Evaluating urban PM₁₀ pollution benefit induced by street cleaning activities

Abstract

Despite their burden in urban particulate air pollution, road traffic non-exhaust emissions are often uncontrolled and information about the effectiveness of mitigation measures on paved roads is still scarce. The present study is aimed to evaluate the effectiveness of mechanical sweeping/water flushing treatments in mitigating urban road dust resuspension and to quantify the real benefit in terms of ambient PM₁₀ concentrations. To this aim a specific campaign was carried out in a heavily trafficked central road of Barcelona (Spain), a Mediterranean city suffering from a traffic-related pollution, both for a high car density and a frequent lack of precipitation. Several street washings were performed by means of mechanical sweepers and pressure water during night in all traffic lanes and sidewalks. PM₁₀ levels were simultaneously compared with four reference urban background air quality stations to interpret any meteorological variability. At the downwind measurement site, PM₁₀ concentrations registered a mean daily decrease of 8.8 μg m⁻³ during the 24 h after street washing treatments. However 3.7–4.9 μg m⁻³ of such decrease were due to the meteorological variability detected at the upwind site, as well as at two of the reference sites. This reveals that an effective decrease of 4–5 μg m⁻³ (7–10%) can be related to street washing efficiency. Mitigation of road dust resuspension was confirmed by investigating the chemical composition of airborne-PM₁₀ filters. Concentrations of Cu, Sb, Fe and mineral matter decrease significantly with respect to concentrations of elemental carbon, used as tracer for exhaust diesel emissions. High efficiency of street washing in reducing road dust loads was found by performing periodic samplings both on the treated and the untreated areas.

Introduction

Road traffic emissions include not only gases and particulate matter released from motor exhausts, but also “non-exhaust” particles derived from the wear and tear of vehicles (Schauer et al., 2006) and road surface, as well as from resuspension due to the turbulence generated by vehicle wheels. Deposited particles on road pavement may come from various sources, such as carbonaceous particles from exhaust emissions and brake pads, metalliferous particles from the wear of brake/disc/rotor system and mineral particles from the abrasion of road pavement and other natural and anthropogenic sources. Once resuspended, these particles are generally defined as non-exhaust or road dust emissions. Non-exhaust emissions in urban environments currently represent a PM source comparable to, or even greater than exhaust emissions (Amato et al., 2009a, Kousoulidou et al., 2008, Kristensson et al., 2004, Abu-Allaban et al., 2003, Jaecker-Voirol and Pelt, 2000] mostly for PM₁₀ (particles with aerodynamic diameter <10 μm). Furthermore, whereas continuing advances in cleaner fuels, tailpipe emission abatement technology, and urban traffic controls are acting to decrease exhaust emissions, no such tendency exists for non-exhaust emissions which are often uncontrolled and represent a major hurdle to accomplishing EU limit values (Harrison et al., 2008, Dijkema et al., 2008, Goodwin et al., 2002). This is especially evident in Mediterranean and Scandinavian countries where respectively the lack of precipitation and the use of studded tyres significantly raise road dust resuspension (Hussein et al., 2008, Querol et al., 2004). The current situation is due in part to the lack of knowledge regarding road dust emissions, their air quality burden and effective control. Inventories of road dust load remain scarce, not least due to the fact that sampling is dangerous, logistically difficult, and time consuming. For this reason road dust collection is generally performed in non-active lanes and sidewalks, and mostly deals with the silt size fraction (<75 μm). Only few studies have investigated chemical and spatial properties of deposited PM₁₀ particles on urban environments (Schauer et al., 2006, Amato et al., 2009b, Ang et al., 2008, Han et al., 2007, Zhao et al., 2006). Accurately estimating the air quality burden of resuspension is a difficult task not least due to the road dust chemical complexity (Amato et al., 2009a, Areskoug et al., 2004, Kantamaneni et al., 1996, Zimmer et al., 1992), however this remains important if we are to establish truly effective measures to mitigate emissions. In this context a key initial task is to evaluate the effectiveness of potential mitigation strategies, in order to provide environmental authorities with sufficient information to establish prospective measures that reduce human exposure to non-exhaust traffic emissions.

Reducing exposure is an important issue since the potential health burden of re-entrained road dust particles is high, mostly due to their content in heavy metals. Schlesinger et al. (2006) indicated that transition metals such as Cu, Zn, Fe, Ni, Cr and Mn, which are biochemically active in redox reactions, are likely related to PM toxicity. Ostro et al. (2007) found a consistent association between Cu, K, Ti and Zn in PM and cardiovascular mortality, and Cu, V, Ti and Zn in PM with respiratory mortality in California. Moreover several toxicological studies also indicate that coarse particles can elicit inflammatory effects (Schins et al., 2004, Schwarze et al., 2007). A recent study in Sweden, found that PM₁₀ generated by erosion of road pavement by studded tyres provoked an inflammatory responses in cells as potent as the response caused by diesel particles (Gustafsson et al., 2008). Jalava et al., 2007, Jalava et al., 2008 and Happo et al. (2007) determined that in Mediterranean cities, coarse particles have higher inflammatory effect than the other PM size fractions. Association between high levels of coarse man-made particles and daily mortality in Barcelona (Spain) has been shown by L. Perez et al. (2008) who also found a worsening during outbreaks of Saharan dust. In a more recent work Perez et al. (2009) found that cardiovascular and cerebrovascular mortality were associated with increased levels of both PM₁ and PM_2.5–10.

In order to reduce road dust emissions from paved roads either preventive or mitigation strategies can be adopted. Preventive strategies aim to avoid dust deposition in the first place, such as paving the access to unpaved lots and covering truck loads. Mitigation measures attempt instead to remove or bind those particles already deposited. Street cleaning by pressurized water for instance can displace dust from asphalt texture and wash particles down the sewage system. Furthermore when water adheres to deposited particles, it increases their mass and surface tension forces, decreasing the likelihood of suspension and transport, especially as cohesion of wetted particles often persists after the water has evaporated due to the formation of aggregates (Watson et al., 2000). However, water flushing has been tested almost exclusively on unpaved roads (Kleeman and Cass, 1999, Gillies et al., 1999, Watson et al., 2000) and its effectiveness on paved roads has not been definitely stated (Norman and Johansson, 2006, Chang et al., 2005). Norman and Johansson (2006) tested several measures on Swedish paved roads concluding that, for the specific weather conditions of Sweden, the most efficient way to reduce PM₁₀ levels in the long-term was to reduce the use of studded tyres while only a marginal reduction was obtained by street washing. Chang et al. (2005) found a higher reduction (30%) on total suspended particles by the combined use of sweeping/washing techniques in Taiwan concentrations but no reduction in PM₁₀ was observed. To our knowledge, no studies are currently available for evaluating effectiveness of street washing in PM₁₀ levels reduction from paved urban areas in temperate climate countries. Given this general lack of knowledge, urban dust resuspension continues to be a well recognised but poorly quantified problem, especially in relatively dry cities such as those surrounding the Mediterranean (Rodríguez et al., 2004, Querol et al., 2004). Barcelona is a somewhat typical Mediterranean city in terms of traffic-related PM pollution, suffering both from a high car density and a frequent lack of precipitation. The metropolitan area of Barcelona is characterized by high road traffic density (6100 vehicles per km²) and by a wide range of industrial activities such as metallurgy and cement plants, as well as two power stations and two city waste incinerators. Traffic density is a consequence of the high population concentration (101 km² with 4.5 million of people entering in the city from the metropolitan area). The urban architecture, characterized by square-blocks with narrow streets, commonly with trees, reduces the dispersion potential of pollutants. The scarce precipitation (average of 500 mm per year) favours the accumulation and resuspension of deposited particulate matter. In addition to local emissions, African dust outbreaks reach Barcelona in the order of 4–5 days per month increasing 2–3 μg m⁻³ the annual PM₁₀ mean (Escudero et al., 2005).

PM levels in the urban background reflect the unsatisfactory level of atmospheric pollution, exceeding the daily limit value from the European Air Quality Directive 1999/30/CE (50 μg PM₁₀ m⁻³) between 44 and 75 times on average per year (data from air quality network). Approximately 80% of such exceedances are due to anthropogenic PM contributions with high proportions of mineral dust (N. Perez et al., 2008, Querol et al., 2001a, Querol et al., 2004). Amato et al. (2009a) estimated that non-exhaust emissions from traffic contribute to background ambient PM₁₀ levels in a comparable amount of exhaust emissions (17% and 21% respectively) on an annual basis. This proportion is much lower in PM_2.5 (8% and 32%). The overall road traffic sector is therefore the main source of pollution, being responsible for 46% of PM₁₀, 51% of PM_2.5 and 48% of PM₁. Given these figures, the strategies designed to reduce PM pollution in Barcelona need to focus at least in part on mitigating road dust emissions.