Sensitivity of local air quality to the interplay between small- and large-scale circulations: a large-eddy simulation study

Abstract

Abstract. Street-level urban air pollution is a challenging concern for modern urban societies. Pollution dispersion models assume that the concentrations decrease monotonically with raising wind speed. This convenient assumption breaks down when applied to flows with local recirculations such as those found in topographically complex coastal areas. This study looks at a practically important and sufficiently common case of air pollution in a coastal valley city. Here, the observed concentrations are determined by the interaction between large-scale topographically forced and local-scale breeze-like recirculations.Analysis of a long observational dataset in Bergen, Norway, revealed that the most extreme cases of recurring wintertime air pollution episodes were accompanied by increased large-scale wind speeds above the valley. Contrary to the theoretical assumption and intuitive expectations, the maximum NO 2 concentrations were not found for the lowest 10 m ERA-Interim wind speeds but in situations with wind speeds of 3 m s −1 . To explain this phenomenon, we investigated empirical relationships between the large-scale forcing and the local wind and air quality parameters. We conducted 16 large-eddy simulation (LES) experiments with the Parallelised Large-Eddy Simulation Model (PALM) for atmospheric and oceanic flows. The LES accounted for the realistic relief and coastal configuration as well as for the large-scale forcing and local surface condition heterogeneity in Bergen. They revealed that emerging local breeze-like circulations strongly enhance the urban ventilation and dispersion of the air pollutants in situations with weak largescale winds. Slightly stronger large-scale winds, however, can counteract these local recirculations, leading to enhanced surface air stagnation.Furthermore, this study looks at the concrete impact of the relative configuration of warmer water bodies in the city and the major transport corridor. We found that a relatively small local water body acted as a barrier for the horizontal transport of air pollutants from the largest street in the valley and along the valley bottom, transporting them vertically instead and hence diluting them. We found that the stable stratification accumulates the street-level pollution from the transport corridor in shallow air pockets near the surface. The polluted air pockets are transported by the local recirculations to other less polluted areas with only slow dilution. This combination of relatively long distance and complex transport paths together with weak dispersion is not sufficiently resolved in classical air pollution models. The findings have important implications for the air quality predictions over urban areas. Any prediction not resolving these, or similar local dynamic features, might not be able to correctly simulate the dispersion of pollutants in cities.

A very high-resolution assessment and modelling of urban air quality

“…In the most extreme cases, known as temperature inversions, the air temperature is increasing with height (Wolf et al, 2014), trapping the turbulence, and therefore pollutants, in a shallow layer near the surface. Even in such conditions, there could be non-negligible turbulence as well as horizontal transport of pollutants driven by local circulations (Wolf-Grosse et al, 2017a). If the calm weather conditions persist sufficiently long, concentrations of air pollutants may reach levels in excess of regulatory thresholds for air pollution (Bergen Kommune, 2019;European Commission, 2019), while their spatial pattern would be highly heterogeneous.…”

“…Thus, the local circulations are likely to determine air quality for the districts' populations. The effect of the local circulations could be accounted for in the PALM simulations (Wolf-Grosse et al, 2017a) but not in statistical models relying on coarse spatial resolution (∼ 1 km) mesoscale models.…”

The large-scale circulation during air quality hazards in Bergen, Norway

“…The RHT station is downstream of the area with the highest local emissions. The exact local transport pathway along the valley axis plays an important role here in addition to local stagnation events (Wolf-Grosse et al, 2017).…”

“…The first explanation questions the direct physical link between the large and the local scales. It is possible that the highly local stagnation inside the valley is only connected through an intermediate step to the large-scale circulation that is resolved by the coarse-resolution global climate models (Outten and Esau, 2013;Wolf-Grosse et al, 2017). This could be a circulation steered by the topography or land-sea differences on the scale of tens of kilometres (Barstad et al, 2008).…”

“…We test if a condition of persistence in the proxy for air pollution episodes can improve its predictive skill and assess the relation between the duration of air pollution episodes and their severity. In addition, we identify the flow above the Bergen valley and the largescale flow features connected to air pollution episodes, and test Wolf-Grosse et al, 2017). Moreover, models tend to predict too deep atmospheric boundary layers (Davy and Esau, 2014;Zilitinkevich et al, 2013).…”