Evaluating Ultrafine Particles and PM2.5 in Microenvironments with Health Perspectives: Variability in Concentrations and Pollutant Interrelationships

Regulation has been applied to the fine particles (PM2.5) but not to particle number concentrations (PNC). We use a mobile platform to measure PNC and PM2.5 in four microenvironments (diesel plume, urban freeway, urban street, and rural freeway). A total of 38661 pairs of measurements in two years (winter 2018 and autumn 2020) are used to evaluate variability in the pollutant concentrations and their interrelationships. Source-discerned total PNC (PNCtot) and temporaladjusted PM2.5 ( increment PM2.5) are calculated and evaluated. Results showed that the average PNCtot in winter (4.8 x 104 pt cm-3) were over two times higher than autumn (0.36 x 104-0.56 x 104 pt cm-3). Moreover, the traffic emissions (PNCd,tr) contribute 30% of the PNC throughout the study while solid fuel burning (PNCd,sfb) could be a major contributor only in winter (29%). Seasonal variability in PNCd,tr and PNCd,sfb was found, with 2-3 times higher median PNCd,tr and 7 times higher median PNCd,sfb in winter compared to autumn. Similarly, PM2.5 in winter (109 & mu;g m-3) was 3-5 times higher than autumn, while increment PM2.5 (40 & mu;g m-3) was 3-6 times higher. In winter, the PM2.5 and increment PM2.5 showed higher concentrations in urban street and rural freeway similar to PNCd,sfb but opposite to the trend of PNCtot and PNCd,tr. The correlation coefficient (R2) is investigated as three combinations (i.e., PNCtot vs. PM2.5, PNCd,tr vs. increment PM2.5, PNCd,sfb vs. increment PM2.5). Here, the R2 showed a comparable seasonal trend (winter lower than autumn) and similar magnitude as the literature, but no strong correlation (R2 < 0.15) was found. This stresses the fact that mitigation measures of PM2.5 do not necessarily reduce PNC and monitoring networks evaluate PM2.5 exposure are unlikely to represent PNC exposure. The concentration ratios in the three combinations are found to vary with microenvironments and seasons. This variability implies that control policies should be diversified with pollutant types and energy usage of the city.