• Statistical source apportionment and multivariate analyses emerged as core tools for decoupling urban/industrial versus natural aerosol sources, using tracer patterns and cross-regional comparisons. [1], [6], [10], [16], [19].

• Atmospheric photochemistry frameworks framed pollutant transformation, emphasizing oxidation of SO2, ozone, hydrogen peroxide, and organic carbon in clouds and the troposphere. [7], [9], [11], [20].

• Aerosol size distribution and composition became a key diagnostic for urban haze versus background air, with emphasis on sampling, size-resolved chemistry, and urban-rural contrasts. [8], [14], [15], [16], [17].

• Global and regional emission inventories and metal tracers framed long-range transport, Arctic haze questions, and differentiation of natural versus anthropogenic sources. [1], [10], [19].

• Organic constituents and VOCs of atmospheric particulate matter highlighted natural plant-derived VOCs, biogenic-organic matter, and complex organic carbon budgets. [3], [14], [17], [18].