“…Nitrous acid (HONO) is an important trace gas in the troposphere. − The key importance of HONO in tropospheric chemistry comes from the fact that it produces OH radicals via photolysis. − The hydroxyl radical is a key oxidant in the troposphere responsible for the cleaning of almost all the hazardous materials present in the atmosphere ranging from carbon monoxide (CO) to volatile organic compounds (VOCs). − In fact, after ozone, the photolysis of HONO is the major source of OH radicals in the atmosphere. − It is worth mentioning that in certain regions and under certain atmospheric conditions, the contribution of nitrous acid to the atmospheric budget of OH radicals can be more significant than ozone. For example, during the winter season in polluted urban areas (high levels of nitrogen oxide (NOx)), the contribution of HONO to the OH radical production can be higher than ozone. , In addition, in a household environment, OH production is dominated by HONO rather than ozone. , Therefore, an understanding of sources, sinks, and transformations of HONO in the atmosphere is crucial for assessing its impacts on the formation of OH radicals.…”