Pollution characteristics in a dusty season based on highly time-resolved online measurements in northwest China

To investigate the pollution characteristics and potential sources in a dusty season, an online analyzer was used to measure trace gases and major water-soluble ions in PM₁₀ from April 1st to May 29th, 2011 in Lanzhou. The average concentrations of HONO, HNO₃, HCl, SO₂ and NH₃ were 0.93, 1.16, 0.48, 9.29 and 5.54 μg/m³, respectively, and 2.8, 2.76, 8.28 and 2.48 μg/m³ for Cl^-, NO₃^-, SO₄^2- and NH₄⁺. In the non-dust period, diurnal variations of SO₄^2-, NO₃^- and their gaseous precursors showed similar change trend. NH₄⁺ showed unimodal pattern whereas NH₃ illustrated a bimodal pattern. HCl and Cl^- showed an opposite diurnal pattern. In the dust event, temporal profiles of HCl and Cl^-, SO₂ and SO₄^2- all presented similar change trend, and SO₄^2- and Cl^- preceded dust ions (Ca²⁺ and Mg²⁺) 13 h. The ratios of NO₃^- to SO₄^2- were 0.65 in the non-dust period and 0.31 in the dust event. In the dust event, the sulfur oxidation ratio (SOR) was a factor of 1.33 greater than that in the non-dust period, and [SO₄^2-]/[SO₂] was 2.31 times of that in the non-dust period. The source apportionment using Probabilistic Matrix Factorization (PMF) suggested that fugitive dust (58.09%), secondary aerosols (33.98%), and biomass burning (7.93%) were the major sources in the non-dust period whereas dust (67.01%), salt lake (29.68%), biomass burning (0.8%), and motor vehicle (2.51%) were the primary sources in the dust event. Concentration weighted trajectory (CWT) model indicated that NO₃^-, Cl^- and K⁺ could be regarded as local source species, the potential sources of Na⁺, Mg²⁺ and Ca²⁺ concentrated in the two large areas with the one covered in the junction areas of Xinjiang, Qinghai and Gansu and another one covered the places around in Lanzhou, the potential sources of SO₄^2- were mainly localized in the areas adjacent to Lanzhou.