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.