Seasonal variation of secondary organic aerosol tracers in Central Tibetan Plateau

Abstract

Abstract. Secondary organic aerosol (SOA) affects the earth's radiation balance and global climate. High-elevation areas are sensitive to global climate change. However, at present, SOA origins and seasonal variations are understudied in remote high-elevation areas. In this study, particulate samples were collected from July 2012 to July 2013 at the remote Nam Co (NC) site, Central Tibetan Plateau and analyzed for SOA tracers from biogenic (isoprene, monoterpenes and β-caryophyllene) and anthropogenic (aromatics) precursors. Among these compounds, isoprene SOA (SOAI) tracers represented the majority (26.6 ± 44.2 ng m−3), followed by monoterpene SOA (SOAM) tracers (0.97 ± 0.57 ng m−3), aromatic SOA (SOAA) tracer (2,3-dihydroxy-4-oxopentanoic acid, DHOPA, 0.25 ± 0.18 ng m−3) and β-caryophyllene SOA tracer (β-caryophyllenic acid, 0.09 ± 0.10 ng m−3). SOAI tracers exhibited high concentrations in the summer and low levels in the winter. The similar temperature dependence of SOAI tracers and isoprene emission suggested that the seasonal variation of SOAI tracers at the NC site was mainly influenced by the isoprene emission. The ratio of high-NOx to low-NOx products of SOAI (2-methylglyceric acid to 2-methyltetrols) was highest in the winter and lowest in the summer, due to the influence of temperature and relative humidity. The seasonal variation of SOAM tracers was impacted by monoterpenes emission and gas-particle partitioning. During the summer to the fall, temperature effect on partitioning was the dominant process influencing SOAM tracers' variation; while the temperature effect on emission was the dominant process influencing SOAM tracers' variation during the winter to the spring. SOAM tracer levels did not elevate with increased temperature in the summer, probably resulting from the counteraction of temperature effects on emission and partitioning. The concentrations of DHOPA were 1–2 orders of magnitude lower than those reported in the urban regions of the world. Due to the transport of air pollutants from the adjacent Bangladesh and northeastern India, DHOPA presented relatively higher levels in the summer. In the winter when air masses mainly came from northwestern India, mass fractions of DHOPA in total tracers increased, although its concentrations declined. The SOA-tracer method was applied to estimate secondary organic carbon (SOC) from these four precursors. The annual average of SOC was 0.22 ± 0.29 μgC m−3, with the biogenic SOC (sum of isoprene, monoterpenes and β-caryophyllene) accounting for 75 %. In the summer, isoprene was the major precursor with its SOC contributions of 81 %. In the winter when the emission of biogenic precursors largely dropped, the contributions of aromatic SOC increased. Our study implies that anthropogenic pollutants emitted in the Indian subcontinent could be transported to the TP and have an impact on SOC over the remote NC.

Molecular characterization of organic aerosols in the Kathmandu Valley, Nepal: insights into primary and secondary sources

“…5d) while the lowest during winter. Their seasonal variation was in agreement with the ambient temperature (Fig S1a), which can influence the isoprene emissions and the photochemical processes (Shen et al, 2015;Wang et al, 2008). The annual average concentration was higher than the 400 urban sites reported from Beijing (44.3 ng m -3 ) and Kunming (108 ng m -3 ) (Ding et al, 2016a), even one to two orders of magnitude higher than that from global oceans and the Arctic (Hu et al, 2013).…”

“…5). Though the major emissions of aromatics come from solvent and fossil fuel use, biomass burning is also considered as an possible source 465 in some sites of the world (Shen et al, 2015). There was a good correlation between DHOPA and levoglucosan ( Fig.…”

“…This method has been applied successfully in many aerosol studies (Fu et al, 2010;Shen et al, 2015;Ding et al, 2016a). The mass fraction of tracer compounds in SOC (FSOC) for an individual precursor was calculate based on the smog chamber simulations.…”

The importance of non-fossil sources in carbonaceous aerosols in a megacity of central China during the 2013 winter haze episode: A source apportionment constrained by radiocarbon and organic tracers

“…Analytical procedures were described in detail in a previous study (Ding et al, 2012), and are provided in the Supporting Information. The relative standard deviation (RSD) for this reconstructed SOC in our lab was 51 ± 11% (Shen et al, 2015).…”

Chemical characteristics of submicron particles at the central Tibetan Plateau: insights from aerosol mass spectrometry

“…3), MO-OOA was likely closely related to long-range transport of air mass from southwest, while LO-OOA could be derived from relatively shorter distance transport such as from a marine air mass from south and regional background aerosol during the nighttime. Shen et al (2015) reported that there was a significant aerosol source from biogenic emissions during summer near the Nam Co station. As shown in Fig.…”