Abstract. East Asia has experienced rapid development with increasing carbon monoxide (CO) emission in the past decades. Therefore, uplifting CO from the boundary layer to the free troposphere in East Asia can have great implications on regional air quality around the world. It can also influence global climate due to the longer lifetime of CO at higher altitudes. In this study, three cases of high CO episodes in the East China Sea and the Sea of Japan from 2003 to 2005 are examined with spaceborne Measurements of Pollution in the Troposphere (MOPITT) data, in combination with aircraft measurements from the Measurement of Ozone and Water Vapor by Airbus In-Service Aircraft (MOZAIC) program. High CO abundances of 300–550 ppbv are observed in MOZAIC data in the free troposphere during these episodes. These are among the highest CO abundances documented at these altitudes. On average, such episodes with CO over 400 ppbv (in the 2003 and 2004 cases) and between 200 and 300 ppbv (in the 2005 case) may occur 2–5 and 10–20% in time, respectively, in the respective altitudes over the region. Correspondingly, elevated CO is shown in MOPITT daytime data in the middle to upper troposphere in the 2003 case, in the lower to middle troposphere in the 2004 case, and in the upper troposphere in the 2005 case. Through analyses of the simulations from a chemical transport model GEOS-Chem and a trajectory dispersion model FLEXPART, we found different CO signatures in the elevated CO and distinct transport pathways and mechanisms for these cases. In the 2003 case, emissions from large forest fires near Lake Baikal dominated the elevated CO, which had been rapidly transported upward by a frontal system from the fire plumes. In the 2004 case, anthropogenic CO from the North China Plain experienced frontal lifting and mostly reached ~ 700 hPa near the East China Sea, while CO from biomass burning over Indochina experienced orographic lifting, lee-side-trough-induced convection, and frontal lifting through two separate transport pathways, leading to two distinct CO enhancements around 700 and 300 hPa. In the 2005 case, the observed CO of ~ 300 ppbv around 300 hPa originated from anthropogenic sources over the Sichuan Basin and the North China Plain and from forest fires over Indochina. The high CO was transported to such altitudes through strong frontal lifting, interacting with convection and orographic lifting. These cases show that topography affects vertical transport of CO in East Asia via different ways, including orographic uplifting over the Hengduan Mountains, assisting frontal lifting in the North China Plain, and facilitating convection in the Sichuan Basin. In particular, topography-induced lee-side troughs over Indochina led to strong convection that assisted CO uplifting to the upper troposphere. This study shows that the new daytime MOPITT near-infrared (NIR) and thermal-infrared (TIR) data (version 5 or above) have enhanced vertical sensitivity in the free troposphere and may help qualitative diagnosis of vertical transport processes in East Asia.
Sources of nitrous acid (HONO) in the upper boundary layer and lower free troposphere of the North China Plain: insights from the Mount Tai Observatory
