What do iv and v represent in terms of national emission standards?

[Pires]

National iv: IV is a Roman numeral, corresponding to the Arabic numeral 4, so National IV is the National 4 standard. National v: National 5 standard. National iv, National v, and National vi standards are all formulated to control vehicle exhaust emissions, including carbon monoxide, hydrocarbons, nitrogen oxides, solid suspended particles, etc. Below is relevant information about emission standards: 1. The National 4 standard was implemented nationwide on July 1, 2010. Compared to National 3, it upgraded the post-emission treatment system, reducing pollutant emissions by 50% to 60% compared to National 3. 2. The National 5 standard was implemented nationwide on January 1, 2018. Compared to National 4, it reduced nitrogen oxide emissions by 25% and added emission limits for non-methane hydrocarbons and PM, reducing atmospheric pollution and helping to prevent smog. 3. The National 6A standard was implemented nationwide on July 1, 2019. Compared to National 5, the National 6A standard tightened the CO limit for gasoline vehicles by 30%; the National 6B standard tightened CO and NOx limits by 50% and 42%, respectively. This means the National 6 standard is fully aligned with international standards.