An investigation into unpaved road emissions from a UK surface limestone quarry using cylindrical adhesive pad collectors and image analysis

Demand for increased aggregates extraction combined with ongoing pressure to adhere to existing and future environmental regulatory requirements requires sustained research and investigation into associated particulate emissions. Open pit quarries commonly operate 'blast and haul' extraction methods, implying that any increase in production will lead to increases in quarry traffic and hence unpaved haul road particulate emissions. This paper presents an analysis and discussion of experimental data collected during a roadside particulate emission sampling study conducted within a major UK limestone quarry. The particulate samples were collected using a total of six cylindrical adhesive pad type collectors located symmetrically, at equally spaced intervals upwind and downwind of an unpaved truck haul road. A total of three sampling studies were conducted, each representing approximately 1 hour of road activity and between 6 and 8 vehicle passes. The first two sampling periods were conducted on an initially unwetted haul road. The third sampling period was conducted following road wetting by a bowser. The surface characteristics of the haul road surface were obtained by the collection of a series of swept surface material samples. An analysis of these samples collected prior to and during each of the sampling periods established the surface moisture and silt content of the unpaved road surface. Weather data were obtained using a permanently sited metrological station located on a hill above the quarry, together with a portable weather station located upwind of the haul road to providing accurate localized data. The dust samples collected on the adhesive pads were sized using an image analysis (IA) based method. The IA method employed advanced image enhancement functions to characterize the captured particulates in terms of total count, captured mass, particle aspect ratio and size distribution. These characteristics were compared for each collector sample over each of the three sampling periods. The study concluded that the IA method was able to provide a useful and consistent range of sample characteristics, although the limit of the resolution of the sizing of the particulates sampled was found to be approximately 5 μm. It was found that there was minimal variation in the sampled particulate size distribution and aspect ratio up to a collection distance of 29 m from the roadside in the downwind direction. A clear attenuation in the capture of particles was identified in terms of particle count and total captured mass reporting to measured upwind background levels at 29 m from source. The directionality of the particulate concentrations collected on the pads was found to vary significantly. It was concluded that IA offers a viable method for the analysis of samples collected from cylindrical adhesive pad collectors.