 The primary objective of this study is to evaluate the intermediatelong-term safety of inhaled nebulized AIR001 administered according to 3 treatment arms 80 milligrams mg 4 times daily, 46 mg 4 times daily, or 80 mg once daily in subjects with World Health Organization WHO Group 1 Pulmonary Arterial Hypertension PAH who have completed the 16-week AIR001-CS05 study. 