Fig. 2: The schematic of the concomitant pollution in the negative response mechanism and samples of experimental analysis.

a Conceptual schematic of the mechanism of concomitant pollution in the negative response mechanism, based on the common trends observed across the nine studied events. This schematic diagram illustrates the trade-off relationship between pollution levels and buffering capacity. As pollution intensifies, the buffering capacity (yellow curve) exhibits a declining trend. The total pollution burden consists of secondary pollution (pink curve) and concomitant pollution (blue curve). b, c Field emission transmission electron microscopy experiment. The samples during the cleaning period were collected at a fine particulate matter (PM_2.5) of 31 μg m⁻³ on October 30, 2023, while the samples during the pollution period were collected at a PM_2.5 of 185 μg m⁻³ on the same day. All other sampling conditions were the same. d Probability density distribution (Bin width = 10) of the concomitant pollution induced by a slight change in pH (ΔpH = 0.5) under the NH₄⁺/NH₃-dominated buffering regime during the clean period. The blue bars represent the distribution of this concomitant pollution. The x-axis indicates the magnitude of the concomitant pollution triggered by the pH perturbation, while the y-axis represents the corresponding probability density.