Direct measurements of NOsub3/sub reactivity in and above the boundary layer of a mountaintop site: identification of reactive trace gases and comparison with OH reactivity

摘要

We present direct measurements of the summertime total reactivity of NOsub3/sub towards organic trace gases, ksubOTG/subsupNOsub3/sub/sup, at a rural mountain site (988 m a.s.l.) in southern Germany in 2017. The diel cycle of ksubOTG/subsupNOsub3/sub/sup was strongly influenced by local meteorology with high reactivity observed during the day (values of up to 0.3 ssup?1/sup) and values close to the detection limit (0.005 ssup?1/sup) at night when the measurement site was in the residual layer and free troposphere. Daytime values of ksubOTG/subsupNOsub3/sub/sup were sufficiently large that the loss of NOsub3/sub 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 NOsub3/sub reactivity. Averaged over the daylight hours, more than 25 % of NOsub3/sub was removed via reaction with biogenic volatile organic compounds (BVOCs), implying a significant daytime loss of NOsubx/sub and the formation of organic nitrates due to NOsub3/sub chemistry. Ambient NOsub3/sub concentrations were measured on one night and were comparable to those derived from a stationary-state calculation using measured values of ksubOTG/subsupNOsub3/sub/sup. We present and compare the first simultaneous, direct reactivity measurements for the NOsub3/sub 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 NOsub3/sub reactivity was weak as would be anticipated given their divergent trends in rate constants with many organic trace gases.