Nitrite in orographic cloud as an indicator of nitrous acid in rural air.

Continuous measurements of nitrite concentrations in orographic cloud have been combined with pH measurements to estimate the gas-phase concentration of nitrous acid (HONO) during late autumn 1990 at Great Dun Fell (847 m) in the north of England. Estimated HONO concentrations were up to 0.5 ppbV (0.5 × 10−9 atm), and on average were ca 10% of the NOx(NO + NO2) concentration. Temporal variations showed no correlation with simultaneous measurements of NO2, suggesting that the nitrite measured in cloud water came from dissolution of gas-phase HONO rather than from heterogeneous reaction of NO2 in cloud. On three occasions, when measurements continued from afternoon into evening, estimated HONO concentrations increased after sunset, as expected from the relatively rapid photolytic destruction of HONO during the day. If gas-phase concentrations of HONO are typically 10% of NOx in rural areas, dry deposition of HONO may make a significant contribution to nitrogen deposition in upland forests, up to 3 kg N ha−1 yr−1.