Rapid growth of methane and carbon monoxide in the Asian Summer Monsoon Anticyclone core region during 2002–2023

We analyse the growth of carbon monoxide (CO) and methane (CH₄) in the Asian Summer Monsoon Anticyclone (ASMA) core region during 2002–2023 and highlight the flow of horizontal air mass within the region using HYSPLIT model. The core ASMA region is delineated using geopotential height (GPH) and wind vectors at 100 hPa. The anticyclone starts its growth in June, attains maximum strength with wide spatial coverage across Asia during July and August, and then declines in September. In the ASMA core region in 2023, CO levels at the tropopause rose by almost 23% (42 to 52 ppbv), while CH₄ increased by 7.6% (1.70 to 1.83 × 10³ ppbv) with respect to year 2002. Similar increments were observed at 200 hPa (CO, 38.9%; CH₄, 7.3%) and 70 hPa (CO, 9.4%; CH₄, 4.9%). The cumulative mass flux (CMF) demonstrates significant vertical air mass movement (> 150 kg·m⁻¹·s⁻¹ at 90–100 hPa), while the horizontal mass transport indicates that 44% of trajectories at 100 hPa originated from Central/East China and 32% from Indo-Gangetic sources, in addition to some long-range air from West Asia. Findings suggest that the ASMA layered dynamics may amplify surface emission impacts, with stratospheric CH₄ growth rates (0.82 ppbv/yr) tripling global background trends. Considering the observed CO and CH₄ increases within the ASMA, our findings suggest that targeted CH4 and CO emission cuts in key source regions are urgently needed to curb near-term warming of upper troposphere and lower stratosphere.