Nitrous acid in the urban area of Rome.

Nitrous acid (HNO2) and a large variety of other components were simultaneously measured in the city centre of Rome (Italy) simultaneously during the NITROCAT ground based field experiment in May/June 2001. The highest HNO2 concentrations were found under high-pressure conditions with high nocturnal atmospheric stability and high values of pollutants. After night-time formation and accumulation up to 2 ppb HNO2 were observed. The measurements confirm that during the first hours after sunrise, when hydroxyl radical (OH) production rates from other sources (photolysis of ozone and formaldehyde (HCHO)) are slow, HNO2 photolysis is the most important primary OH source in the lowest part of the troposphere; up to 1-4 x 107 OH cm-3 s-1 were estimated for that time from this source. This contributes considerably to the initiation of the photochemistry for the day. The unexpected high daytime concentrations of few hundred ppt observed by DOAS as well as by the two in-situ wet collection techniques (wet denuder/IC, coil sampling/HPLC) possibly influence ozone chemistry during the entire day. The heterogeneous on-surface production of HNO2 (and consequently of HNO3) provides also a new-type acidity formation influencing directly the biosphere and the materials. About 20% of the total nitrite was found on atmospheric aerosols. The HNO2 measurements agree well for the different in-situ measurement techniques and the spatial integration DOAS simultaneously performed over several weeks in the real atmosphere and during reaction chamber experiments.