Seasonal and diurnal variations in dust characteristics on the northern slopes of the Tien Shan – Grain-size, mineralogy, chemical signatures and isotope composition of attached nitrate

• Dust monitoring at seasonal and diurnal scale at the northern slopes of Tien Shan.
• Particle size composition was measured for 31 grain size classes at 1-min resolution.
• Particles >2.5 μm were collected using a high-volume-impactor at 3-day intervals.
• Analytical data including the isotope composition of the attached NO3 are presented.
• We infer relationships between dust composition and hemispherical circulation.

A dust monitoring program was initiated in the summer of 2010 to (i) improve the informative value of aeolian components of varved lake sediments in Central Asia for palaeoclimatic interpretations, (ii) evaluate the impact of aeolian influx on modern lakes in the region and (iii) obtain data for a future comparison with dust east of the high mountain belt formed by the Alay, Pamir, Tien Shan and Altai mountains. We collected the coarse (>2.5 μm) dust fraction on the northern slopes of the Tien Shan 42°40′49.69″N, (74°41′37.36″E, 1740 m asl) using a high-volume slit-impactor at 3-day sampling intervals. We present data on the mineralogical composition, particle-size distribution, soluble salts and nitrate isotope composition of the collected dust. The short-term and seasonal changes in dust concentration and composition are discussed in the context of high temporal resolution measurements of meteorological parameters and particle counts for 31 grain-size bands.Throughout the study period, CaCO3 was a major dust constituent (average particle frequency 14%). Between July 2010 and October 2012, the average content of soluble salts was 10 wt.%; mole percentages of water-leachable anions were 60% NO3, 30% SO4, 10% Cl. Ca was the dominant leachable cation (>90%). The collected dust comprised (i) gypsum which forms pedogenically in the topsoils of arid regions and (ii) secondary gypsum originating from the interaction of sulphuric acid aerosols with CaCO3 in the atmosphere. Variable proportions of (i) and (ii) and the extent of Ca(NO3)2 formation (verified by chemical mass budgets) were documented in the Ca/Sr and Ca/SO4 ratios of the aqueous leachates.The isotopic compositions of the dust nitrate in the majority of the samples clustered between −10 and +10‰ for δ15N[NO3] (VSMOW) and +50 and +100‰ for δ18O[NO3] (Air). The δ18O[NO3] values of a majority of the collected samples exhibited a weak positive correlation with the NO3 load of the collected dust (R2 = 0.148). The time series of the δ15N[NO3] values varied around a basement level of −6‰, which indicates that NOx from mobile sources is a major precursor of the dust-NO3. The distinct, positive departures from these values correlated with increases in the content of soot. We conclude that the related δ15N[NO3] peaks reflect increased NOx contributions from coal burning. The possible origin of the dust was evaluated using satellite images and backward trajectory calculations for a few example monitoring intervals. Data synthesis in the context of hemispherical atmospheric circulation models, including the data for 2013, the results from ongoing chemical and isotope analyses of the non-soluble solid residues of the collected materials and the statistical handling of an extended data set will further elucidate the relationships involving atmospheric circulation, atmospheric dust load, dust sources and the characteristics of the study region.