Radon as a tracer of atmospheric influences on traffic-related air pollution in a small inland city

Abstract

A B S T R A C T One year of radon, benzene and carbon monoxide (CO) concentrations were analysed to characterise the combined influences of variations in traffic density and meteorological conditions on urban air quality in Bern, Switzerland. A recently developed radon-based stability categorisation technique was adapted to account for seasonal changes in day length and reduction in the local radon flux due to snow/ice cover and high soil moisture. Diurnal pollutant cycles were shown to result from an interplay between variations in surface emissions (traffic density), the depth of the nocturnal atmospheric mixing layer (dilution) and local horizontal advection of cleaner air from outside the central urban/industrial area of this small compact inland city. Substantial seasonal differences in the timing and duration of peak pollutant concentrations in the diurnal cycle were attributable to changes in day length and the switching to/from daylight-savings time in relation to traffic patterns. In summer, average peak benzene concentrations (0.62 ppb) occurred in the morning and remained above 0.5 ppb for 2 hours, whereas in winter average peak concentrations (0.85 ppb) occurred in the evening and remained above 0.5 ppb for 9 hours. Under stable conditions in winter, average peak benzene concentrations (1.1 ppb) were 120% higher than for well-mixed conditions (0.5 ppb). By comparison, summertime peak benzene concentrations increased by 53% from well-mixed (0.45 ppb) to stable nocturnal conditions (0.7 ppb). An idealised box model incorporating a simple advection term was used to derive a nocturnal mixing length scale based on radon, and then inverted to simulate diurnal benzene and CO emission variations at the city centre. This method effectively removes the influences of local horizontal advection and stability-related vertical dilution from the emissions signal, enabling a direct comparison with hourly traffic density. With the advection term calibrated appropriately, excellent results were obtained, with high regression coefficients in spring and summer for both benzene (r 2 Â0.90Á0.96) and CO (r 2 Â0.88Á0.98) in the two highest stability categories. Weaker regressions in winter likely indicate additional contributions from combustion sources unrelated to vehicular emissions. Average vehicular emissions during daylight hours were estimated to be around 0.503 (542) kg km (2 h (1 for benzene (CO) in the Bern city centre.

Characterizing the State of the Urban Surface Layer Using Radon‐222

“…A more detailed seasonal analysis of stability influences on all components of the radiation balance, surface energy budget, and urban canyon turbulence will be the subject of a future investigation, with a view to providing detailed benchmarking data sets for the development, testing, and evaluation of urban canopy model parameterization schemes. Regarding the need for improved urban carbon budgets and pollution monitoring to help constrain urban climate effects (Grimmond et al, ), the combination of radon‐based stability classification and radon‐derived effective mixing heights has been proven to greatly assist in relating observed urban pollution concentrations to source strengths (Williams et al, ) and could prove useful in this endeavor independently of dedicated lidar or ceilometer observations.…”

“…Historically, difficulties in reliably relating observed nocturnal radon concentrations to a particular mixing state at a given site were largely attributable to a lack of appropriate conditioning of single-height radon observations and researchers not accounting for fetch effects. Subsequently, direct observations of radon gradients (e.g., Chambers et al, 2011;Williams et al, 2013), interpretation of temporal changes in radon concentration (e.g., Fujinami & Esaka, 1987;Perrino et al, 2001), and the estimation and removal of fetch effects (e.g., Chambers, Podstawczynska, et al, 2016;Chambers et al, 2015;Williams et al, 2016) have been demonstrated to solve this problem.…”

“…From year to year the average radon source function is quite consistent, since the half-life of radon's parent, radium-226, is 1,600 years. To account for seasonal changes in soil moisture, as well as freezing and snow cover, on the radon source function, stability classifications ( Figure 1b) need to be derived separately for each season (e.g., see Williams et al, 2016). Once defined, however, these seasonal classifications can be used unchanged retrospectively, or for future analyses, for many decades.…”

“…Since radon has a relatively consistent and spatially distributed source function, assuming an estimate of the source function is available, near-surface radon concentrations can be used in conjunction with a simple box model (equation (3)) to estimate the "effective mixing height" under stable nocturnal conditions (e.g., Chambers, Podstawczynska, et al, 2016;Fontan et al, 1979;Griffiths et al, 2013;Guedalia et al, 1980;Sesana et al, 2003;Williams et al, 2016):…”

Atmospheric mercury in the southern hemisphere tropics: seasonal and diurnal variations and influence of inter-hemispheric transport

“…Figure 2 also shows that fetch-component radon concentrations begin to drop in both years around September-October, which HYSPLIT trajectories show is coincident with air mass origin shifting away from the Australian continent and towards the northern Arafura and Timor seas. Throughout the wet season fetch-component radon remains low, though not at baseline levels (Zahorowski et al, 2013;Chambers et al, 2016b), implying that there is still some terrestrial influence on incoming air masses from the Australian continent or surrounding islands to the north. Wetseason wind data (Fig.…”