Direct measurements of NO 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 inline-formulaNO₃ towards organic trace gases, inline-formula $M4inlinescrollmathml{k}_{\mathrm{normal\; OTG}}^{chem{\mathrm{normal\; NO}}_{\mathrm{normal\; 3}}}$ 25pt19ptsvg-formulamathimgae91486e0338273d98571ea12cbd498a acp-18-12045-2018-ie00001.svg25pt19ptacp-18-12045-2018-ie00001.png , at a rural mountain site (988 m a.s.l.) in southern Germany in 2017. The diel cycle of inline-formula $M5inlinescrollmathml{k}_{\mathrm{normal\; OTG}}^{chem{\mathrm{normal\; NO}}_{\mathrm{normal\; 3}}}$ 25pt19ptsvg-formulamathimge8f17a710a9c5b8473f3f65552c89306 acp-18-12045-2018-ie00002.svg25pt19ptacp-18-12045-2018-ie00002.png was strongly influenced by local meteorology with high reactivity observed during the day (values of up to 0.3 sinline-formula⁻¹) and values close to the detection limit (0.005 sinline-formula⁻¹) at night when the measurement site was in the residual layer and free troposphere. Daytime values of inline-formula $M8inlinescrollmathml{k}_{\mathrm{normal\; OTG}}^{chem{\mathrm{normal\; NO}}_{\mathrm{normal\; 3}}}$ 25pt19ptsvg-formulamathimg134614b32cb8e6e3a068a222f7becfc1 acp-18-12045-2018-ie00003.svg25pt19ptacp-18-12045-2018-ie00003.png were sufficiently large that the loss of inline-formulaNO₃ 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 inline-formulaNO₃ reactivity. Averaged over the daylight hours, more than 25 % of inline-formulaNO₃ was removed via reaction with biogenic volatile organic compounds (BVOCs), implying a significant daytime loss of inline-formulaNO_x and the formation of organic nitrates due to inline-formulaNO₃ chemistry. Ambient inline-formulaNO₃ concentrations were measured on one night and were comparable to those derived from a stationary-state calculation using measured values of inline-formula $M15inlinescrollmathml{k}_{\mathrm{normal\; OTG}}^{chem{\mathrm{normal\; NO}}_{\mathrm{normal\; 3}}}$ 25pt19ptsvg-formulamathimga16f602a9c457d0dd407f657230840d7 acp-18-12045-2018-ie00004.svg25pt19ptacp-18-12045-2018-ie00004.png . We present and compare the first simultaneous, direct reactivity measurements for the inline-formulaNO₃ 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 inline-formulaNO₃ reactivity was weak as would be anticipated given their divergent trends in rate constants with many organic trace gases.