Luftgestützte Messung von HO x -Radikalkonzentrationen mittels Laser-induzierter Fluoreszenz auf einem Zeppelin NT: Untersuchung der atmosphärischen Oxidationsstärke der unteren Troposphäre

The OH radical is the major atmospheric oxidant that dominates the photochemical degradation of trace gases and pollutants in the atmosphere. The consumed OH can be recycled through the HO2 radical by reacting with NO, thereby forming ozone. Thus, OH and HO2 are ideal parameter in order to test the current understanding of the atmospheric degradation of trace gases. In this work, an instrument for the measurement of OH and HO2 radicals by laser induced fluorescence was developed and deployed on board a Zeppelin NT. The measurements presented here were conducted in July and August 2012 in the region Emilia Romagna in Northern Italy. The measurement platform Zeppelin NT allowed the observation of a comprehensive set of chemical and physical parameter within the lower troposphere between 75 - 900m above ground. During the measurement flights, strong trace gas gradients were observed in the early morning that could be explained by the layering within the Planetary Boundary Layer. Typically, low trace gas concentrations were found in the residual layer in high altitudes whereas the highest OH reactivities up to 10 s-1 and NOx mixing ratios up to 10 ppbv were observed in the mixed layer which is strongly influenced by ground emissions. The linear correlation between observed OH and j(O1D) with a slope of 4.4 × 1011 cm-3s is comparable to other field measurements in continental regions. Additionally, the observed OH depends nonlinearly on NOx resulting in enhanced mean OH concentrations by a factor of 2 for NOx mixing ratios between 1.5 and 2.0 ppbv. Observed mean HO2 concentrations in air masses with [NO] 1 ppbv. For the first time, this HOx dependency on NOx was observed locally when crossing vertical and horizontal NOx gradients. Box model calculations based on the Regional Atmospheric Chemistry Mechanism reproduce the measured OH for [NOx] < 3 ppbv. For higher NOx mixing ratios, the model overestimates OH for more than 50% of the data points. The model predicts HO2 within the uncertainty of the measurements and the model. The prediction for OH could be improved by implementing a newly proposed gas phase machanism forming HONO (Li et al., 2014). The analysis of the HOx data does not hint at a significant NO independent, non-classical OH-recycling during the measurement flights performed in Italy.