Impact of peninsular Southeast Asian biomass-burning smoke on chemical apportionment of aerosol light extinction at a high-altitude remote environment in the western North Pacific

Abstract

The Lulin Atmospheric Background Station (LABS; 2862 m) sits directly downwind of peninsular Southeast Asia, capturing the fingerprints of regional environmental shifts and serving as a silent witness to the changing chemistry of our atmosphere. A study focusing on the springs of 2014 and 2015 reveals a stark narrative of how human-driven biomass-burning (BB) interacts with climate and air quality. During BB periods, PM2.5 concentrations were significantly higher in 2014 (17.2 ± 8.2 μg m−3) and 2015 (21.1 ± 9.8 μg m−3) than during non-biomass-burning (NBB) periods (7.3 ± 3.7 and 4.8 ± 2.4 μg m−3, respectively). PM2.5 mass consisted mainly of organic matter (OM; 37–66%) and (NH4)2SO4 (19–37%). Diagnostic ratios confirmed the dominance of BB, with K+/OC ratios (0.09–0.10) matching signatures of agricultural waste burning. K+/EC ratios were noticeably higher during BB periods (0.28–0.37) than during NBB periods (<0.20). Aerosol optical properties reflected mass changes: the light extinction coefficient (bext) increased by a factor of 1.7 during BB days in 2014 (114 ± 56 vs. 68 ± 30 Mm−1) and was 5 times higher in 2015 (130 ± 58 vs. 24 ± 13 Mm−1). Chemical apportionment of bext using the revised Interagency Monitoring of Protected Visual Environments (IMPROVE) algorithm identified OM as the main contributor to visibility reduction (63–72% and 34–54% during BB and NBB periods, respectively). Although the revised IMPROVE algorithm showed high reliability during BB periods (R2 = 0.86–0.91), it involved a complex mix of mineral dust and secondary inorganics. Findings highlight the significant influence of regional BB on PM2.5 and visibility at a remote site, while also pointing out the limitations of standard extinction models in low-concentration, multi-source background environments.