio could
be used to assess the relative extent of aqueous processing by comparing
inferred OXL concentrations between air masses, with the implicit
assumption that sampled SO42- mainly
originates from aqueous processing, which is expected to be particularly
true but not limited to near clouds. We emphasize that the
OXL:SO42- ratio applies specifically
to aqueous-processed aerosol (including via clouds or wet aerosol
particles) and that an estimation of total SOA from this ratio requires
additional information about the ratio of OXL and SOA. Furthermore,
gas-phase oxidation is an important source of uncertainty in the ratio
as it can be the dominant SO42-pathway at times and may partly explain
OXL:SO42- variability between
campaigns.
Examining the OXL:SO42- ratio in other
parts of the world and seasons would be beneficial to further gauge its
variability as well as to identify other potentially confounding
factors. Future analysis employing the multi-seasonal ACTIVATE campaign
will provide a valuable dataset for investigations of this nature.