Development and Application of a Relaxed Eddy Accumulation (REA) System to Measure Vertical Fluxes of Nitrous Acid (HONO)

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During recent years Nitrous Acid (HONO) has been identified as a dominant source of OH radicals in the lower atmosphere. Since the OH radical is responsible for the degradation of most air pollutants and for the formation of harmful photooxidants, e.g. ozone, the identification and quantification of the sources of HONO are of major importance. Based on laboratory studies mainly ground surface sources of HONO were postulated to explain the un-expected high daytime levels in the lower atmosphere. However, resulting flux measurements of HONO over ambient surfaces are only rarely available. In addition, only the aerodynamic gradient method is typically applied, caused by the absence of fast and sensitive HONO measurements methods, resulting in significant uncertainties in the quantification of vertical fluxes of HONO in the atmosphere. Thus, a REA (Relaxed Eddy Accumulation) system should be developed and tested in the present project. A two channel instrument should be used which is based on the LOPAP (Long Path Absorption Photometer) approach and which is coupled to a micrometeorological inlet system. Two fast magnetic valves controlled by an ultrasonic anemometer are used to probe the two channels for up- and downdraft air masses. In addition, a third measurement channel is used to correct for chemical interferences. Besides setting-up of the REA instrument, a new software for controlling the inlet valves and for micrometeorological data logging will be developed. The new REA system will be first tested and optimized for sensitive flux measurements at the BUW. To the end of the project, HONO fluxes and supporting other parameters will be measured over bare soil at an agriculture field site in Grignon (France) in two campaigns to identify and quantify daytime sources of HONO in the atmosphere. The obtained data should be compared with results from a former DFG-project of the applicant in which HONO fluxes were determined by the more simple aerodynamic gradient method at the same measurement site.