Abstract
Abstract. We present direct measurements of the summertime total reactivity of NO3 towards organic trace gases, kOTGNO3, at a rural mountain site (988 m a.s.l.) in southern Germany in 2017. The diel cycle of kOTGNO3 was strongly influenced by local meteorology with high reactivity observed during the day (values of up to 0.3 s−1) and values close to the detection limit (0.005 s−1) at night when the measurement site was in the residual layer and free troposphere. Daytime values of kOTGNO3 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 compounds (BVOCs), implying a significant daytime loss of NOx and the 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 kOTGNO3. 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 OH and NO3 reactivity was weak as would be anticipated given their divergent trends in rate constants with many organic trace gases.
Highlights
Hydroxyl (OH) and nitrate radicals (NO3) play a centrally important role in cleansing the atmosphere of trace gas emissions resulting from both anthropogenic and biogenic activity (Lelieveld et al, 2004, 2016; Ng et al, 2017)
The NO2 mixing ratios used as input to calculate the NO3 reactivity were taken from the cavity ring-down spectroscopy (CRDS) instrument, with data gaps filled by cavity phase-shift (CAPS) measurements
The highest values of kONTOG3 were detected with north-easterly winds (Fig. S2) coincident with the warmest days of the campaign and the highest biogenic emissions
Summary
Hydroxyl (OH) and nitrate radicals (NO3) play a centrally important role in cleansing the atmosphere of trace gas emissions resulting from both anthropogenic and biogenic activity (Lelieveld et al, 2004, 2016; Ng et al, 2017). The large NO3 mixing ratios at night-time and the large rate constants for the reaction of NO3 with several unsaturated biogenic VOCs result in NO3 being the dominant sink of many BVOCs (Wayne et al, 1991; Atkinson, 2000; Atkinson and Arey, 2003a, b; Brown and Stutz, 2012; Liebmann et al, 2017, 2018), especially those whose emission is mainly temperature dependent and continues at night-time, e.g. monoterpenes (Hakola et al, 2012). Previous work on NO3 reactivity has revealed a strong meteorological influence on the NO3 lifetime, especially when air masses are decoupled from the surface layer into which reactive trace gases (NO and BVOC) are emitted at night (Brown et al, 2007b, 2011; Sobanski et al, 2016; Liebmann et al, 2018). We compare NO3 reactivity to simultaneous measurements of OH reactivity over the same period
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