Densification versus urban sprawl. Modeling the impact of two urban growth scenarios on air quality

This study analyzes the impact of urban form on air pollution for two urban growth scenarios in 2040 for Quito, Ecuador: Urban sprawl (low-density outward expansion) and densification (the population increase occurs within the 2017 city boundaries). We estimate concentrations of carbon monoxide (CO), nitrogen oxides (NOx), nitrogen dioxide (NO2), ozone (O3), and particulate matter with an aerodynamic diameter equal to or less than 2.5 & mu;m (PM2.5). Two downscaled global emission pathways (ECLIPSE) that present a lower/upper range are applied: 1) Current legislation emissions, CLE, and 2) Maximum technically feasible reductions, MTFR. The population is expected to increase by 50%, from 3 million in 2017 to 4.5 million in 2040. The sprawl scenario considerably increases the urban area by 52%. However, these new areas hold only 9% of the population. Both pathways present considerably lower emissions for 2040 than in 2017. Emissions for all sectors, except trans-portation, are the same for both scenarios. Transport emissions for the densification scenario are 32% lower than for urban sprawl.Due to reduced emissions, the 2040 scenarios result in lower CO, NOx, and PM2.5 concentrations for both pathways compared to 2017. This drop is more marked for the MTFR pathway compared to CLE. Contrary to our expectations, the two scenarios result in similar annual concentrations at the metropolitan level. Even when emission variations are considerable, differences between the scenarios are relatively small, particularly for CO,O3, and PM2.5, due to high regional contributions. However, the spatial distribution of concentrations within urban areas differs significantly; thus, the sprawl scenario results in a larger surface with higher CO, NOx, NO2, and PM2.5 levels and the opposite for O3. Still, due to the population distribution, the densification scenario exposes more residents to higher CO, O3, and PM2.5 values, and the sprawl scenario exposes more inhabitants to higher concentrations of NO2 and NOx.We find an improvement in air quality for both scenarios in 2040 compared to 2017. The annual average NO2 concentrations exceed local regulations in a small urban area only for the CLE pathway. In contrast, for PM2.5, the area and population exposed to levels that exceed the local regulation are higher than that for NO2 and more significant for the densification than the sprawl scenario and for the CLE than the MTFR pathway.