Case C-174/21: Judgment of the Court (Third Chamber) of 16 March 2023 — European Commission v Republic of Bulgaria (Failure of a Member State to fulfil obligations — Directive 2008/50/EC — Ambient air quality — Judgment of the Court establishing a failure to fulfil obligations — Article 260(2) TFEU — Obligation to take the necessary measures to comply with such a judgment — Failure to fulfil that obligation alleged by the European Commission — Lack of clarity of the letter of formal notice as to whether the judgment still had to be complied with on the reference date — Principle of legal certainty — Inadmissibility)