# AERQ--A Web-Based Decision Support Tool for Air Quality Assessment
## 2 Materials and Methods
### Application of AERMOD to the Cagliari Metropolitan Area  
Cagliari's climate is typical of the Mediterranean, with its mild winters and hot, dry summers. The region's complex topography includes industrial and rural areas, marshlands, small hills, and coastal regions. The city lies over and around the slope of the historic district of Castello and nine other limestone hills of the middle-to-late Miocene, with unique heights of a little more than 100 m (330 ft) above sea level on the long plains of Campidano. As highlighted by [PERSON] et al. (2023) [30], the complex geographical settings near the sea complicate AERMET's calculation of the micrometeorological conditions required for AERMOD. In the test site, AERMOD was used to model air dispersion over a rectangular domain covering the municipality of Cagliari and some surrounding metropolitan areas. A regular grid of 2500 receptor points was employed, with a spacing of 200 m between receptors. Road segments were modeled as line sources, with a total of 1989 sources (Figure 1). Memory usage and computational time for the AERMOD run depend on the number of sources and receptors, but the total computation time was reduced by employing parallel simulation strategies. The ISTAT (Italian National Institute of Statistics) [31] big data warehouse was used to acquire vehicle information. Data are organized by theme, presented in multidimensional tables, and with a wide range of standard metadata. This dataset was used for a preliminary statistical analysis of vehicle types, environmental categories, Euro standards, fuel types, and CO\({}_{2}\) emissions per kilometer. In Cagliari, 131,742 vehicles are registered, with 90,881 actively circulating or authorized to circulate with valid insurance. It is worth noting that Cagliari's vehicle fleet is among the oldest in Italy, with approximately 50% of cars older than 15 years and engines classified below Euro 4. To estimate the spatiotemporal distribution of harmful emissions from city traffic, it is crucial to associate specific vehicle emissions with an estimated traffic flow distribution on Cagliari's streets, at the highest possible resolution. A network of 167 low-cost, IoT-based sensors, managed by 98 stations, collects real-time data on circulating vehicles, classifying them as long, short, and motorcycles. These data are available through the Municipality of Cagliari's API portal [32]. The system faces challenges, such as data gaps and occasional inaccuracies in station measurements, with metadata providing hourly updates on data reliability. Emission loads were calculated by combining traffic data with specific emission factors [33; 34]. Emission factor methods involve using standardized coefficients to estimate the amount of pollutants released per unit of fuel consumed or distance traveled by vehicles and other sources. These methods, as described by the IPCC report in 2019 [35], simplify the calculation of air pollution loads by applying average emission rates to various activities or sources, facilitating more manageable air quality assessments. Traffic data provide key insights into the number and types of vehicles passing through a road segment over time. This includes details on vehicle categories such as passenger cars, trucks, and buses, as well as their density at different hours. Each vehicle type has specific emission factors, which indicate the amount of pollutants (e.g., CO\({}_{2}\), NO\({}_{2}\), or particulate matter) emitted per unit of activity, such as per kilometer driven. To estimate emissions, the number of vehicles in each category is multiplied by their corresponding emission factors, yielding the total emissions for that road segment within  
Figure 1: This shows (**a**) the main roads included in the model, color-coded based on the hourly average number of circulating vehicles (treated as linear pollution sources), (**b**) the categories of vehicles (e.g., motorcycles, trucks, cars) operating in Cagliari, and (**c**) the types of vehicle emissions (e.g., Euro 0, Euro 1, etc.).