Nitrous acid is an important precursor for OH radicals in the polluted troposphere. The heterogeneous conversion of NO2 to HONO, however, is currently not well understood. Measurements of HONO and NO2 in Phoenix in summer 2001 using long path DOAS show ratios of chemically formed secondary to 2> that rarely exceeded 3%. During two nocturnal dust storm events, however, a significant increase of this ratio was observed. The unprecedented high ratios near 19% suggest a highly efficient NO2 to HONO conversion process on mineral dust particles. The particle composition in Phoenix is similar to other mineral dusts, implying that the enhanced NO2 conversion could be an important HONO, and therefore also OH, source in regions where pollution and dust storms coincide.