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 (PM 2.5 ) but not to particle number concentrations (PNC). We use a mobile platform to measure PNC and PM 2.5 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 ( PNC tot ) and temporal-adjusted PM 2.5 ( ∆ PM 2.5 ) are calculated and evaluated. Results showed that the average PNC tot in winter (4.8 × 10 4 pt cm –3 ) were over two times higher than autumn (0.36 × 10 4 –0.56 × 10 4 pt cm –3 ). Moreover, the traffic emissions ( PNC d,tr ) contribute 30% of the PNC throughout the study while solid fuel burning ( PNC d,sfb ) could be a major contributor only in winter (29%). Seasonal variability in PNC d,tr and PNC d,sfb was found, with 2–3 times higher median PNC d,tr and 7 times higher median PNC d,sfb in winter compared to autumn. Similarly, PM 2.5 in winter (109 µ g m –3 ) was 3–5 times higher than autumn, while ∆ PM 2.5 (40 µ g m –3 ) was 3–6 times higher. In winter, the PM 2.5 and ∆ PM 2.5 showed higher concentrations in urban street and rural freeway similar to PNC d,sfb but opposite to the trend of PNC tot and PNC d,tr . The correlation coefficient (R 2 ) is investigated as three combinations (i.e., PNC tot vs. PM 2.5 , PNC d,tr vs. ∆ PM 2.5 , PNC d,sfb vs. ∆ PM 2.5 ). Here, the R 2 showed a comparable seasonal trend (winter lower than autumn) and similar magnitude as the literature, but no strong correlation (R 2 < 0.15) was found. This stresses the fact that mitigation measures of PM 2.5 do not necessarily reduce PNC and monitoring networks evaluate PM 2.5 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.