 The primary objective of this study is to evaluate the efficacy of inhaled nebulized AIR001 administered, for 16 weeks, according to 3 treatment arms 80 mg once daily, 46 mg 4 times daily, or 80 mg 4 times daily in subjects with World Health Organization WHO Group 1 Pulmonary Arterial Hypertension PAH, as determined by change in Pulmonary Vascular Resistance PVR from Baseline to Week 16 measured immediately post completion of AIR001 nebulization as soon as feasible. 