[Characterization of Two Heavy Pollution Episodes in Tianjin in 2020]

High-resolution online monitoring data from January to February in 2020 was used to study the characterization of two heavy pollution episodes in Tianjin in 2020; the heavy pollution episode that lasted from January 16 to 18, 2020 (referred to as episode Ⅰ) and that from February 9 to 10, 2020 (referred to as episode Ⅱ) were analyzed. The results showed that two heavy pollution episodes were influenced by regional transportation in the early stage and local adverse meteorological conditions in the later stage. During these episodes, the average wind speed was low, the average relative humidity was close to 70%, and relative humidity approached the saturated, the boundary layer heights were below 300 m, and the horizontal and vertical diffusion conditions were poor. Compared to episode Ⅰ, the concentration of pollutants decreased during episode Ⅱ, especially for the concentration of NO₂. During the episode Ⅱ, the concentrations of PM_2.5 and CO were higher in the north of Tianjin. The chemical component concentrations and their mass ratios to PM_2.5 changed significantly in both episodes; the concentrations of secondary inorganic ions (NO₃^-, SO₄^2-, and NH₄⁺), elemental carbon (EC) and Ca²⁺were higher in episode Ⅰ, the concentrations of organic carbon (OC) and Cl^- slightly increased in episode Ⅱ; and the concentrations of K⁺were higher in episode Ⅱ. Compared to episode Ⅰ, because of the increase in the combustion sources and significant reductions in the number of vehicles, the mass ratios of SO₄^2-, OC, and K⁺ to PM_2.5 increased while the mass ratios of NO₃^- and EC to PM_2.5 decreased in episode Ⅱ; the mass ratios of NH₄⁺ and Cl^- to PM_2.5 were relatively higher due to the continuity of the industrial production processes; the mass ratios of Ca²⁺ to PM_2.5 were lower in two heavy pollution episodes because construction activities were halted. Source apportionment of PM_2.5 was performed using the positive matrix factorization (PMF) model. In episode Ⅰ, the major sources of PM_2.5 in Tianjin were secondary sources, industrial and coal combustion, vehicle exhaust, crustal dust, fireworks and biomass burning, with contributions of 53.8%, 20.2%, 18.6%, 6.3%, and 1.1%, respectively. In episode Ⅱ, the same sources were identified in the PMF analysis with contributions of 48.3%, 28.2%, 8.7%, 2.6%, and 12.2%, respectively. Compared to episode Ⅰ, the contributions of industrial and coal combustion, fireworks and biomass burning increased, and the contributions of secondary sources, vehicle exhaust, and crustal dust decreased in episode Ⅱ; contributions of vehicle exhaust and crustal dust decreased by 53.2% and 58.7%, respectively.