Most pollutants emitted into the atmosphere by natural sources or human activities are removed by atmospheric oxidants (OH and NO3 radicals, O3, Cl atom, etc.) via chemical oxidation processes. For instance, the hydroxyl free radical (OH), as a primary "cleansing agent," removes 85% CO, 90% CH4, 30% SO2, 50% NO2, and most volatile organic compounds (VOCs) from the atmosphere. It is well known that, besides ozone (O3) and formaldehyde (CH2O), nitrous acid (HONO in gas phase and HNO2 in liquid phase) acts as an important source of OH radical. HONO is usually considered to be an important OH source by its photolysis in the early morning. Recent measurements revealed, however, a strongly enhanced formation of HONO during daytime via unknown mechanisms, the photolysis of which accounts for up to 60% of the integrated hydroxyl radical source strengths, resulting in the photolysis of HONO being viewed as a much more important source of daytime OH radical than previously thought.
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