Observations of high rates of NO2-HONO [NO subscript 2 - HONO] conversion in the nocturnal atmospheric boundary layer in Kathmandu, Nepal

摘要

Nitrous acid (HONO) plays a significant role inthe atmosphere, especially in the polluted troposphere. Itsphotolysis after sunrise is an important source of hydroxylfree radicals (OH). Measurements of nitrous acid and otherpollutants were carried out in the Kathmandu urban atmosphereduring January–February 2003, contributing to thesparse knowledge of nitrous acid in South Asia. The resultsshowed average nocturnal levels of HONO (1.7±0.8 ppbv),NO2 [NO subscript 2] (17.9±10.2 ppbv), and PM10(0.18±0.11 mgm−3 [mg m superscript -3]) in urbanair in Kathmandu. Surprisingly high ratios of chemicallyformed secondary [HONO] to [NO2] [NO subscript 2] (up to 30%) werefound, which indicates unexpectedly efficient chemical conversionof NO2 [NO subscript 2] to HONO in Kathmandu. The ratios of[HONO]/[NO2] [NO subscript 2] at night were found to be much higher thanpreviously reported values from measurements in urban air inEurope, North America and Asia. The influences of aerosolsurface, ground reactive surface, and relative humidity onNO2-HONO [NO subscript 2 - HONO] chemical conversion were discussed. The highhumidity, strong and low inversion layer at night, and highaerosol pollution burden in Kathmandu may explain the particularlyefficient conversion of NO2 [NO subscript 2] to HONO.