10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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Correlation between PM2.5 and Particle Number Concentrations in Four Major Cities: Toronto, Los Angeles, Helsinki and London
ALMA LORELEI DE JESUS, Md Mahmudur Rahman, Mandana Mazaheri, Luke Knibbs, Helen Thompson, Greg J. Evans, Cheol H. Jeong, Markku Kulmala, Tuukka Petäjä, Krista Luoma, Hilkka Timonen, Jarkko Niemi, Roy M. Harrison, David Beddows, Constantinos Sioutas, Mohammad Sowlat, Lidia Morawska, Queensland University of Technology
Abstract Number: 1097 Working Group: Health Related Aerosols
Abstract Gaining more knowledge on how different particulate metrics are related would help in successfully controlling particulate matter (PM) concentrations in the ambient air. This study focused on the mass and number concentrations, particularly PM2.5 or mass concentration of particles with diameter of <2.5 µm in order to illustrate that mitigating PM2.5 would not necessarily reduce the ultrafine particles (UFP) concentration. Particles with diameter <0.1 µm are best quantified by the particle number concentration (PNC). The two parameters, PM2.5 and PNC are affected by different drivers, therefore may vary spatially and temporally between cities. PM2.5 is relatively more homogenous within an air shed while PNC is more variable depending mainly on the distribution of the combustion emission sources. To better understand these two important metrics and demonstrate their similarities and differences, this study aims to provide quantitative information on the relationship between ambient PM2.5 and PNC in four cities: Toronto, Canada; Los Angeles, USA; Helsinki, Finland; and London, UK. All these cities are located in the temperate region though Helsinki and Toronto are classified under Moist Continental Mid-Latitude Climate while London and Los Angeles are classified under Moist Subtropical Mid-Latitude climate based on the Köppen-Geiger system. Urban areas are particularly interesting because high population density implies that considerable amount of anthropogenic pollutants are produced in any city.
One-year hourly averages of PM2.5 and PNC were acquired and analysed. Data analysis include: comparing hourly average of PM2.5 and PNC using descriptive statistics, statistical models and non-parametric tests; and identifying the factors that drive the relationship. Our results showed that the annual median of PM2.5 concentrations for Toronto, Helsinki and London were similar (6.6, 6.9 and 7.8 µg.m-3, respectively) but Los Angeles was a bit higher at 16 µg.m-3. The elevated PM2.5 in Los Angeles was due to formation of secondary aerosols influenced by the area’s meteorological characteristics according to Hasheminassab et al. (2014). London and Los Angeles had almost the same annual median of PNC (7.2 and 7.3 x 103 #.cm-3, respectively) compared to Toronto and Helsinki that were high (1.5 and 1.1 x 104 #.cm-3, respectively). This can be attributed to the monitoring site category since it was an urban background station for London and Los Angeles and a roadside station for Helsinki and Toronto; therefore, traffic was directly affecting PNC. The annual medians of both PM2.5 and PNC were compared and statistical tests were carried out to assess significant differences between cities. The results indicate significant differences in PM2.5 and PNC between cities.
Regardless of the difference in climate, all cities showed similar patterns in the diurnal and seasonal variations. The diurnal trend of the PNC in all cities showed traffic peaks in the morning and evening, which was not the case for PM2.5. On the other hand, a seasonal trend was observed for both PM2.5 and PNC where concentrations were high during cold months and lower during the warm months. Establishing the relationship between these two parameters is important to determine the more reliable parameter to be used in exposure and epidemiological studies as well as identifying the effective regulatory control measures. There were weak to moderate correlations between PM2.5 and PNC across all cities. Therefore, both parameters must be considered in implementing regulatory control measures for ambient PM.
Reference: Hasheminassab, S., Daher, N., Saffari, A., Wang, D., Ostro, B. D., & Sioutas, C. (2014). Spatial and temporal variability of sources of ambient fine particulate matter (PM2.52.5) in California. Atmospheric Chemistry and Physics, 14(22), 12085. doi:http://dx.doi.org/10.5194/acp-14-12085-2014.