Inclusion Criteria:  acute hypercapnic exacerbation of chronic obstructive pulmonary disease with pH < 7.35 pCO2 > 45mmHg age ≥ 18 years written informed consent  Exclusion Criteria:  immediate need for intubation (acc. to intubation criteria in this protocol) pH < 7.15 BMI ≥ 35 kg/m² established home-NIV or home-CPAP end-stage disease with DNI/DNR order diseases that could influence the primary endpoint: e.g. acute heart infarction, cardiogenic lung edema, acute and massive lung embolism (hypertensive), chronic dialysis with metabolic acidosis, unstable rib fracture influencing ventilation, injury to the face prohibiting use of a face mask acute disease that precludes participation in the trial tracheotomized patients psychological/mental or other inabilities to supply required informed consent participation in other interventional trials suspected lack of compliance