Satellite observations show that carbon monoxide (CO) concentration centers exist in the tropopause region of the Tibetan Plateau, while their sources and formation mechanism still remain uncertain. In this paper, the 3-D chemical transport model GEOS-Chem is used to conduct sensitivity analysis in 2016. Combined with the analysis data and satellite data, the contribution of three important emission sources (South Asia, East Asia and Southeast Asia) and two important chemical reaction species (CH₄ and nonmethane volatile organic compounds (NMVOCs)) to CO in the upper troposphere and lower stratosphere (UTLS) are studied. The results show that in the Asian monsoon region CO emissions originating from the surface are transported to the upper troposphere via a deep convection process and then enter the Asian Summer Monsoon (ASM) anticyclone. The strong ASM anticyclone isolates the mixing process of air inside and outside the anticyclone, upon entry of carbon monoxide-rich air. In the lower stratosphere, the intensity of the ASM anticyclone declines and the air within the anticyclone flows southwestward with monsoon circulation. We found that in the summer Asian monsoon region, South Asia exhibited the highest carbon monoxide concentration transported to the UTLS. CH₄ imposed the greatest influence on the CO concentration in the UTLS region. According to the model simulation results, the CO concentrations in the Asian monsoon region at 100 hPa altitudes were higher than those in other regions at the same latitudes. Regarding effects, 43.18% originated from CH₄ chemical reactions, 20.81% originated from NMVOC chemical reactions, and 63.33% originated from surface CO emissions, while sinks yielded a negative contribution of −27.32%. Regarding surface CO emissions, East Asia contributed 13.56%, South Asia contributed 39.27%, and Southeast Asia contributed 7.15%.

Aerosol extinction profiles at 550 nm were retrieved by applying multi-axis differential optical absorption spectroscopy (MAX-DOAS) and lookup table. Then the tropospheric NO₂ and HCHO vertical column densities were retrieved using a two-step method from 28 July to 5 August of 2015 in Shanghai. The retrieved results were compared with the satellite products, and then their diurnal variation was observed. A consistency check was performed before the inversion to obtain a correction factor. Based on the sensitivity of geometric angles to oxygen dimer air mass factor (O₄ AMF, AMF is the ratio of the slanted column density to the vertical column density), the parameterization scheme of geometric angles in the lookup table is optimized. The results show that the aerosol increased significantly in the afternoon. The diurnal variation of tropospheric NO₂ and HCHO vertical column densities (VCDs) are bimodal and unimodal patterns respectively, and their values are higher than those of GOME-2 and OMI satellite products. A process of aerosol reduction and recovery are related to ground particulates and meteorological elements. The chemical sensitivity of local ozone production also has a clear diurnal variation.

Based on the Langley method and the EuroSkyRad (ESR) pack retrieval scheme, we carried out the retrieval of the aerosol properties for the CE–318 sunphotometer observation data from March 2012 to February 2014 in Kunming, China, and we explored the possible mechanisms of the seasonal variations. The seasonal variation of the aerosol optical depth (AOD) was unimodal and reached a maximum in summer. The retrieval analysis of the Angstrom exponent (α) showed the aerosol types were continental, biomass burning (BB), and urban/industrial (UI); the content of the desert dust (DD) was low, and it may have contained a sea–salt (SS) aerosol due to the influence of the summer monsoon. All the aerosol particle spectra in different seasons showed a bimodal structure. The maximum and submaximal values were located near 0.2 μm and 4 μm, respectively, and the concentration of the aerosol volume was the highest in summer. In summer, aerosol particles have a strong scattering power but a weak absorption power; this pattern is the opposite in winter. The synergistic effect of the East Asian monsoon and the South Asian monsoon seasonal oscillations can have an important impact on the variation of the aerosol properties. The oscillation variation characteristic of the total vertical columnar water vapor (CWV) and the monsoon index was completely consistent. The aerosol types and sources in the Yunnan–Kweichow Plateau and the optical radiation properties were closely related to the monsoon circulation activities during different seasons and were different from other regions in China.

The El Niño-Southern Oscillation (ENSO) and the distinctive topographic features of the Tibetan Plateau (TP) exert a significant influence on the stratosphere-to-troposphere transport (STT) process. Their combined effects further amplify the spatiotemporal variability of tropospheric ozone (O₃) concentrations. To investigate the impact of ENSO on total column ozone (TCO) over the TP and the underlying mechanisms, this study employs the Weather Research and Forecasting Model with Chemistry (WRF-Chem). Results demonstrate that the single-synthesis simulation using WRF-Chem outperforms the multi-year simulation of the Whole Atmosphere Community Climate Model (WACCM) in capturing tropospheric TCO, zonal wind, potential height, and temperature. Over the TP, except in autumn, the variations in TCO within the lower stratosphere and troposphere exhibit opposite phases during El Niño and La Niña years. Specifically, TCO in the lower stratosphere is primarily regulated by the Brewer-Dobson circulation (BDC) and potential height. In contrast, TCO in the troposphere is shaped not only by potential height but also by STT processes and regional vertical circulation. The thermal effect of the TP plays a pivotal role in modulating the subtropical jet stream (STJ), potential height, and vertical circulation—with notable phase reversals observed between El Niño and La Niña years. Furthermore, the thermal effect of the TP acts in synergy with the Hadley Circulation (HC) to drive changes in the STJ, thereby exerting a significant impact on the spatiotemporal distribution of tropospheric O₃ over the TP.

This study adopts MAX-DOAS technology and optimal estimation methods to analyze the vertical profiles of aerosols, HONO, and O₃ in Kunming during winter 2024, thereby clarifying their vertical distribution characteristics and diurnal variation rules. Aerosols are predominantly concentrated in the near-surface layer, with concentrations decreasing gradually with increasing altitude; their concentrations rise in the morning, reach a peak at 10 a.m., and then decline, with the maximum extinction coefficient reaching 1.3 km⁻¹. HONO concentrations decrease exponentially with height, being the highest around sunrise, the lowest at noon, and then gradually accumulating, with the maximum concentration exceeding 3.14 × 10¹⁰ molec·cm⁻³. O₃ achieves its maximum concentration in the 2.6–4 km altitude range in the afternoon (the highest concentration > 1.16 × 10¹² molec·cm⁻³), while near-surface O₃ concentrations increase steadily from morning onward. Model analysis demonstrates that without considering HONO, OH radicals shift from net production to net loss in the morning, leading to reduced O₃ production rates and a decrease of approximately 8 ppb in the maximum O₃ concentration. These findings highlight the significant vertical and diurnal variations in the three pollutants, emphasize HONO’s crucial role in OH radical generation and O₃ formation, provide valuable scientific support for air pollution control, and confirm HONO’s important impact on regional atmospheric chemistry and the distribution of key pollutants.