Sources and photochemistry of volatile organic compounds in the remote atmosphere of western China: results from the Mt. Waliguan Observatory

Abstract

The chemistry of the natural atmosphere and the influence by long-range transport of air pollution are key issues in the atmospheric sciences. Here we present two intensive field measurements of volatile organic compounds (VOCs) in late spring and summer of 2003 at Mt. Waliguan (WLG, 36.28° N, 100.90° E, 3816 m a.s.l.), a baseline station in the northeast part of the Qinghai-Tibetan Plateau. Most VOC species exhibited higher concentrations in late spring than in summer. A typical diurnal variation was observed with higher nighttime levels, in contrast to results from other mountainous sites. Five different air masses were identified from backward trajectory analysis showing distinct VOC speciation. Air masses originating from the central Eurasian continent contained the lowest VOC levels compared to the others that were impacted by anthropogenic emissions from China and the Indian subcontinent. A photochemical box model based on the Master Chemical Mechanism (version 3.2) and constrained by a full suite of measurements was developed to probe the photochemistry of atmosphere at WLG. Our results show net ozone production from in situ photochemistry during both late spring and summer. Oxidation of nitric oxide (NO) by the hydroperoxyl radical (HO₂) dominates the ozone production relative to the oxidation by the organic peroxy radicals (RO₂), and the ozone is primarily destroyed by photolysis and reactions with the HO_x (HO_x = OH + HO₂) radicals. Ozone photolysis is the predominant primary source of radicals (RO_x = OH + HO₂ + RO₂), followed by the photolysis of secondary oxygenated VOCs and hydrogen peroxides. The radical losses are governed by the self and cross reactions among the radicals. Overall, the findings of the present study provide insights into the background chemistry and the impacts of pollution transport on the pristine atmosphere over the Eurasian continent

Characteristics of Summertime Volatile Organic Compounds in the Lower Free Troposphere: Background Measurements at Mt. Fuji

“…Because of the relatively long atmospheric lifetime of ethane (Table 1), it contributed most of the measured alkane concentration in both years. This result is similar to those obtained at other mountain stations, irrespective of locations and seasons (Greenberg et al, 1996;Schmitt and Volz-Thomas, 1997;Sharma et al, 2000;Swanson et al, 2003;Helmig et al, 2008;Suthawaree et al, 2010;Xue et al, 2013). Alkyne, as represented solely by ethyne, had the lowest contribution among all VOC types, accounting for 5.3%-6.1% of total VOCs.…”

“…However, Khwaja and Narang (2008) also reported a low ratio of 0.36 during nighttime for free tropospheric subsidence at Whiteface Mountain in the northeastern United States. Xue et al (2013) observed an even lower ratio of 0.25 at Mt. Waliguan in summer.…”

“…Studies have also shown highly inconsistent concentrations of alkenes in summertime at mountain sites, such as 397 ppt at Happo, Japan (Sharma et al, 2000); 167.8 ppt at Mt. Waliguan, China (Xue et al, 2013); 32 ppt at Izana, the Canary Islands (Schmitt and Volz-Thomas, 1997); and 5 ppt in the free troposphere at Mauna Loa, Hawaii (Greenberg et al, 1996). The halocarbons showed much lower variability than did VOCs, regardless of whether they were controlled according to the Montreal Protocol (MP; Fig.…”

“…Measurements have been obtained using aircrafts (Blake et al, 1996;Bechara et al, 2008;Baker et al, 2010) and tethered balloons (Helmig et al, 1998;Spirig et al, 2004) operated at different heights. Ground-based measurements of VOCs at high-altitude stations have also been conducted in various locations, such as North America (Swanson et al, 2003;Khwaja and Narang, 2008;Burley et al, 2016), Europe (Karl et al, 2001;Li et al, 2005;Helmig et al, 2008;Lanz et al, 2009;Lo Vullo et al, 2016a, b), Africa (Schmitt and Volz-Thomas, 1997), East Asia (Sharma et al, 2000;Suthawaree et al, 2010;Xue et al, 2013;Yang et al, 2016), and Hawaii (Greenberg et al, 1992(Greenberg et al, , 1996 the Japan Meteorological Agency (JMA). Yearlong and multiple-year measurements of tropospheric ozone Dokiya et al, 2001), water-soluble ions from aerosol samples (Suzuki et al, 2008), and rainwater ) have been successfully obtained at the FRS.…”

Ambient air benzene at background sites in China's most developed coastal regions: Exposure levels, source implications and health risks

“…Monitoring of benzene levels at more background sites would, therefore, not only reveal background exposure levels, but also reflect the impact of regional social-economic changes. Unfortunately, monitoring at background or remote locations is scarce and sporadic in China (Guo et al, 2004;Tang et al, 2009;Suthawaree et al, 2010;Xue et al, 2013;Zhang et al, 2014). To fully understand the exposure levels and the potential health risks, long-term measurements are needed.…”