Figure 2 . Altitude-resolved linear regressions of dust species
collected by the PILS during CAMP2Ex (BB samples
excluded) colored by OXL:SO42-. Red
and blue dashed lines denote literature-based ratios for dust (Park et
al., 2004; Švédová et al., 2019; Q. Wang et al., 2018) and sea salt
(Chesselet et al., 1972), respectively.
3.3. Impact of biomass burning on
OXL:SO42-
Although strong OXL and SO42-correlations may be interpreted as a signal of aqueous processing
(Sorooshian, Varutbangkul, et al., 2006; Yao et al., 2003; Yu et al.,
2005), the presence of BB emissions must also be considered as a source
of both SO42- and OXL (Narukawa et
al., 1999; Yang et al., 2014). When present, BB emissions led to
enhanced OXL:SO42- ratios and
correlations (Fig. 1). Size-resolved
OXL:SO42- show similar enhancements
during BB periods (Fig. S3b). In the presence of BB emissions, the
OXL:SO42- ratio is known to increase
from 0.05 (non-BB) to 0.18 (BB) in Sydney (Swan & Ivey, 2021) and from
0.03 – 0.069 (non-BB) to 0.072 – 0.15 (BB) in Hong Kong (Zhou et al.,
2015). Differences in BB-related
OXL:SO42- between environments may be
attributed to factors including biomass type (Christian et al., 2003),
wet scavenging during transport (Marelle et al., 2015), combustion phase
(Kondo et al., 2011; Pósfai et al., 2003), and sampled size range (i.e.,
PM4 for CAMP2Ex, PM1for NiCE, PM18 for CHECSM). During
CAMP2Ex, BB-impacted data suggested two subpopulations
that differ slightly in their
OXL:SO42- slopes (Fig. 1h). Both
populations were sampled during a large biomass burning event (15 Sep
2019; RF9) but differ in terms of location (~330 km
apart), composition, and number concentration (not shown), pointing to
clear differences within the CAMP2Ex BB-impacted
population left for future work. Regardless of BB differences between
campaigns, such a pronounced impact on the
OXL:SO42- ratio in those respective
datasets demonstrates the importance of accounting for BB when
exploiting the OXL:SO42- ratio for
analysis and modeling purposes relevant to secondary aerosol formation
processes.