Direct measurements of NO.sub.3 reactivity in and above the boundary layer of a mountaintop site: identification of reactive trace gases and comparison with OH reactivity

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