Transport pathways of carbon monoxide from Indonesian fire pollution to a subtropical high-altitude mountain site in western North Pacific

Abstract. Dry conditions associated with El Niño and a positive Indian Ocean Dipole (IOD) are known to have caused major fire pollution events and intense carbon emissions over a vast spatial expanse of Indonesia in October 2006 and 2015. During these two events, a substantial increase in carbon monoxide (CO) mixing ratio was detected by in-situ measurements at Lulin Atmospheric Background Station (LABS, 23.47° N 120.87° E, 2,862 m ASL) in Taiwan, the only background station in the subtropical western North Pacific region. Compared to the long-term October mean (2006–2021), CO was elevated by ~47.2 ppb (37.2 %) and ∼36.7 ppb (28.9 %) in October 2006 and 2015, respectively. In this study, plausible pathways for CO transport from Indonesia to LABS are delineated by using MOPITT CO observations and MERRA-2 reanalysis products (winds and geopotential height (GpH)). Two simultaneously occurring transport pathways were identified: (i) horizontal transport in the free troposphere and (ii) vertical transport through the Hadley circulation (HC). The GpH analysis of both events revealed the presence of a high-pressure anticyclone over the northern part of the South China Sea (SCS), which played an important role in the free tropospheric horizontal transport of CO. In this scenario, CO in the free troposphere is transported on the western edge of the high-pressure system and then driven by subtropical westerlies to LABS. Simultaneously, uplifted CO over Indonesia can enter the HC and transfer to subtropical locations such as LABS. The vertical cross-section of MOPITT CO and MERRA-2 vertical pressure velocity supported the transport of CO through the HC. Further, the results revealed a distinct HC strength in two events (higher in 2006 compared to 2015) due to the different El Niño conditions. Overall, the present findings can provide some insights into understanding the regional transport of pollution over Southeast Asia and the role of climate conditions on transport pathways.