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The tropospheric concentrations of nitrogen dioxide (NO 2) and formaldehyde (HCHO) have high spatio-temporal variability, and in situ observations of these trace gases are still scarce, especially in remote background areas. We made four similar circling journeys of mobile multi-axis differential optical absorption spectroscopy (MAX-DOAS) measurements in the Three Rivers' Source region over the Tibetan Plateau in summer (18–30 July) 2021 for the first time. The differential slant column densities (DSCDs) of NO 2 and HCHO were retrieved from the measured spectra, with very weak absorptions along the driving routes. The tropospheric NO 2 and HCHO vertical column densities (VCDs) were calculated from their DSCDs by the geometric approximation method, and they were further filtered to form reliable data sets by eliminating the influences of sunlight shelters and the vehicle's vibration and bumpiness. The observational data show that the tropospheric NO 2 and HCHO VCDs decreased with the increasing altitude of the driving route, whose background levels ± standard deviations were 0.40 ± 1.13×1015 molec. cm -2 for NO 2 and 2.27±1.66×1015 molec. cm -2 for HCHO in July 2021 over the Three Rivers' Source region. The NO 2 VCDs show similar geographical distribution patterns between the different circling journeys, but the levels of the HCHO VCDs are different between the different circling journeys. The elevated NO 2 VCDs along the driving routes usually corresponded to enhanced transport emissions from the towns crossed. However, the spatial distributions of the HCHO VCDs depended significantly on natural and meteorological conditions, such as surface temperature. By comparing TROPOMI satellite products and mobile MAX-DOAS results, we found that TROPOMI NO 2 and HCHO VCDs have large positive offsets in the background atmosphere over the main area of the Three Rivers' Source. Our study provides valuable data sets and information of NO 2 and HCHO over the Tibetan Plateau, benefitting the scientific community in investigating the spatio-temporal evolution of atmospheric composition in the background atmosphere at high altitudes, validating and improving the satellite products over mountain terrains, and evaluating the model's ability to simulate atmospheric chemistry over the Tibetan Plateau. [ABSTRACT FROM AUTHOR]

Biomass burning play a key role in the global carbon cycle by altering the atmospheric composition, and affect regional and global climate. Despite its importance, a very few high-resolution records are available worldwide, especially for recent climate change. This study analyzes levoglucosan, a specific tracer of biomass burning emissions, in a 38-year ice core retrieved from the Shulehe Glacier No. 4, northeastern Tibetan Plateau. The levoglucosan concentration in the Shulehe Glacier No. 4 ice core ranged from 0.1 to 55 ng mL−1, with an average concentration of 8 ± 8 ng mL−1. The concentrations showed a decreasing trend from 2002 to 2018. Meanwhile, regional wildfire activities in Central Asian also exhibited a declining trend during the same period, suggesting the potential correspondence between levoglucosan concentration of the Shulehe Glacier No. 4 ice core and the fire activity of Central Asia. Furthermore, a positive correlation also exists between the levoglucosan concentration of the Shulehe Glacier No. 4 ice core and the wildfire counts in Central Asia from 2002 to 2018. While backward air mass trajectory analysis and fire spots data showed a higher distribution of fire counts in South Asia compared to Central Asia, but the dominance of westerly circulation in the northern TP throughout the year. Therefore, the levoglucosan in the Shulehe Glacier No. 4 provides clear evidence of Central Asian wildfire influence on Tibetan Plateau glaciers through westerlies. This highlights a great importance of ice core data for wildfire history reconstruction in the Tibetan Plateau Glacier regions. [Display omitted] • The average concentration of levoglucosan in ice core showed a decreasing trend from 2002 to 2018. • The levoglucosan of ice core may be affected by fire activities of Central Asia. • The levoglucosan concentration in ice core can record past fire activity. [ABSTRACT FROM AUTHOR]

Stable isotopic compositions in atmospheric water vapor (δ18O V , δD V and d-excess V) are effective tracers that provide new insights into the local and regional hydrological cycle on the Tibetan Plateau. One-year continuous measurements of the isotopic compositions in near-surface atmospheric water vapor on the Southeastern Tibetan Plateau are reported. Our results demonstrated the relative impacts of moisture source shifts, large scale moisture transport, and local processes on the variability of the isotopic compositions in atmospheric water vapor. The observed seasonal variations in δ18O V and d-excess V exhibit low values in the monsoon period and high values in the non-monsoon period due to the predominance of large-scale atmospheric circulations. Local processes significantly modulate diurnal cycles of δ18O V and d-excess V. The sharp decrease in δ18O V and increase in d-excess V is related to the synoptic process. Moreover, the seasonality of moisture sources and transport, including moisture uptake and upstream rainout strongly affects the atmospheric moisture and the isotopes. These findings will improve our understanding of the regional hydrological cycle on the Southeastern Tibetan Plateau and provide reference to validate the simulated stable water isotopes. • One-year continuing measurement of stable isotopes in water vapor on the Southeastern Tibetan Plateau are presented. • Significant seasonality and diurnal cycles of the stable isotopes in water vapor are demonstrated. • Water vapor sources, local processes and synoptic events impact the variability of stable isotopes on different time scales. [ABSTRACT FROM AUTHOR]

The Tibet Plateau (TP) plays an essential role in modulating regional and global climate, and its influence on climate is affected also by human-related processes, including changes in atmospheric composition. However, observations of atmospheric composition, especially vertical profile observations, remain sparse and rare on the TP, due to extremely high altitude, topographical heterogeneity and grinding environment. Accordingly, the forcing and feedback of atmospheric composition from rapidly changing surrounding regions to regional environmental and climate change in the TP remains poorly understood. This paper introduces a high time-resolution (~15 min) vertical profile observational dataset of atmospheric composition (aerosol, NO2, HCHO and HONO) on the TP for more than one year (2017-2019) using a passive remote sensing technique. The diurnal pattern, vertical distribution and seasonal variations of these pollutants are documented here in detail. The sharing of this dataset would benefit scientific community in exploring source-receptor relationships, forcing and feedback of atmospheric composition on the TP to regional and global climate. It also provides potentials to improve satellite retrievals, and to facilitate the development and improvement of models in cold regions. The dataset is freely available at Zenodo [ABSTRACT FROM AUTHOR]

Black carbon (BC) constitutes a pivotal component of atmospheric aerosols, significantly impacting regional and global radiation balance, climate, and human health. In this study, we evaluated BC data in two prominent atmospheric composition reanalysis datasets: the Modern-Era Retrospective analysis for Research and Applications, Version 2 (MERRA-2) and the Copernicus Atmosphere Monitoring Service (CAMS), and analyzed the causes of their deviations. This assessment is based on observational data collected from 34 monitoring stations across China from 2006 to 2022. Our research reveals a significant and consistent decline in BC concentrations within China, amounting to a reduction exceeding 67.33%. However, both MERRA-2 and CAMS reanalysis data fail to capture this declining trend. The average annual decrease of BC in MERRA-2 from 2006 to 2022 is only 0.06 μg/m3 per year, while the BC concentration in CAMS even increased with an average annual value of 0.014 μg/m3 per year. In 2022, MERRA-2 had overestimated BC concentration by 20% compared to observational data, while CAMS had overestimated it by approximately 66%. In the regional BC concentration analysis, the data quality of the reanalysis data is better in the South China (R M = 0.59, R C = 0.53), followed by the North China (R M = 0.50, R C = 0.42). Reanalysis BC data in Northwest China and the Tibetan Plateau are difficult to use for practical analysis due to their big difference with observation. In a comparison of the anthropogenic BC emissions inventory used in the two atmospheric composition reanalysis datasets with the Multi-resolution Emission Inventory model for Climate and air pollution research (MEIC) emissions inventory, we found that: Despite the significant decline in China's BC emissions, MERRA-2 still relies on the 2006 emissions inventory, while CAMS utilizes emission inventories that even show an increasing trend. These factors will undoubtedly lead to greater deviations between reanalysis and observational data. [Display omitted] • The BC of MERRA-2 and CAMS was comprehensively evaluated for the first time in China. • Neither MERRA-2 nor CAMS reproduce observed BC downward trend from 2006 to 2022. • The use of the BC emission inventory with wrong annual trend is an important reason. [ABSTRACT FROM AUTHOR]

Transmission electron microscopy (TEM) was employed to obtain morphology, size, composition, and mixing state of background aerosols with diameter less than 1 µm in the northern Qinghai-Tibet Plateau (QTP) during 15 September to 15 October 2013. Individual aerosol particles mainly contained secondary inorganic aerosols (SIA - sulfate and nitrate) and organics during clean periods (PM2.5 mass concentration less than 2.5 μg m-3). The presence of K-Na-Cl associated with organics and an increase in soot particles suggest that an intense biomass burning event caused the highest PM2.5 concentrations (>30 μg m-3) during the study. A large number fraction of the fly-ashcontaining particles (21.73%) suggests that coal combustion emissions in the QTP significantly contributed to air pollutants at the medium pollution level (PM2.5: 10-30 μg m-3). We concluded that emissions from biomass burning and from coal combustion both constantly contribute to anthropogenic particles in the QTP atmosphere. Based on size distributions of individual particles at different pollution levels, we found that gas condensation on existing particles is an important chemical process for the formation of SIA with organic coating. TEM observations show that refractory aerosols (e.g., soot, fly ash, and visible organic particles) likely adhere to the surface of SIA particles larger than 200 nm due to coagulation. Organic coating and soot on surface of the aged particles likely influence their hygroscopic and optical properties, respectively, in the QTP. To our knowledge, this study reports the first microscopic analysis of fine particles in the background QTP air. [ABSTRACT FROM AUTHOR]