Context

• A day after Canada announced it would expel a Chinese diplomat over allegations of involvement in a campaign to intimidate a Canadian politician, China has declared a Canadian diplomat in Shanghai as persona non grata.

What is persona non grata?

• Persona non grata is a Latin phrase which means “unwelcome person.” 
• In diplomacy, it refers to a diplomat or foreign person whose entering or remaining in a certain country has been prohibited by that country.
• The designation received diplomatic meaning at the 1961 Vienna Convention for Diplomatic Relations. 
• Article 9 of the treaty mentions that a country can declare any member of a diplomatic staff persona non grata “at any time and without having to explain its decision.”
• Soon after the declaration, the person concerned usually returns to their home nation. In case they fail to do so within a reasonable period, the country “may refuse to recognise the person concerned as a member of the mission.” 
• The article also says that a person can be declared persona non grata even before arriving in a country.

When is it used?

• There aren’t any fixed rules regarding when a country can declare a foreign person persona non grata as it doesn’t need to give an explanation according to Article 9 of the Vienna Convention.
• Historically speaking, countries have used it to express their discontent with the actions of other nations. 
• During the Cold War, it became a “tit-for-tat” sanction as both the US and the Soviet Union liberally declared each other’s diplomats persona non grata.

About Vienna Convention 

• The Vienna Convention on Diplomatic Relations (VCDR) was signed in 1961 as an international treaty that defines a framework for diplomatic relations between independent countries. 
• It specifies the privileges of a diplomatic mission that enable diplomats to perform their function without fear of coercion or harassment by the host country. 
• This forms the legal basis for diplomatic immunity. Its articles are considered a cornerstone of modern international relations. 
• The Convention is nearly universally ratified, with Palau and South Sudan being the exceptions.

References:

https://indianexpress.com/article/explained/everyday-explainers/china-canada-face-off-what-is-persona-non-grata-8600426/#:~:text=persona%20non%20grata%3F-,Persona%20non%20grata%20is%20a%20Latin%20phrase%20which%20means%20%E2%80%9Cunwelcome,been%20prohibited%20by%20that%20country.