Direct measurements of NO 3 -reactivity in and above the boundary layer of a mountain-top site: Identification of reactive trace gases and comparison with OH-reactivity

We present direct measurements of the summertime, total reactivity of NO 3 towards organic trace gases, k OTG NO 3 , at a rural mountain site (988 m a.s.l.) in southern Germany in 2017. The diel cycle of k OTG NO 3 was strongly influenced by local meteorology with reactivity high during the day (values of up to 0.3 s -1 ) but usually close to the detection limit (0.005 s -1 ) at night when the measurement site was in the residual layer/free troposphere. Daytime values of k OTG NO 3 were sufficiently large that the loss of NO 3 due to reaction with organic trace gases competed with its photolysis and reaction with NO. Within experimental uncertainty, monoterpenes and isoprene accounted for all of the measured NO 3 -reactivity. Averaged over the daylight hours, more than 25 % of NO 3 was removed via reaction with biogenic volatile organic compounds (BVOCs), implying a significant daytime loss of NO x and formation of organic nitrates due to NO 3 chemistry. Ambient NO 3 concentrations were measured on one night and were comparable to those derived from a stationary state calculation using measured values of k OTG NO 3 . We present and compare the first simultaneous, direct-reactivity measurements for the NO 3 and OH radicals. The decoupling of the measurement site from ground level emissions resulted in lower reactivity at night for both radicals, though the correlation between OH- and NO 3 -reactivity was weak as would be anticipated given their divergent trends in rate constants with many organic trace gases.