Air quality in Bergen during atmospheric blocking situations

Atmospheric conditions observed during late November and early December last year in Bergen are known as the atmospheric blocking. During days with blocking, easterly and north-easterly weak winds prevail. In Bergen, blocking situations have been frequently observed in January-February 2010 and return now in November. Blocking, as a meteorological phenomenon, causes large public resonance as its persistence over several days and even weeks deteriorate the air quality in some parts of the city. The air quality in Bergen is monitored by Statens Vegvesen and Klima and Forurensning directorate (http://www.luftkvalitet.info). Unfortunately, regular observations are available only from two stations (Radhuset and Danmarkplas), which do not cover the larger Bergen area.

G.C. Rieber Climate institute at the Nansen Environmental and Remote Sensing Center is currently involved in a number of research projects aiming on understanding of micro-meteorology and air pollution in urban areas. Although the focus of those projects is on European megacities and some other specific locations, we decided to respond on the problematic air quality situation in Bergen. The very fine scale turbulence-resolving model PALM has been run with resolution of 30 m over the larger Bergen area utilizing the satellite digital elevation data from the ASTER project and initial and boundary conditions from radio-sounding in Stavanger at 00 EST 23 November 2010. Since the Bergen emission inventory is not available, we used estimation of emission from “Handlingsplan for bedre luft i Bergen 2007” provided by Bergen Komunne and Norconsult. The lack of detailed information for the model calibration causes some inaccuracy and uncertainties in our simulations.

The simulations results are given in Figure below. Figure depicts a normalized concentration of road pollution (red shading) accumulated over 3 hours between 06 a.m. and 09 a.m. and corresponding averaged wind (vectors) within the lowest 30 m from the surface. Despite uncertainties and lack of data, Figure not only reasonable reproduced the visually observed (see Photo) and measured air quality hazard in the Bergen valley but also suggests dynamical explanation of the situation. Indeed, even our preliminary simulations indicated that the area between Minde and the Bergen centre should have the worst pollution problem with the maximum pollution near Danmarksplass. The simulations disclose that the combination of orographical blocking and katabatic drainage flow (blue arrows) developing in this area must be responsible for air stagnation and pollution build-up. The strong clear sky radiation cooling in the valley causes cold air subsidence along the hill slopes and the near surface northward flow. At the same time the mean air flow is directed southward as atmospheric soundings reveal. Above Nordness and Laksevag those flows meet and lock polluted air in the city center. The strong radiative inversion, seen both in the model and in the data from Geophysical Institute, prevent normal air mixing in the deep layer. The inversion top is clearly seen on the photo as the height of improved visibility.

Our simulations answer why other parts of the larger Bergen area do not affected so severely. The reason is largely in a favorite southward slope of other valleys. For example, the estimations of the emission rate are large in both the Fyllingsdalen and Loddefjord (Vestkanten) areas, but the simulated concentrations are less than one half of those near Danmarksplass. In fact, the mean northerly wind assists the drainage flow to flash out the pollution toward open water bodies.

Nansen Environmental and Remote Sensing Center models have significant potential to be tapped in the assessment of the Bergen air quality and in supporting the policy measures. More detailed simulations for the considered day in November are under way. However, in order to become routine consultancy instrument, the high resolution models should be calibrated over far more detailed and comprehensive meteorological and chemical observations in different parts of Bergen. This project could be organized around Bergen Komunne as coordinator of data providers (Geophysical Institute, Norconsult, Vegvesen, Meteorological Institute), modelers (NERSC) and users (Helsedirectoratet, Hospitals, Kommune departments) for the benefit of the Bergen city inhabitants.

Figure caption; Simulation results for the near surface relative concentration (red shading) and the air flow (vectors). Blue vectors correspond to dranaige flow, dark vectors – to geostrophically driven flow. The maximum concentration accumulated around Danmarksplass has been scaled to unity. The coast line is shown as bold black contour and the elevations are shown as contours in every 100 m. Observe that, the elevations from ASTER database is not exactly correspond to the topographical map. Smaller water bodies are typically missed.

News reportage at NRK Hordaland on 5. January is available at (MP3-file):

ftp://ftp.nersc.no/Press/nrk-hord-05-01-2011.mp3