Abstract
Abstract. Sulfate and nitrate compounds can greatly increase the hygroscopicity of mineral particles in the atmosphere and consequently alter the particles' physical and chemical properties. Their uptake on long-distance-transported Asian dust particles within mainland China has been reported to be substantial in previous studies, but the production was very inefficient in other studies. We compared these two salts in particles collected from a synoptic-scale, mid-latitude, cyclone-induced dust storm plume at the Tengger Desert (38.79° N, 105.38° E) and in particles collected in a postfrontal dust plume at an urban site in Xi'an (34.22° N, 108.87° E) when a front-associated dust storm from the Tengger Desert arrived there approximately 700 km downwind. The results showed that the sulfate concentration was not considerably different at the two sites, while the nitrate concentration was slightly larger at the urban site than that at the desert site. The estimated nitrate production rate was 4–5 ng µg−1 of mineral dust per day, which was much less than that in polluted urban air. The adiabatic process of the dust-loading air was suggested to be the reason for the absence of sulfate formation, and the uptake of background HNO3 was suggested to be the reason for the small nitrate production. According to our investigation of the published literature, the significant sulfate and nitrate in dust-storm-associated samples within the continental atmosphere reported in previous studies cannot be confirmed as actually produced on desert dust particles; the contribution from locally emitted and urban mineral particles or from soil-derived sulfate was likely substantial because the weather conditions in those studies indicated that the collection of the samples was started before dust arrival, or the air from which the samples were collected was a mixture of desert dust and locally emitted mineral particles. These results suggest that the production of nitrate and sulfate on dust particles following cold fronts is likely limited when the particles move from the desert to populated areas within the continent. For an accurate quantification of sulfate and nitrate formed on long-distance-transported desert dust particles at downwind populated areas in eastern China, dust collection efforts are indispensable to minimize any possible influence by locally emitted particles or at least to ensure that the samples are collected after dust arrival.
Highlights
Mineral dust particles constitute a substantial fraction of atmospheric aerosol mass and play various roles in atmospheric physics and chemistry (Bi et al, 2011; Dentener et al, 1996; Fu et al, 2009; Sokolik and Toon, 1996; Tegen et al, 1996)
The NH+4 concentration was lower than the detection limit in the first sample and increased slightly in the second and third samples. These results indicate that the composition of particles in the postfrontal air was close to the state of dust particles at the desert areas, whereas the composition was gradually affected by local emissions afterwards
To further examine the situation of previously reported sulfate and nitrate formation on dust storm particles at populated areas in eastern China, we investigated all published papers we found on this subject and carefully checked the records of sample collection and the available meteorological conditions when the samples were collected in those studies from the papers and official public websites for historical meteorological records
Summary
Mineral dust particles constitute a substantial fraction of atmospheric aerosol mass and play various roles in atmospheric physics and chemistry (Bi et al, 2011; Dentener et al, 1996; Fu et al, 2009; Sokolik and Toon, 1996; Tegen et al, 1996) Dust particles at their source areas are mainly composed of quartz, clays, micas, feldspars, carbonates (primarily calcite, CaCO3) and other minor minerals (Usher et al, 2003). Some measurements of the chemical composition of longdistance-transported dust particles have shown that most of the dust particles were not altered chemically and were externally mixed with species produced in the air via gasto-particle reactions, such as sulfate and nitrate (Denjean et al, 2015; Song et al, 2005). We compare the concentrations and mass fractions of sulfate and nitrate in the samples at the two sites and examine the production of nitrate and sulfate on desert dust particles after the particles were transported from the desert to the populated area in an attempt to understand the chemistry and aging on dust
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