A relaxed eddy accumulation (REA) LOPAP system for flux measurements of nitrous acid (HONO)

Abstract. In the present study a relaxed eddy accumulation (REA) system for quantification of vertical fluxes nitrous acid (HONO) was developed and tested. The is based on three-channel long-path absorption photometer (LOPAP) instrument, which two channels are used updrafts downdrafts, respectively, third one correction chemical interferences. instrument coupled to REA gas inlet, an ultrasonic anemometer controls fast magnetic valves probe LOPAP depending wind direction. A software (PyREA) developed, measurement cycles, regularly alternates between REA, zero parallel ambient measurements. addition, assignment downdrafts physical periodically alternated, correct differences in interferences different air masses. During study, only small were identified excluding significant errors when using interference channel. laboratory experiments, high precision independence dilution-corrected HONO concentrations length valve switching periods demonstrated. field campaign performed order test new REA-LOPAP at TROPOS monitoring station Melpitz, Germany. range -4×1013 molecules m−2 s−1 (deposition) +1.0×1014 (emission) obtained. typical diurnal variation observed with low, partly negative during night-time higher positive around noon. After intensive rain period emissions daytime continuously increasing, explained by drying uppermost ground surfaces. Similar other campaigns, highest correlation flux product NO2 photolysis frequency concentration (J(NO2)⋅[NO2]), implies formation photosensitized conversion organic surfaces, such as humic acids. Other postulated mechanisms also discussed but tentatively ranked being minor importance campaign.