Chemical and Physical Characterization of Traffic Particles in Four Different Highway Environments in the Helsinki Metropolitan
Total Page：16
File Type：pdf, Size：1020Kb
Atmos. Chem. Phys., 16, 5497–5512, 2016 www.atmos-chem-phys.net/16/5497/2016/ doi:10.5194/acp-16-5497-2016 © Author(s) 2016. CC Attribution 3.0 License. Chemical and physical characterization of traffic particles in four different highway environments in the Helsinki metropolitan area Joonas Enroth1,2, Sanna Saarikoski3, Jarkko Niemi4,5, Anu Kousa4, Irena Ježek6, Griša Mocnikˇ 6,7, Samara Carbone3,a, Heino Kuuluvainen8, Topi Rönkkö8, Risto Hillamo3, and Liisa Pirjola1,2 1Metropolia University of Applied Sciences, Department of Technology, Helsinki, Finland 2University of Helsinki, Department of Physics, Helsinki, Finland 3Finnish Meteorological Institute, Atmospheric Composition Research, Helsinki, Finland 4Helsinki Region Environmental Services Authority HSY, Helsinki, Finland 5University of Helsinki, Department of Environmental Sciences, Helsinki, Finland 6Aerosol d.o.o., Ljubljana, Slovenia 7Jožef Stefan Institute, Ljubljana, Slovenia 8Tampere University of Technology, Department of Physics, Tampere, Finland anow at: University of São Paulo, Department of Applied Physics, São Paulo, Brazil Correspondence to: Liisa Pirjola (liisa.pirjola@metropolia.fi, liisa.pirjola@helsinki.fi) Received: 16 December 2015 – Published in Atmos. Chem. Phys. Discuss.: 18 January 2016 Revised: 7 April 2016 – Accepted: 16 April 2016 – Published: 3 May 2016 Abstract. Traffic-related pollution is a major concern in ur- erage emission factors appeared to be lower for the CN and ban areas due to its deleterious effects on human health. The higher for the NO2 than ten years ago. The reason is likely characteristics of the traffic emissions on four highway en- to be the increased fraction of light-duty (LD) diesel vehicles vironments in the Helsinki metropolitan area were measured in the past ten years. The fraction of heavy-duty (HD) traffic, with a mobile laboratory, equipped with state-of-the-art in- although constituting less than 10 % of the total traffic flow, strumentation. Concentration gradients were observed for all was found to have a large impact on the emissions. traffic-related pollutants, particle number (CN), particulate mass (PM1), black carbon (BC), organics, and nitrogen ox- ides (NO and NO2/. Flow dynamics in different environ- ments appeared to be an important factor for the dilution of 1 Introduction the pollutants. For example, the half-decay distances for the traffic-related CN concentrations varied from 8 to 83 m at Vehicle exhaust emissions constitute major sources of ultra- different sites. The PM1 emissions from traffic mostly con- fine particles (UFP; below 100 nm in diameter), black car- sisted of organics and BC. At the most open site, the ratio bon (BC), organic carbon (OC), and NO2 in urban environ- of organics to BC increased with distance to the highway, ments (e.g., Pey et al., 2009; Morawska et al., 2008; Jo- indicating condensation of volatile and semi-volatile organ- hansson et al., 2007; Lähde et al., 2014). Although during ics on BC particles. These condensed organics were shown the last 15 years particle mass emissions have been signif- to be hydrocarbons as the fraction of hydrocarbon fragments icantly reduced, due to the tightened emission regulations in organics increased. Regarding the CN size distributions, and improvements in vehicle technology, the number emis- particle growth during the dilution was not observed; how- sions of the smallest UFP particles (below 50 nm in diameter) ever the mass size distributions measured with a soot particle have been observed to be significant (Rönkkö et al., 2013; aerosol mass spectrometer (SP-AMS), showed a visible shift Kumar et al., 2010). Besides the exhaust emissions partic- of the mode, detected at ∼ 100 nm at the roadside, to a larger ipating in chemical and physical transformation processes size when the distance to the roadside increased. The fleet av- in the atmosphere, affecting urban visibility and global cli- mate (IPCC, 2013), they have harmful health effects. Ultra- Published by Copernicus Publications on behalf of the European Geosciences Union. 5498 J. Enroth et al.: Traffic particles in four different highway environments fine particles can penetrate deep into the human pulmonary Besides dilution and aerosol dynamics, traffic fleet and and blood-vascular systems increasing the risk of asthma, re- flow rate (e.g., Zhu et al., 2009; Beckerman et al., 2008), duced lung function, cardiovascular disease, heart stroke, and background concentrations (Hagler et al., 2009), and atmo- cancer (Pope III and Dockery, 2006; Sioutas et al., 2005; Ket- spheric chemical and physical processes (Beckerman et al., tunen et al., 2007; Su et al., 2008; Alföldy et al., 2009). The 2008; Clements et al., 2009), all affect pollutant concen- European Environment Agency has estimated that fine parti- trations near the highways. Hagler et al. (2009) and Jan- cles caused around 430 000 premature deaths in Europe and häll (2015) found that local topography and land use, par- around 1900 in Finland in 2012 (EEA, 2015). In particular, ticularly noise barriers and roadside vegetation, can also be people who live, work or attend school near major roads have important factors for determining the concentrations. In addi- an increased health risk. tion, the measurement results depend on sampling techniques During the last decade, pollutant gradients near highways and instruments used in studies. have been extensively examined in the USA (Zhu et al., 2009; Using single-particle mass spectrometry the characteris- Clements et al., 2009; Hagler et al., 2009; Canagaratna et al., tics of vehicle emissions have been studied in the past decade 2010; Durant et al., 2010; Padró-Martínez et al., 2012; Mas- on a dynamometer (e.g., Sodeman et al., 2005; Shields et soli et al., 2012), in Canada (Beckerman et al., 2008; Gilbert al., 2007) and near a highway (e.g., Lee et al., 2015). Only et al., 2007), in Australia (Gramotnev and Ristovski, 2004), a few of the published studies have investigated changes in India (Sharma et al., 2009), and in Finland (Pirjola et al., in exhaust-particle chemical composition during dispersion. 2006, 2012; Lähde et al., 2014). Generally, all of these stud- Clements et al. (2009) collected high-volume PM2:5 samples ies showed that the pollutant concentrations were higher near at distances of 35 and 65 m from a major highway. They highway than further from the roadside, sharply decreasing discovered that, unlike the particle-bound elemental carbon to background levels within 300–500 m downwind. How- (EC), organic carbon (OC) concentrations increased with dis- ever, Gilbert et al. (2007) discovered that the NO2 concentra- tance downwind. Instead, Durant et al. (2010) report a time- tion decreased during the first 200 m distance from the edge dependent decrease in the concentrations of particulate or- of the highway but beyond 200 m downwind started to in- ganics (Org) and hydrocarbon-like organic aerosol (HOA) crease, indicating that factors other than the highway traffic up to 200–300 m downwind from a highway during morn- influenced the increased NO2 concentration. ing hours. Massoli et al. (2012) present spatial–temporal gra- These studies showed that the concentration levels and dients of the HOA and oxygenated organic aerosol (OOA) gradient shapes of UFP and other primary vehicular emis- concentrations summed with the refractory BC (rBC) up to sions near major roads depend in a complex way on many 500 m downwind from a highway. The sum of HOA and factors, including meteorological conditions such as atmo- rBC mass concentration decreased with increasing distance spheric stability, temperature, wind speed, wind direction, whereas the sum of OOA and rBC was constant. The size and surface boundary layer height (Durant et al., 2010). Di- distributions of organics and rBC pointed out that the fresh lution is a very crucial process, additionally it is accompa- soot mode peaking at ∼ 100 nm was slightly coated by the nied by aerosol dynamics processes such as nucleation, co- HOA material, whereas the accumulation soot mode peaking agulation, condensation, evaporation, and deposition (Kumar at ∼ 500 nm was heavily coated by the OOA material, rep- et al., 2011 and references therein). In the diluting and cool- resenting the background aerosol. The change in the chemi- ing exhaust, new particles are formed by homogeneous nu- cal composition of traffic particles with distance has several cleation during the first milliseconds (Kittelson, 1998), af- causes. The exhaust from the vehicles is hot when emitted ter which they immediately grow by condensation of con- but it cools quickly as it is mixed with ambient air. Cooling densable vapors. Low temperature favors nucleation and con- promotes the condensation of organic vapors on particles, but densation, whereas evaporation becomes important during as the exhaust is diluted with ambient air, the concentration high ambient temperature. On the other hand, the majority of gaseous semivolatile organic compounds (SVOCs) is re- of volatile organic compounds is emitted by vehicles during duced, leading to the evaporation of SVOCs from particles in cold starts (Weilenmann et al., 2009). Consequently, Padro- order to maintain phase equilibrium (Robinson et al., 2010). Martinez et al. (2012) report that the gradient concentrations Therefore there is an ongoing competition between differ- were much higher in winter than in summer, even 2–3 times ent processes in the emission plume: new particle formation higher as observed by Pirjola et al. (2006). Relative humidity