## 1 Introduction
### Sources and Concentrations of Ultrafine Particles  
With the rising concerns around the environmental and health effects of air pollution, [PERSON] and [PERSON] note that there has been growing attention toward particulate matter, especially strategies aimed at minimizing sources and related activities [15]. [PERSON] et al. [16] conducted a 10-day intensive measurement campaign of particulate matter in a high-traffic urban area of Budapest, Hungary. Using an aethalometer model and positive matrix factorization, they identified six major sources of ultrafine particles, three of which were directly attributable to traffic emissions, most notably from heavy-duty diesel trucks. Traffic-related sources were found to contribute approximately 31% of total submicron PM, making them the dominant contributor to the most harmful particles observed during the study.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Category** & **Diameter** & **Common** & **Health Relevance** & **EPA Standard (NAQS)** & \begin{tabular}{c} **Typical Background** \\ **Concentration** \\ \end{tabular} \\ \hline PM\({}_{10}\) & \(\leq\)10 \(\mu\)m & \begin{tabular}{c} Coarse \\ particles \\ \end{tabular} & \begin{tabular}{c} Reach upper \\ airways \\ (nose/throat) \\ \end{tabular} & \begin{tabular}{c} 24 h:150 \(\mu\)g/m\({}^{3}\) (not to be \\ exceeded more than once per \\ year on average over 3 years) \\ \end{tabular} & \begin{tabular}{c} Rural: \(\sim\)10–30 \(\mu\)g/m\({}^{3}\) \\ Urban: \(\sim\)20–60 \(\mu\)g/m\({}^{3}\) \\ \end{tabular} \\ \hline PM\({}_{2.5}\) & \(\leq\)2.5 \(\mu\)m & Fine particles & \begin{tabular}{c} Penetrate deep \\ into lungs \\ \end{tabular} & \begin{tabular}{c} Annual: 12 \(\mu\)g/m\({}^{3}\) (primary); \\ 15 \(\mu\)g/m\({}^{3}\) (secondary) \\ \end{tabular} & \begin{tabular}{c} Rural: \(\sim\)5–15 \(\mu\)g/m\({}^{3}\) \\ Urban: \(\sim\)10–30 \(\mu\)g/m\({}^{3}\) \\ \end{tabular} \\ \hline PM\({}_{0.1}\) & \(\leq\)0.1 \(\mu\)m & \begin{tabular}{c} Ultrafine \\ particles \\ \end{tabular} & \
\begin{tabular}{c} Reach alveoli, \\ enter bloodstream \\ \end{tabular} & Not regulated under NAAQS &
\begin{tabular}{c} Non-diesel/traffic-area \\ (UFP background Morgan- \\ town) \(<\) 2400 particles/cm\({}^{3}\) \\ \end{tabular} \\ \hline \hline \end{tabular}
\end{table}
Table 1: Principal classification of EPA National Ambient Air Quality Standards (NAAQS) compared with UFP criteria.  
A global review by [PERSON] et al. (1998) showed that concentrations of ultrafine particles are much higher near traffic highways and airports, which affirmed findings of previous studies. That same review also found that UFP concentrations within an 8 to 10 km radius of airports were four to five times higher than typical background levels. Finally, a similar trend was observed in urban settings where proximity to busy traffic highways or regions with industrial activities had higher concentrations of UFPs when compared with residential areas with low traffic. A national-scale empirical model of UFP concentrations in the United States developed by [PERSON] et al. (1998) also revealed a similar trend in which busy highway traffic, commercial land use, and urban-related variables accounted for much of the spatial variability in UFP concentrations throughout the continental US. This analysis of \(\sim\)6 million census blocks across the U.S. from 2016 to 2017 showed that UFP concentrations could be found to range between 1800 and 26,600 particles/cm\({}^{3}\), often located near highways, airports, and industrial regions.