The effect of salting with magnesium chloride on the concentration of particular matter in a road tunnel

This work presents an analysis of the effect of dust treatment with a magnesium chloride solution on the concentration of airborne particulate matter inside a road tunnel. During the winter 2004/2005, the road inside the tunnel of Strømsås was salted 43 times with magnesium chloride solution, of which 27 times with the amount 20gm-2 and 16 with 40gm-2. In addition, the road was swept 11 times and washed twice. Simultaneously, one recorded the concentration of PM10PM10 and PM2.5PM2.5 hourly. The model used in the analysis is a generalised additive model (GAM) on log-scale, having traffic counts, diverse meteorological conditions and the pollution reducing actions as predictor variables.The analysis revealed no clear effect from sweeping and washing. The impact of the dust binding medium on the concentrations of particulate matter is, however, clearly propitious. As one would expect, it is largest immediately after applying the magnesium chloride, and diminishes steadily afterwards. The duration of the effect is estimated to 10 days, but with a rather large uncertainty (95% confidence intervals between 3 and 16 days). We estimated an effect (reduced pollution level) of 70%70% on the concentration of the coarse particles PM10PM10–PM2.5PM2.5 and 56%56% on the concentration of PM10PM10. The estimated effect on the fine particles PM2.5PM2.5 is a modest 17%17%, and barely significant.