Study Finds Weather Conditions Have Significant Effect on PM Concentrations

5 December 2008

Dry winter weather and a low mixing layer height (MLH) for pollutants from vehicle exhausts lead to the highest concentrations of PM₁₀ in cities, according to a team of German scientists writing in the January 2009 issue of the International Journal of Environment and Pollution. Their findings suggest that traffic controls, other than an outright ban for several days at a time, would have little effect on pollution concentrations.

The mixing layer is the portion of the atmosphere close to the surface layer where air pollutants get diluted, without leaving this layer. The height of the mixing layer is a gauge of the vertical dilution of air pollution, and is affected by vertical temperature and moisture profiles in the lower atmosphere.

Jutta Rost of the Meteorological Institute, at the University of Freiburg, and colleagues there and at the Fraunhofer Institute for Transportation and Infrastructure Systems, in Dresden, and the Federal State Institute for Environmental Protection, in Baden-Wuerttemberg, carried out a retrospective analysis of the atmospheric conditions that affected PM₁₀ levels in four cities (Freiburg, Karlsruhe, Mannheim and Stuttgart) in South-West Germany during the period from 2001 to 2005.

The four cities exhibit different emission conditions, particularly various levels of traffic load. For each city, the team obtained PM₁₀ data from roadside stations and urban background (UB) stations within the official air quality monitoring network in the Federal State of Baden-Wuerttemberg (South-West Germany. This provided them with two distinct types of official urban air quality data against which they could validate their findings.

They then looked at atmospheric exchange conditions as represented by sunlight levels, air temperature, wind speed, rainfall, and the height at which PM₁₀ particles and other pollutants are mixing with the atmosphere.

The results of the statistical analysis indicate that precipitation and mixing layer height are the two main meteorological variables that influencing concentrations of PM₁₀ particles at road level within cities. The data from both types of measuring stations gave the same results.

Extremely high PM₁₀ concentrations were recorded during stable stratification and low turbulent exchange during stagnant synoptic situations. The highest PM₁₀ concentrations were observed on days with low h_ML [mixing layer height] values. These situations occurred most commonly in the winter half year, whereas PM₁₀ concentrations only seldom exceeded the short-term PM₁₀ standard of 50 µg m^-3 in the summer half year.

In winter, high pressure situations are characterized by a negative net radiation balance and hence stable stratification even during day-time, while in summer the net radiation balance is negative in the night-time, but positive during day-time. Therefore turbulent exchange is present during summer high-pressure situations leading to moderate PM₁₀ concentrations.

...The impact of the weather conditions on PM₁₀ concentrations is larger than the roadside increment due to the site-specific emission from vehicular traffic. On the basis of these results, it seems to be unlikely that local traffic limitations will have a significant influence on PM₁₀ concentrations. To protect from a potential violation of the short-term PM₁₀ standard at the roadside stations, large areas need to be banned from traffic a few days in advance at certain synoptic situations. These measures would be effective only for short periods of time under stable atmospheric conditions, but most effective contemporaneously to precipitation events.

—Rost et al. 2009

The team hopes to develop a forecasting model of PM₁₀ levels that could be used to advise people at most risk of breathing problems on when to avoid city centres and other urban areas. The work also has implications for ensuring that air quality in urban environments is maintained at levels safe for public health.

Resources

• Jutta Rost, Thomas Holst, Elke Sähn, Matthias Klingner, Katja Anke, Dieter Ahrens, and Helmut Mayer (2009) Variability of PM₁₀ concentrations dependent on meteorological conditions. International Journal of Environment and Pollution, 36, 3-18

• S Emeis, K Schäfer, C Münkel (2008) Long-term observations of the urban mixing-layer height with ceilometers. IOP Conf. Series: Earth and Environmental Science 1 doi: 10.1088/1755-1307/1/1/012027