Nitrous acid (HONO) is a crucial precursor to hydroxyl radicals (OH) and plays significant roles in tropospheric photochemistry and atmospheric oxidation capacity. However, the quantitative understanding of HONO formation across diverse environments remains limited, especially in the marine atmosphere. In this study, we quantified HONO formation in contrasting environments: an urban site in the Yellow River Delta, a mountain site in North China, and two coastal sites in Qingdao with different di