Article 2.5.2.3.3. The necessary measures shall be taken to ensure that the critical level for protecting vegetation stated in Appendix 2.5.3.13 and assessed in accordance with Appendix 2.5.3.3, Part A, are observed. If fixed measurements are the only source of data for assessing air quality, the number of sampling points may not be less than the minimum number stated in Appendix 2.5.3.5, Part C. If these data are supplemented by data obtained through indicative measurements or modelling, the minimum number of sampling points may be reduced by up to 50%, provided the assessed concentrations of the polluting substance in question can be established in accordance with the data quality targets stated in Appendix 2.5.3.1, Part A.