## 4 Discussion  
This study's findings provide reasonable evidence to conclude that further study may be warranted to better understand how the number of trucks may significantly impact the correlation between speed and particle concentration, particularly for the UFP size range factor, in determining the concentration of UFPs close to roads. The difference between Brockway Avenue and Beechurst Avenue, the two sampling locations, was noted for the entire UFP size range counts on the NanoScan. Truck activity was responsible for 60.35% of the variation in UFP concentration at Brockway according to the correlation between truck count and 15.4 nm UFP particle concentration (Figure 3). This result is consistent with previous studies showing that emissions from diesel engines are the main source of ultrafine particles, especially in regions with high truck traffic [49]. The increased loading of on-road trucks also directly influences engine operation by raising the torque demand  
Figure 10: Multivariate analysis of Beechurst Avenue UFP concentration with a Pearson correlation coefficient (R) of 0.4 (reaching its maximum correlation coefficient value for particle counts in the 64.9 nm size bin). Figure 9: Multivariate analysis of Brockway Avenue UFP concentration with a Pearson correlation coefficient (R) of 0.77 (reaching its maximum correlation coefficient value for particle counts in the 86.6 nm size bin). required to maintain truck momentum. This higher torque demand necessitates greater fuel injection rates and may also alter injection timing, both of which significantly affect the combustion process and emissions [50]. The study's central hypothesis that heavy-duty diesel truck activity increases UFP levels in near-road environments was also supported by the Beechurst results, which showed a positive correlation between truck count and UFP concentrations. By demonstrating that truck traffic is a strong predictor of local air pollution, a crucial component in evaluating the public health concerns associated with diesel emissions, this study advances the larger research goal. Due to significant roadway variations, the correlation at Beechurst Avenue was weaker than at Brockway Avenue. The extent of UFP pollution elevation caused by terrain is one important issue. The uphill and downhill movement of diesel trucks at Brockway Avenue in steep terrain affects UFP emissions and accumulation, so topography is significant in this research field. Trucks need greater engine power when they ascend steep slopes, which causes more fuel combustion and higher UFP concentrations in complex terrains. The increased loading of on-road trucks directly influences engine operation by raising the torque demand required to maintain truck momentum. This higher torque demand necessitates greater fuel injection rates and may also alter injection timing, both of which significantly affect the combustion process and emissions [50]. Beechurst benefits from easier truck mobility and a smoother traffic flow due to its flat topography and improved road conditions because the flat terrain promotes natural air flow and helps to efficiently dilute emissions. Enhanced road conditions permit the better dispersion of pollutants. UFP pollutants are more effectively dispersed at Beechurst than at Brockway, which has rougher roads and a more complex topography. The overall concentration of ultrafine particles (UFPs) is lower even though truck emissions continue to contribute to UFP concentrations. Despite these variations, Beechurst's positive correlation nevertheless supports the study's methodology and emphasizes the steady influence of diesel truck emissions on UFP pollution. The results highlight how crucial it is to take site-specific topographical aspects into account when assessing the quality of the air close to roads. The Multiple-Path Particle Dosimetry (MPPD) model [46] yields similar estimates to those provided previously by [PERSON] et al.