Non-exhaust emissions, the definition, measurement, regulatory limit, and network distribution

Exhaust emissions have been targeted for more than 20 years with worldwide restrictions. The EE includes mostly gaseous air pollutants including Greenhouse Gas, NOx, CO, PM, etc. Through setting regulatory emission limits, manufacturers are forced to improve the quality of fuel, the technology of internal combustion engines, and the design of emission after-treatment (catalytic converter & particle filter, etc.), so that the combustion efficiency can be elevated and the tailpipe emissions can be well controlled.
However, actually, there is another type of emissions that is generated from the mechanical wear, between brake pairs and the tire/road surface. Basically, NEE can be categorized as brake wear, tire wear, road wear, and resuspended dust. It is intuitive, but not until recently has this type of emissions been highlighted, partially due to the decrease of exhaust emissions. Another reason is the adverse health impact of NEE. For example, brakewear brings out heavy metal components from metallic pads, and, recently, PAH from non-asbestos organic pads. Tire wear generates polymers that can be either dispersed in the atmosphere or deposited on the ground, entering the water cycle of the earth. As the increasing share of electric vehicles increases, exhaust emissions, especially for light-duty vehicles, are reduced in terms of proportion of total traffic emissions. In contrast, NEE has become dominant, as has been proved in Europe, such as the UK, Germany, and Spain.
According to the prediction of the European Union Commission, NEE will take 90% of the total road traffic emissions by 2050. In 2022, the EU presented the upgraded vehicle emission limit, EURO 7, in which brake wear and tire wear are officially set restricted. The most recent update of the EURO7 is that the proposal has been approved and is expected to take into force in 2025 (according to different vehicle types).
The new limit is pretty aggressive: for ICEV LDVs, the brake wear limit is 7 mg/vehicle/kilometer and 3 mg/veh/km for electric LDVs. The tire wear is defined as tire abrasion, and the timeline of different types of vehicles undergoing tire type-approval tests is defined.
Accompanying the new limit, a regulatory emission test procedure is also designed, in which the drive cycle follows the World Light Vehicle Test Procedure (WLTP), and the test procedure is defined in the UN Global Technical Regulation No. 24 on brake emissions. Back for EE, EURO limits include two types of tests: dynamometer test for pre-sale type-approval and real driving emission (RDE) test (with portable emission measurement system, PEMS) for use phase certificate. Given the limit of NEE, the projection of future scenarios needs to be supported by trustful emission models. The most widely used emission models developed by regulatory authorities include MOVES (EPA), COPERT (EU), and EMFAC (CARB). Unlike EE, NEE studies rarely investigate the spatial emission distribution across the road network. To understand what factors drive the NEE, particularly, brake wear on-road, we adopted the framework of MOVES, modified its methodology to match with the most recent measurement, and localized the settings of market and local drive characteristics. The driving data we used is a classic driving record dataset, Didi ride-hailing records, in November 2016 in Chengdu, China. The market information, including the brake material, brake type, and vehicle sizes, was obtained from e-commerce and vehicle registration websites.
Here are the results: