Urban Tracer Dispersion and Infiltration into Buildings Over a 2-km Scale

J C Matthews,H G Silva,A Bacak,M D Wright,M Flynn,M Priestley,C J Percival,T J Bannan,D E Shallcross,D Martin

doi:10.1007/s10546-019-00498-5

Abstract

Field experiments were undertaken in the summer of 2015 in Manchester, UK, to investigate the dispersion behaviour and infiltration into buildings of gas-phase pollutants over horizontal distances of 1–5 km. Inert cyclic perfluorocarbon tracers were released for 15 min at either one or three release points and samples taken in locations indoors and outdoors up to 2 km downwind. Background measurements of these cyclic perfluorocarbons range between 5.6 and 12.6 parts per quadrillion (ppq). On most occasions, tracer concentrations are higher on the sixth floor than at ground level. Tracer concentrations persist in the least well-ventilated rooms after concentrations return to background levels outdoors. The highest tracer concentrations, 329 ppq above background, occur at dawn on 23 July from a sixth-floor sampling position during thermally stable conditions. At low wind speeds, tracer is detected upwind of the prevailing wind direction; on 24 July, tracer is detected to the north-west of the release point for north-north-east wind direction. A simple street network model does not predict tracer concentrations at low wind speeds over the km scales in this investigation due to tracer likely escaping the urban canopy. Predictions from a simple correlation model overestimate concentrations originating from distant sources, which is believed to be due to infiltration into buildings along the journey from source to receptor. A Gaussian plume model predicts the highest tracer concentrations for most receptor points on 23 July when the lowest Obukhov length of – 26 m was measured, agreeing with tracer measurements.

Highlights

1.1 Rationale and ScopeFlow through the complex geometries of cities plays an important role in determining air quality (Martin et al 2008), with well-tested dispersion models required to understand many aspects of urban air pollution
Results for each tracer were deemed significant if the measured concentration was greater than the mean background concentration paired to the sample concentration, plus two standard deviations across all background concentrations obtained for that tracer. (Results that were non-significant are listed as NS in the Table.) There is a 1 ppq analytical uncertainty and a 5% systematic uncertainty in all measurements due to the quoted accuracy of the primary standard
The highest concentrations were measured when the tracer was released at sunrise; up to 330 ppq of PMCH concentration above background was measured at the sixth floor of the Simon Building from a release 1.9 km away (R2)

Summary

Introduction

1.1 Rationale and ScopeFlow through the complex geometries of cities plays an important role in determining air quality (Martin et al 2008), with well-tested dispersion models required to understand many aspects of urban air pollution. Several studies have investigated airflow (e.g. Martin et al 2011a and references therein) through urban geometries, and derived parametrizations that describe downwind concentrations of pollutants for a given release rate, wind speed, and building geometry (Wood et al 2009; Martin et al 2010a). Simmonds et al (2002) measured PMCH and mPDMCH concentrations at Mace Head, Ireland in 2001; with values of 5.5 (± 0.3) ppq of PMCH, and 10.7 (± 0.3) ppq and 8.5 (± 0.2) ppq of the cis and trans isomers of the mPDMCH tracer respectively. Urban Tracer Dispersion and Infiltration into Buildings Over. Of 8 ppq in 2007 and predicted an increase of 0.2 ppq per year If we extrapolate this value we can tentatively estimate that the concentration of PMCH should have reached 9.6 ppq by 2015. Hanna et al (1982) simplified the second regime using Monin–Obukhov similarity theory and described the ground-level maximum concentration Cmax as a function of distance from the source x as

Methods

Results

Conclusion