High spatial resolution WRF-Chem modeling in Quito, Ecuador†

The WRF-Chem model was applied for gas and aerosol chemistry in Quito, Ecuador, at a high horizontal resolution of 2 km. WRF-Chem was chosen due to its full coupling of meteorological and chemical processes, which is particularly suitable for complex topography and urban-scale simulations. Emission inventories were taken from EDGAR for the outer domains (32 and 8 km horizontal resolution), and local emission estimates were used for the innermost domain (2 km resolution) as initial estimates. The base year of simulation was 2018, and two months were chosen: April and December. WRF-Chem results were tested at five air quality stations across the Quito metropolitan area. To reduce bias between modeled and observed concentrations, Quito 2011 baseline emissions for CO, NO_x, SO₂, and PM_2.5 were adjusted by factors of 1.5, 0.75, 0.30 and 3.0 approximately, resulting in annual emission estimates of 300, 27, 1.5 and 7.5 kilotonnes per year (kton per year) for CO, NO_x (expressed as NO₂ equivalent), SO₂ and PM_2.5, respectively. The model run with these adjusted emissions showed good performance for CO, NO_x, SO₂, and O₃ (r ∼0.4–0.8), but performance was lower for PM_2.5 (r ∼0.4–0.5), particularly in the afternoon. This is ascribed mainly to an underestimation of secondary organic aerosol formation. The impact of biogenic VOC emissions on ozone and PM_2.5 is positive but small (+3–8%), and the inclusion of aerosol radiative feedback is minor (∼−0.5%), because of the relatively small ambient PM_2.5 concentrations in Quito.