Impact of Gobi desert dust on aerosol chemistry of Xi'an, inland China during spring 2009: differences in composition and size distribution between the urban ground surface and the mountain atmosphere

Abstract

Abstract. Composition and size distribution of atmospheric aerosols from Xi'an city (∼ 400 m, altitude) in inland China during the spring of 2009 including a massive dust event on 24 April were measured and compared with a parallel measurement at the summit (2060 m, altitude) of Mt. Hua, an alpine site nearby Xi'an. EC (elemental carbon), OC (organic carbon) and major ions in the city were 2-22 times higher than those on the mountaintop during the whole sampling period. Compared to that in the non-dust period a sharp increase in OC was observed at both sites during the dust period, which was mainly caused by an input of biogenic organics from the Gobi desert. However, adsorption/heterogeneous reaction of gaseous organics with dust was another important source of OC in the urban, contributing 22 % of OC in the dust event. In contrast to the mountain atmosphere where fine particles were less acidic when dust was present, the urban fine particles became more acidic in the dust event than in the non-dust event, mainly due to enhanced heterogeneous formation of nitrate and diluted NH 3 . Cl − and NO − 3 in the urban air during the dust event significantly shifted toward coarse particles. Such redistributions were further pronounced on the mountaintop when dust was present, resulting in both ions almost entirely staying in coarse particles. On the contrary, no significant spatial difference in size distribution of SO 2− 4 was found between the urban ground surface and the mountain atmosphere, which dominated in the fine mode (< 2.1 µm) during the nonevent and comparably distributed in the fine (< 2.1 µm) and coarse (> 2.1 µm) modes during the dust event.

Particle acidity and sulfate production during severe haze events in China cannot be reliably inferred by assuming a mixture of inorganic salts

“…Such a correlation was also reported by other researchers Yu et al, 2005), suggesting the co-occurrence and internally mixing state of both compounds in the atmosphere. In addition, the previous field measurements also revealed that WSOM in China is not only enriched in carboxylic acids (including oxalic acid) but also in other organic species, including carbonyls, amines, and water-soluble nitrogen-containing organic compounds (G. Wang et al, 2010;Wang et al, 2013;Zheng et al, 2015;Yao et al, 2016;F. Liu et al, 2017).…”

“…Such an assumption is clearly invalid, as aerosols typically contain a large portion of WSOM in China (Fig. 3), including organic nitrogen species (G. Wang et al, 2010;Wang et al, 2013) and acids (G. Wang et al, 2009Wang et al, , 2010Wang et al, 2006). Also, organic acids engage in particle-phase reactions with the basic species (i.e., NH 3 and amines), significantly enhancing the particle hygroscopicity and reducing the acidity (GomezHernandez et al, 2016).…”

Aircraft measurements of polar organic tracer compounds in tropospheric particles (PM&lt;sub&gt;10&lt;/sub&gt;) over central China

“…Figure 8 shows the vertical profiles of major organic species among the identified organics. It is noticeable that, with increasing altitude, concentrations of most organic species (e.g., levoglucosan, mannitol, phthalic acid, MBTCA, and pinic acid) decrease in both seasons, owing to the atmospheric dilution during upward transport from the ground surface (Simoneit et al, 1991;Gao et al, 2003;Feng et al, 2006;Fu et al, 2010;Wang et al, 2013). During the ACEAsia campaign, a large organic carbon aerosol source was observed in the free troposphere over the downwind regions of China (Heald et al, 2005), which is proposed to be mainly from isoprene-derived SOA (Henze and Seinfeld, 2006).…”

One-year observations of carbonaceous and nitrogenous components and major ions in the aerosols from subtropical Okinawa Island, an outflow region of Asian dusts

“…In November we obtained relatively high OC / EC ratio. This may be caused by long-range atmospheric transport of biomass burning plumes from Southeast Asia because biomass burning is very common in the region in autumn (Wang et al, 2013b) and the air masses mostly came from Southeast Asia including Philippines (Fig. 2d).…”