Direct measurements of NO3-reactivity in and above the boundary layer of a mountain-top site: Identification of reactive trace gases and comparison with OH-reactivity

Jonathan M. Liebmann, Jennifer B. A. Muller, Dagmar Kubistin, Anja Claude, Robert Holla, Christian Plaß-Dülmer, Jos Lelieveld, John N. Crowley
2018 Atmospheric Chemistry and Physics Discussions  
<p><strong>Abstract.</strong> We present direct measurements of the summertime, total reactivity of NO<sub>3</sub> towards organic trace gases, <i>k</i><sub>OTG</sub><sup>NO<sub>3</sub></sup>, at a rural mountain site (988<span class="thinspace"></span>m<span class="thinspace"></span>a.s.l.) in southern Germany in 2017. The diel cycle of <i>k</i><sub>OTG</sub><sup>NO<sub>3</sub></sup> was strongly influenced by local meteorology with reactivity high during the day (values of up to 0.3<span
more » ... up to 0.3<span class="thinspace"></span>s<sup>-1</sup>) but usually close to the detection limit (0.005<span class="thinspace"></span>s<sup>-1</sup>) at night when the measurement site was in the residual layer/free troposphere. Daytime values of <i>k</i><sub>OTG</sub><sup>NO<sub>3</sub></sup> were sufficiently large that the loss of NO<sub>3</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 NO<sub>3</sub>-reactivity. Averaged over the daylight hours, more than 25<span class="thinspace"></span>% of NO<sub>3</sub> was removed via reaction with biogenic volatile organic compounds (BVOCs), implying a significant daytime loss of NO<sub>x</sub> and formation of organic nitrates due to NO<sub>3</sub> chemistry. Ambient NO<sub>3</sub> concentrations were measured on one night and were comparable to those derived from a stationary state calculation using measured values of <i>k</i><sub>OTG</sub><sup>NO<sub>3</sub></sup>. We present and compare the first simultaneous, direct-reactivity measurements for the NO<sub>3</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 NO<sub>3</sub>-reactivity was weak as would be anticipated given their divergent trends in rate constants with many organic trace gases.</p>
doi:10.5194/acp-2018-324 fatcat:zff5sy24njcxbli7l4lqsfhkwm