Dominant Processes of HONO Derived from Multiple Field Observations in Contrasting Environments

The sources of atmospheric nitrous acid (HONO) have been frequently investigated in recent decades. However, the dominant mechanisms of HONO formation and its significant daytime source in contrasting environments remain largely unknown. In this study, 46 observational data sets were collected worldwide, and the underlying processes driving diurnal HONO patterns were extracted on the basis of principal component analysis (PCA). We identified three dominant processes: a NOx process (PC1) that was dominant in polluted inland areas, a photodecomposition process (PC2) that was dominant in marine or mountain environments, and a soil-related process (PC3). The three processes together explained >88% of the total variance. HONO variations in coastal regions were governed by either PC1 or PC2, determined by the effects of the sea–land breeze. These PCs were then applied to the identification of unknown HONO sources. The results showed that the unknown source was mainly related to the NOx process in most urban areas; however, it was associated with the photodecomposition-dominated process in coastal regions. The application of PCA to isolate the common behaviors of HONO provides valuable insights into the dominant HONO formation processes in different environments.