Monitoring Volatile Evolution in Disrupting Comet D/2021 A1 (Leonard) with NOEMA and APEX

We report a preperihelion survey of volatile emissions from comet D/2021 A1 (Leonard) with the Northern Extended Millimeter Array (UT 2021 November 5, 21, and December 1) and the Atacama Pathfinder Experiment (UT 2021 December 9–10), spanning heliocentric distances ( r H ) from 1.3 to 0.80 au. We securely detected hydrogen cyanide (HCN) and carbon monosulfide (CS) and placed 3 σ upper limits on CH 3 OH, H 2 CO, and CO abundances. Line kinematics and interferometric spatial constraints indicate that HCN was released at or near the nucleus (parent scale length <300 km), while CS showed higher gas expansion velocities and mixing ratios that increased with decreasing r H —consistent with production from a distributed source. Across our campaign, CS mixing ratios relative to H 2 O increased by a factor of ∼5, from 0.02% ± 0.01% at r H = 1.3 au to 0.10% ± 0.02% by r H = 0.80 au. HCN mixing ratios in our data rose modestly, from 0.04% ± 0.02% at r H = 1.3 au to 0.07% ± 0.02% by r H = 0.81 au. However, contemporaneous measurements from other facilities placed HCN consistently at a higher absolute level (∼0.08%) with additional variability. Once cross-facility measurements were included, the HCN abundance showed no statistically robust monotonic dependence on r H . Variability in both species during the mid-December outbursts and fragmentation suggests that D/2021 A1’s volatile evolution reflected not only solar insolation but also disruption processes, underscoring the value of multi-epoch, multi-instrument monitoring to capture rapid, species-dependent changes.