Field measurements on the attenuation characteristics of PM2.5 and toxic gases in a blasting metro tunnel and evaluation of the re-entry time.

[Display omitted] • PM2.5, toxic gases, air temperature and humidity were measured in a blasting tunnel. • The attenuation rule of PM2.5, CO and NO 2 conformed to an exponential function. • The decay time, peak concentration and corresponding time of pollutants were revealed. • The construction environment in the tunnel was high temperature and high humidity. • The re-entry time in descending order was dust, CO and NO 2. In this study, to reveal the attenuation rules of dust and toxic gases versus time after blasting, nine blasting measurements were conducted in a metro tunnel. Two blasting scenarios were used to investigate the diffusion characteristics of blasting fumes, whilst the remaining seven blasting scenarios were used to analyze the evolution of air parameters [e.g., carbon monoxide (CO), nitrogen dioxide (NO 2) and fine particulate matter (PM2.5) concentration, particle size distribution, and air temperature and humidity] versus time after blasting at different positions. The field measurement results showed that the actual throwing length of the blasting fumes was less than half of its empirical value. The attenuation rules of PM2.5, CO, and NO 2 conformed to an exponential function with a quartic polynomial of time in the exponent. The decay time of CO and NO 2 concentrations was longer than that of PM2.5, and the sizes of particles in the working zone were basically less than 30 μm 5 min after blasting. In addition, the time to peak CO concentration was basically longer than that for PM2.5 and NO 2 , and the peak concentrations of PM2.5, CO, and NO 2 were greatly affected by the supply air volume, explosive mass, forced air duct outlet position, surrounding rock grade, and geological conditions. Moreover, blasting had a weak effect on the air temperature. In contrast, the air temperature increased significantly during slagging operations because of the mechanical heat discharged; the air temperature could be as high as 36.6 ℃, indicating that cooling technologies are needed to achieve an air temperature below 28 ℃. Furthermore, the difficulty of dust control and re-entry time based on dust were significantly greater than that of CO, whilst re-entry time based on CO was significantly larger than that of NO 2 by an order of magnitude, and was generally 1,500–2,500 s after ventilation in China. These findings offer theoretical guidance for the cleaner production and safer management of construction tunnels. [ABSTRACT FROM AUTHOR]