Figure 1 . Linear regressions of OXL and
SO42- mass concentrations across
different field campaigns sorted chronologically. All campaigns are
aircraft-based except for CHECSM, which was ground-based. Statistics for
BB-impacted samples are presented separately in red.
CAMP2Ex high-OXL and
high-SO42- populations are colored in
yellow and purple, respectively, in (h). Axes limits are not
standardized across panels. In addition to Pearson’s R, the median
absolute deviation (MAD) was used as a measure of slope goodness-of-fit.
3.2. Source of the CAMP2Ex high-OXL population
Oxalate and SO42- from
CAMP2Ex show three populations: high-OXL,
high-SO42-, and BB-impacted, each
characterized by a distinct OXL:SO42-slope (Fig. 1h). BB-impacted samples are defined as data collected
during RFs 9 and 10 (15 and 16 September 2019, respectively), which
targeted smoke emissions. The high-OXL population was defined with
OXL:SO42- > 0.3 and the
high-SO42- population with
OXL:SO42- < 0.06; varying
the high-OXL ratio lower threshold within 0.10 – 0.30 and
high-SO42- upper threshold within 0.06
– 0.10 does not impact results of the analyses presented below. Ensuing
discussion about these three populations points to important influences
on the OXL:SO42- ratio.
The high-OXL population during CAMP2Ex (Fig. 1h) was
not observed in the other field campaigns. These high-OXL samples were
mostly sampled within the free troposphere (> 5 km) (Fig.
S5b), altitudes of which were rarely sampled in other field campaigns
with AToM being the exception. A few reasons can explain the
high-altitude, high-OXL samples during CAMP2Ex: (1)
OXL’s lengthier chemical formation pathways compared to
SO42- (Ervens, 2015; Sorooshian, Lu,
et al., 2007), (2) inefficient scavenging of gaseous precursors as air
masses are transported upward (Heald et al., 2005), and (3) gas-phase
OXL and/or its precursors partitioning onto dust particles (Stahl et
al., 2020b; Stahl et al., 2021; Sullivan & Prather, 2007). As the PILS
sampled PM4 during CAMP2Ex, we
hypothesize that the enhanced OXL is due to gas-particle partitioning of
OXL and/or its precursors onto coarse mode particles such as dust or sea
salt (Mochida et al., 2003; Rinaldi et al., 2011; Sullivan & Prather,
2007; Turekian et al., 2003), evidenced by a prominent coarse mode peak
(Dp ~ 2.5 μm) in the size distributions
of high-OXL samples (Fig. S5). Among the two sources, dust is more
likely based on the higher affinity of OXL and/or its precursors to
partition onto dust particles compared to sea salt particles (Stahl et
al., 2020b). Furthermore, efficient wet scavenging of sea salt reduces
its free troposphere concentrations as compared to those of the marine
boundary layer (Murphy et al., 2019; Schlosser et al., 2020).
Though both AToM and CAMP2Ex sampled a wide range of
altitudes, no high-OXL population was observed during AToM. This is
because CAMP2Ex operated near major dust sources such
as the Maritime Continent (Hilario, Cruz, Cambaliza, et al., 2020) and
continental Asia (Matsumoto et al., 2003) while data from AToM represent
more remote marine environments (Table 1). Though the
OXL:SO42- ratio from AToM is indeed
slightly enhanced aloft over the Atlantic (Fig. S4c), this is still a
full order of magnitude lower than that of the high-OXL population from
CAMP2Ex (Fig. 1h).
To more deeply characterize the high-OXL population, we compared several
key variables between the CAMP2Ex high-OXL and
high-SO42- populations (Table S3), all
of which showed statistically significant differences based on the
Mann-Whitney U-test (99% confidence level; p < 0.01).
The following characteristics hint to gas-particle partitioning of OXL
and/or its precursors onto dust aloft as has been documented in other
studies (e.g., Stahl et al., 2020b; Sullivan & Prather, 2007): (1) dust
species such as Ca2+ (Kchih et al., 2015) had
approximately double the mass concentration in high-OXL air as compared
to high-SO42- air (Table S3), (2)
high-OXL air was mostly sampled in the free troposphere (Fig. S5), (3)
ionic crustal ratios in the free troposphere (> 5 km) were
more similar to dust values than those for sea salt based on literature
(Park et al., 2004; Švédová et al., 2019; Q. Wang et al., 2018) (Fig.
2), and (4) a prominent coarse mode peak is observed for high-OXL
samples (Fig. S6). Among the other two campaigns sampling
PM4, elevated
OXL:SO42- values at higher altitudes
were also observed during AToM (Fig. S4); during ACTIVATE, dust was not
prevalent at the altitudes sampled (< 5 km).
We next compared m/z 44AMS and OXL (from PILS) to
assess the possibility of gas-particle partitioning of OXL and/or its
precursors onto coarse mode particles such as dust. m/z44AMS indicates the mass concentration of
oxygenated/aged organic aerosol with the functional group
CO2+ (Q. Zhang et al., 2005), of which
OXL is a subcomponent. As the AMS sampled PM1 and the
PILS sampled PM4 in CAMP2Ex, their
comparison lends insight into how coarse mode particles (i.e., 1–4 μm)
may affect PILS observations. Furthermore, because OXL is one component
of m/z 44AMS, comparing PILS OXL
(PM4) to m/z 44AMS(PM1) can serve as an indicator of coarse mode OXL if
OXL:m/z 44AMS ⪆ 1 as a highly conservative
threshold. The high-OXL population has an OXL:m/z44AMS molar ratio of 2.84 ± 3.95 (Table S3), which
provides strong evidence for coarse mode OXL (1–4 μm). This is
supported by a high OXL:OrgAMS ratio in the high-OXL
population (0.84 ± 0.98) compared to the
high-SO42- population (0.01 ± 0.01).
Interestingly,
OrgAMS:SO42-AMSis similar between the high-OXL (1.08 ± 1.34) and
high-SO42- populations (1.27 ± 1.02).
Since the AMS samples PM1, these findings offer more
evidence that differences between populations lie in the coarse mode.