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The Cassini Plasma Spectrometer (CAPS) Electron Spectrometer (ELS) instrumentonboard Cassini revealed an unexpected abundance of negative ions above 950 kmin Titan's ionosphere. \textit{In situ} measurements indicated the presence ofnegatively charged particles with mass-over-charge ratios up to 13,800\textit{u/q}. At present, only a handful of anions have been characterized byphotochemical models, consisting mainly of C$_n$H$^-$ carbon chain andC$_{n-1}$N$^-$ cyano compounds ($n=2-6$); their formation occurring essentiallythrough proton abstraction from their parent neutral molecules. However,numerous other species have yet to be detected and identified. Considering theefficient anion growth leading to compounds of thousands of \textit{u/q}, it isnecessary to better characterize the first light species. Here, we present newnegative ion measurements with masses up to 200 \textit{u/q} obtained in an\ce{N2}:\ce{CH4} dusty plasma discharge reproducing analogous conditions toTitan's ionosphere. We perform a comparison with high altitude CAPS-ELSmeasurements near the top of Titan's ionosphere from the T18 encounter. Themain observed peaks are in agreement with the observations. However, a numberof other species (\textit{e.g.} \ce{CNN-}, \ce{CHNN-}) previously notconsidered suggests an abundance of N-bearing compounds, containing two orthree nitrogen atoms, consistent with certain adjacent doubly-bonded nitrogenatoms found in tholins. These results suggest that an N-rich incorporation intotholins may follow mechanisms including anion chemistry, further highlightingthe important role of negative ions in Titan's aerosol growth.

Nitrogen-containing Anions and Tholin Growth in Titan's Ionosphere: Implications for Cassini CAPS-ELS Observations