IAQMS-street v2.0: a two-way coupled regional-urban–street-network model system for Beijing, China

Owing to the substantial traffic emissions in urban areas, especially near road areas, the concentrations of pollutants, such as ozone (O₃) and its precursors, have a large gap with the regional averages and their distributions cannot be captured accurately by traditional single-scale air-quality models. In this study, a new version of a regional-urban-street-network model (IAQMS-street v2.0) is presented. An upscaling module is implemented in IAQMS-street v2.0 to calculate the impact of mass transfer to regional scale from street network. The influence of pollutants in street network is considered in the concentration calculation on regional scale, which is not considered in a previous version (IAQMS-street v1.0). In this study, the simulated results in Beijing during August 2021 by using IAQMS-street v2.0, IAQMS-street v1.0, and the regional model (NAQPMS) are compared. On-road traffic emissions in Beijing, as the key model-input data, were established using intelligent image-recognition technology and real-time traffic big data from navigation applications. The simulated results showed that the O₃ and nitrogen oxides (NO_x) concentrations in Beijing were reproduced by using IAQMS-street v2.0 both on regional scale and street scale. The prediction fractions within a factor of two (FAC2s) between simulations and observations of NO and NO₂ increased from 0.11 and 0.34 in NAQPMS to 0.78 and 1.00 in IAQMS-street v2.0, respectively. The normalized mean biases (NMBs) of NO and NO₂ decreased from 2.67 and 1.33 to -0.25 and 0.08. the concentration of NO_x at street scale is higher than that at the regional scale, and the simulated distribution of pollutants on regional scale was improved in IAQMS-street v2.0 compared with that in IAQMS-street v1.0.  We further used the IAQMS-street v2.0 to quantify the contribution of local on-road traffic emissions to the O₃ and NO_x emissions and analyze the effect of traffic-regulation policies in Beijing. Results showed that heavy-duty trucks are the major source of on-road traffic emissions of NO_x. The relative contributions of local traffic emissions to NO₂, NO, and O₃ emissions were 53.41, 57.45, and 8.49%, respectively. We found that traffic-regulation policies in Beijing largely decreased the concentrations of NO_x and hydrocarbons (HC); however, the O₃ concentration near the road increased due to the decrease consumption of O₃ by NO. To decrease the O₃ concentration in urban areas, controlling the local emissions of HC and NO_x from other sources requires consideration.