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.