Evaluation of Local Scale PM Pollution Levels in Typical Street Canyon in Riga ()
1. Introduction
Urban air pollution is essential in public, governmental, self governmental and European concern as well. Some well known air pollution problems, such as high PM10 pollution episodes (e.g. London smog) and long-term steady NO2 pollution levels in connection with meteorological data, human health and economic impact are significant [1]. Monitoring atmospheric particulate matter is a challenge faced by the European Union. Specific rules on this subject are being developed (Directive 2004/107/EC, Directive 2008/ 50/EC) in order to reduce the potential adverse effects on human health caused by air pollution. The atmospheric aerosols are produced by variety of natural (soil dust erosion, sea salt, volcanism, natural forest fires) and anthropogenic (industries, transport, biomass burning, combustion of fossil fuels) activities and are very much important to characterize various lower tropospheric phenomenon. Atmospheric particles with an aerodynamic diameter smaller than 10 μm (PM10) have been put under scrutiny in the past, being easily inhaled and deposited within the respiratory system [2]. Studies show that PM10 plays a role in the incidence and severity of respiratory diseases [3,4] and has significant associations with decline in lung function and cardio-vascular pathologies. A lot of epidemiological studies on the health effects of air pollution were concerned with measuring the link between daily deaths and the kind of severe pollution episodes that occurred in London, England in 1952, Donora, Pennsylvania in 1948, and the Meuse Valley, Belgium in 1930. Using simple methods, these studies established a link between cardiopulmonary mortality and extreme levels of sulfur oxide and particulate matter [5].
In addition, particles in suspension in the atmosphere can play a role in the radiative balance of the earth since they permit the absorption or the reflection of solar radiation, allowing them to alter global climate. Usually in different countries, while a monitoring of the PM measurements is routinely performed at different scales (regional, national scale, site specific), almost nothing is done to finely identify their chemical composition and source origin. Scientific studies have rather privileged sampling strategies based on a limited number of sites associated with a sourcereceptor modelling [6].
Life time of coarse particles is relatively short. They are effectively impacted by gravitational settling and wind turbulence. According to fractional analysis, it could be assumed that PM2.5 life time could range from several days till several weeks that lead it to long range transport [7].
2. Methodology
2.1. Sampling Site Description
All the measurements given here were obtained in different sampling sites in Riga (North-East part of Europe). Two sampling sites (Street Canyon (1) and Street Canyon (2)) were located in city center, in typical street canyon; another two sites were in mixed industrial-heavy traffic impacted areas. Major pollution sources in the study area are traffic with average daily flow intensity 25,000 vehicles per day in street canyons located in city center. Both streets are 15 m wide and are flanked on both sides with about 22 - 30 m high buildings, geographical orientation - 223 degrees (NE-SW direction). Typical traffic flow regime during working days is given in Figure 1. Another two sampling sites (Urban/Traffic Site (1) and Urban/traffic Site (2)) are located close to Riga Free Port activities, such as oil terminals, grain and coal processing, rail activities. Location of air quality monitoring stations and meteorological station are presented in Figure 2.
The meteorological data site (56˚57'2.16''N and 24˚06'57.86''E, height 6 m above sea level for temperature measurements and 26 m above ground level for wind measurements) used was from Riga-University monitoring site located at central part of Riga. Monitoring program covers widest spectra of measurements—standard meteorology (wind speed and direction, air temperature, solar radiation, precipitation, et al.) and some specific measurements—ice condition, snow cover and condition. The climate is maritime and temperate, due to northern location winter temperature extremes could reach −30˚C for short periods, especially during January and February, average winter temperature is −4˚C, continuous snow cover lasts eighty-two days. Due to the proximity of the ocean autumn rains and fogs are frequent. Average summer temperature is 17˚C, with extreme values for 30˚C.
2.2. Sampling Method
PM sampling were done by OPSIS instrument SM200 Beta attenuation particulate monitor (gravimetric sampler), which is an automatic method. Instrument is equipped with PM10/PM2.5 head, inlet intensity 2.3 m3/h, and mass measurement range 0 - 1000 μg/m3. Overview of used monitoring standards is given in Table 1.
3. Results and Discussion
Only data sets with at least 75% completeness of data were used for analysis. For long-term analysis data set of 10 year period (2003-2012) for PM10 and 4 year period (2008-2011) for PM2.5 were used. For source-apportionment analysis results of field survey for eight months period (28.04.2007. −31.12.2007.) was analyzed.
3.1. Long-Term Changes
Figure 3 shows long-term changes of PM10 and PM2.5 concentrations on typical street canyons. It should be noted that quite strong decreasing trend is detected and for the last two years annual limit value (40 mg/m3) aren’t exceeded. Concerning PM2.5 pollution levels target value (25 µg/m3) is exceeded for all measurement period. According to European legislation PM2.5 target value enters into force as limit value at 2015.