## 2 Materials and Methods
### Exposure Monitoring  
An ultrafine particle electrical mobility size analyzer and counter, (TSI Model 3910, Shoreview, MN, USA, referred to simply as the NanoScan) is an advanced, portable, manufacturer-calibrated, particle size analyzer used for quantifying ultrafine particle concentrations across diverse environmental and industrial settings [34]. The 3910 model is noted for its high sensitivity and precision in quantifying particles ranging from 5 to 1000 nanometers, making it a vital instrument for researchers and professionals in environmental science, occupational health, and air quality monitoring [34; 35]. In addition to its abilities, the TSI MODEL 3910 is also known for its small size, battery operation and portability, which is a major advantage for field studies. The NanoScan SMPS was deployed to continuously measure UFP concentrations and size distributions in real-time at each site. This instrument provides high-resolution particle sizing within the 10-420 nm range, with a sampling interval of one minute [36]. It accomplishes this by integrating a TSI Scanning Mobility Particle Sizer (SMPS(tm), TSI Model 3938, Shoreview, MN, USA,) spectrometer into a portable unit approximately the dimensions of a basketball [37]. A bipolar aerosol charger, powered by a long-column differential mobility analyzer (DMA, TSI, Model 3083, Shoreview MN, USA), maintains particles within a defined charge distribution, facilitating the precise categorization of size and number based on electrical mobility [35; 37]. Studies have consistently shown that the NanoScan SMPS Nanoparticle Sizer 3910 is well-suited for applications that involve the sampling and analysis of particulate matter composition and concentrations in the air. The NanoScan has the ability to isolate the counts from one size bin to be used as the prime indicator of total UFP exposure. These are graphed as \"counts\" for the sum of all the bin sizes if no bin size is specified, or, for select sizes, the particle size of the selected bin that was used is appended to the \"count\" title. In theory, a total count could be calculated from the counts in a single bin if the size distribution can be assumed to remain constant. Sampling was done only days forecast to have low wind speeds and no precipitation with little difference between sites and no evidence of ground level valley inversions (Table 2). [PERSON] et al., for instance, conducted a study to validate the accuracy of scanning mobility particle analyzers based on models developed with COMSOL software [38]. A differential mobility analyzer (DMA) consisting of a semi-ellipsoidal gas flow conditioner, a multi-hole ring, and an anti-turbulent slit was used to enhance particle size resolution. The study findings showed that instruments such as the NanoScan that employ a long-column differential mobility analyzer (DMA) have higher accuracy and are suitable for sampling and measuring particle size and concentrations in a specific location. For the current study, the NanoScan monitor was positioned at approximately 1.5 m above ground level, aligning with the breathing zone for pedestrians and commuters [35]. The instrument at each site was positioned upwind, ideally at the center of the location. This ensured the direct exposure of the sampling inlet to the air stream and that samples were collected from suitably representative locations. One of the key features of the NanoScan is its high sensitivity and precision. As noted by [PERSON] and [PERSON], the NanoScan employs scanning mobility particle sizing (SMPS), a mobility principle that allows it to detect and count individual particles [37]. This portable version of the SMPS has been a recent development, especially in the measurement of ultrafine particles, which are significantly smaller in size and thus difficult to detect [39]. The NanoScan utilizes SMPS technology to effectively analyze and measure UFP concentrations