Background aerosol over the Himalayas and Tibetan Plateau: observed characteristics of aerosol mass loading

Abstract

Abstract. To investigate the atmospheric aerosols of the Himalayas and Tibetan Plateau (HTP), an observation network was established within the region's various ecosystems, including at the Ngari, Qomolangma (QOMS), Nam Co, and Southeastern Tibetan (SET) stations. In this paper we illustrate aerosol mass loadings by integrating in situ measurements with satellite and ground-based remote sensing datasets for the 2011–2013 period, on both local and large scales. Mass concentrations of these surface atmospheric aerosols were relatively low and varied with land cover, showing a general tendency of Ngari and QOMS (barren sites) > Nam Co (grassland site) > SET (forest site). Daily averages of online PM2.5 (particulates with aerodynamic diameters below 2.5 µm) at these sites were sequentially 18.2 ± 8.9, 14.5 ± 7.4, 11.9 ± 4.9 and 11.7 ± 4.7 µg m−3. Correspondingly, the ratios of PM2.5 to total suspended particles (TSP) were 27.4 ± 6.65, 22.3 ± 10.9, 37.3 ± 11.1 and 54.4 ± 6.72 %. Bimodal mass distributions of size-segregated particles were found at all sites, with a relatively small peak in accumulation mode and a more notable peak in coarse mode. Diurnal variations in fine-aerosol masses generally displayed a bi-peak pattern at the QOMS, Nam Co and SET stations and a single-peak pattern at the Ngari station, controlled by the effects of local geomorphology, mountain-valley breeze circulation and aerosol emissions. Dust aerosol content in PM2.1 samples gave fractions of 26 % at the Ngari station and 29 % at the QOMS station, or ∼ 2–3 times that of reported results at human-influenced sites. Furthermore, observed evidence confirmed the existence of the aerodynamic conditions necessary for the uplift of fine particles from a barren land surface. Combining surface aerosol data and atmospheric-column aerosol optical properties, the TSP mass and aerosol optical depth (AOD) of the Multi-angle Imaging Spectroradiometer (MISR) generally decreased as land cover changed from barren to forest, in inverse relation to the PM2.5 ratios. The seasonality of aerosol mass parameters was land-cover dependent. Over forest and grassland areas, TSP mass, PM2.5 mass, MISR-AOD and fine-mode AOD were higher in spring and summer, followed by relatively lower values in autumn and winter. At the barren site (the QOMS station), there were inconsistent seasonal patterns between surface TSP mass (PM2.5 mass) and atmospheric column AOD (fine-mode AOD). Our findings implicate that HTP aerosol masses (especially their regional characteristics and fine-particle emissions) need to be treated sensitively in relation to assessments of their climatic effect and potential role as cloud condensation nuclei and ice nuclei.

The Characteristics of the Aerosol Optical Depth within the Lowest Aerosol Layer over the Tibetan Plateau from 2007 to 2014

“…In the northern Indo-Gangetic plain, the occurrence of polluted dust was higher than the dust [28,40]. Additionally, higher smoke aerosols also occurred in the adjacent regions of the southern edge of the TP [13,28]. High local anthropogenic emissions leading to high aerosol loading in the Indo-Gangetic plain are also likely to contribute to the high AODS [2,28,47].…”

“…On the other hand, polluted dust also existed over the TP and had a relatively larger thickness in spring and summer, which could have contributed to the relatively high aerosol layer amounts over the main body of TP in spring [28]. In Figure 3, the AODS was generally low (<0.2) in the main body of the TP in each season, because fewer aerosol loads existed there and the TP acted as a natural barrier leading to low aerosol load in the main body of the TP [13,40]. The highest AODS was found over the Qaidam basin in spring (~0.3) and summer (~0.3), which was consistent with the results of Xu et al [22].…”

Chemical characterization of long-range transport biomass burning emissions to the Himalayas: insights from high-resolution aerosol mass spectrometry

“…1. During the pre-monsoon period, atmospheric pollutants associated with BB emissions in South Asia are generally advected by regional and long-range transport (e.g., westerlies and South Asian monsoon system) to the Himalayas and build up in the southern foothills, where they are then lifted up to high altitude by the Himalayan topography and typical valley wind circulation (Zhao et al, 2013;Cong et al, 2015;Liu et al, 2017). However, the chemical properties of aerosol particles are still not well understood in the Himalayas region due to its remote and harsh environments, challenging weather conditions, and logistical difficulties.…”

Aerosol Optical Properties and Associated Direct Radiative Forcing over the Yangtze River Basin during 2001–2015

“…However, long-term systemic studies on the aerosol optical and radiative characteristics are still insufficient over the whole YRB due to the lack of observation sites, especially over the upstream and midstream regions of YRB. Furthermore, the midstream and downstream regions of YRB and Sichuan Basin have suffered from serious air pollution caused by the industrial production, vehicular emission, straw burning and the long-distance transport of dust particles from North China in spring [8][9][10][34][35][36]. The various aerosol sources and diverse surface types make the aerosol optical and radiative characteristics over YRB more complicated [37].…”