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Abstract:
The reaction between the nitrate radical (NO$_3$) and volatile organic compounds (VOCs) can lead to alkyl nitrates (ANs). Some alkyl nitrates tend to deposit on particles to form secondary organic aerosols (SOA). The formation of SOA via the NO$_3$-initiated oxidation of VOCs with high AN yields such as isoprene or monoterpenes like limonene thus forms an important, irreversible NO$_x$ loss path from the gas-phase and can be of great importance for air quality. This work focusses on direct measurements of VOC-induced NO$_3$ reactivity ($k^{NO_3}$) and ANs by cavity ring-down spectroscopy (CRDS) in field, chamber and indoor measurements:\\ Two chamber studies deal with the NO$_3$ + isoprene system: The first comprises 22 experiments at the large environmental SAPHIR chamber of the research centre in Jülich (FZJ) in scope of the NO3ISOP campaign and investigates the role of NO$_3$ during the primary and secondary oxidation of isoprene. Intercomparison of NO$_3$ reactivities derived from VOC measurements ($\Sigma k_i[VOC]_i$), box-model and nonstationary-state calculations (using NO$_3$, NO$_2$, N$_2$O$_5$ and O$_3$ mixing ratios) verified that the NO$_3$ reactivity measurements were accurate and allowed estimation of the rate constant for the reaction between NO$_3$ and isoprene-derived peroxy radicals (RO$_2$). The second was performed with another Teflon chamber (SCHARK) and focused on the quantification of isoprene-derived ANs (ISOP-NITs) with means of thermal-dissociation CRDS. Detailed experiments revealed that sampling ISOP-NITs through heated quartz glass inlets in the presence of O$_3$ yields to broad thermal dissociation profiles (isotherms) of ISOP-NITs overlapping with that of peroxy nitrates (PNs). A way to circumvent the interference of ISOP-NITs in the detection of PNs is deployment of a Teflon inlet.\\ Measurements of $k^{NO_3}$ were also performed in a field campaign (TO2021) on the summit of the Kleiner Feldberg (KF, 825 m above sea level). $k^{NO_3}$ was mainly determined by monoterpenes and in good agreement with $\Sigma k_i[VOC]_i$. $k^{NO_3}$ contributed with ca. 16\% to the overall NO$_3$ daytime loss and 50-60 \% to the nighttime loss, which is caused by soil emissions of NO. Steady-state calculations allowed assessment of NO$_3$ mixing ratios and intercomparison with NO$_3$ measurements during previous campaigns on the KF.\\ Furthermore, NO$_3$ reactivity and mixing ratios of NO$_2$, O$_3$, NO$_3$ as well as N$_2$O$_5$ were measured for four days inside a laboratory. In the absence of NO, several pptv of N$_2$O$_5$ were detected, which was in good agreement with N$_2$O$_5$ mixing ratios derived from steady-state calculations. The indoor mixing ratios of NO$_3$ and N$_2O_5$ did not follow the typical diel cycle since photolysis is inefficient inside. Unidentified VOCs formed a relevant NO$_3$ sink.
