Dagmar Kubistin

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 NO3 towards organic trace gases, kOTG NO3 , at a rural mountain site (988 m a.s.l.) in southern Germany in 2017. The diel cycle of kOTG NO3 was strongly influenced by local 15 meteorology with high reactivity observed during the day (values of up to 0.3 s) and values close to the detection limit (0.005 s) at night when the measurement site was in the residual layer / free troposphere. Daytime values of kOTG NO3 were sufficiently large that the loss of NO3 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 NO3-reactivity. Averaged over the daylight hours, more than 25% of NO3 was removed via reaction with biogenic volatile organic 20 compounds (BVOCs), implying a significant daytime loss of NOx and formation of organic nitrates due to NO3 chemistry. Ambient NO3 concentrations were measured on one night and were comparable to those derived from a stationary state calculation using measured values of kOTG NO3 . We present and compare the first simultaneous, direct-reactivity measurements for the NO3 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 OHand NO3-reactivity was weak as would be 25 anticipated given their divergent trends in rate constants with many organic trace gases.