Significant HONO formation by the photolysis of nitrates in the presence of humic acids.

The generation of HONO and NO by the photolysis of nitrates in the presence of humic acids (HA) was measured under various conditions. The photolysis experiments of HA, KNO and KNO/HA under simulated sunlight was carried out by a flow tube reactor at ambient temperature and pressure. HONO and NO were major products by the photolysis of KNO. By contrast, the photolysis of HA and KNO/HA mainly generated HONO. HA significantly enhanced the formation of HONO during the photolysis process of KNO. With increasing the KNO mass, the HONO formation rate (R) on KNO/HA increased while the photolysis rate normalized by the KNO mass exhibited an opposite trend. R on KNO/HA linearly increased with irradiation intensity (88-262 W/m) and relative humidity (7-70%), whereas it linearly decreased with the pH (pH = 2-12). In addition, the reaction paths of the HONO formation by the photolysis of nitrates in the presence of HA were proposed according to experimental results. Finally, atmospheric implications of the enhanced HONO formation by the photolysis of nitrates in the presence of HA were discussed.