Characterization of inorganic ions in rainwater in the megacity of Shanghai: Spatiotemporal variations and source apportionment

In the present study, the pH values and the concentrations of nine inorganic ions (SO ₄ ²⁻ , NO ₃ ⁻ , NH ₄ ⁺ , Cl ⁻ , Na ⁺ , Ca ²⁺ , K ⁺ , Mg ²⁺ and F ⁻ ) in rainwater samples collected in Shanghai during 2011–2016 were measured to study their spatiotemporal variations and to identify their sources. The rainwater was severely acidified with an average volume-weighted mean (VWM) pH value of 4.96 during 2011–2016, indicating that Shanghai still suffers from severe acid rain pollution. The VWM concentrations of the measured ions in the rainwater samples decreased by 26–53% during 2011–2012. Then, SO ₄ ²⁻ , NO ₃ ⁻ , NH ₄ ⁺ , Ca ²⁺ and F ⁻ remained at relatively stable levels during 2012–2016, whereas Cl ⁻ , Na ⁺ and Mg ²⁺ fluctuated during the same period. The seasonal variations showed that all of the ions except for NH ₄ ⁺ exhibited the highest VWM concentrations in winter, followed by those in spring and autumn, and the lowest values were in summer. The highest VWM concentrations of these ions in winter were mainly attributed to adverse meteorological conditions, the rare rainfall, as well as greater anthropogenic emissions. The spatial distributions of the VWM concentrations of SO ₄ ²⁻ , NO ₃ ⁻ and NH ₄ ⁺ in rainwater in Shanghai during 2014 were significantly linked to local emission sources, and higher levels of SO ₄ ²⁻ and NO ₃ ⁻ were observed in the industrial and populated areas (i.e., the JS and PD districts), whereas higher VWM concentrations of NH ₄ ⁺ were concentrated in agricultural areas (i.e., the QP and SJ districts). The wet deposition fluxes of SO ₄ ²⁻ , NO ₃ ⁻ and NH ₄ ⁺ were closely related to rainfall. The areas featured by greater rainfall generally presented higher wet deposition fluxes of SO ₄ ²⁻ , NO ₃ ⁻ and NH ₄ ⁺ . On the basis of the positive matrix factorization (PMF) and Enrich factor (EF) analysis, five sources for the inorganic ions in rainwater in urban Shanghai were identified, including secondary formation, crustal/soil dust, construction emissions, marine sources and waste incineration emissions. Our results provided a comprehensive picture of the ionic compositions of rainwater in Shanghai and could help the government to develop targeted regulations to reduce acid deposition in this typical megacity of China.