Neutral molecular cluster formation of sulfuric acid dimethylamine observed in real time under atmospheric conditions

Abstract: For atmospheric sulfuric acid (SA) concentrations the presence of dimethylamine (DMA) at mixing ratios of several parts per trillion by volume can explain observed boundary layer new particle formation rates. However, the concentration and molecular composition of the neutral (uncharged) clusters have not been reported so far due to the lack of suitable instrumentation. Here we report on experiments from the Cosmics Leaving Outdoor Droplets chamber at the European Organization for Nuclear Research revealing the formation of neutral particles containing up to 14 SA and 16 DMA molecules, corresponding to a mobility diameter of about 2 nm, under atmospherically relevant conditions. These measurements bridge the gap between the molecular and particle perspectives of nucleation, revealing the fundamental processes involved in particle formation and growth. The neutral clusters are found to form at or close to the kinetic limit where particle formation is limited only by the collision rate of SA molecules. Even though the neutral particles are stable against evaporation from the SA dimer onward, the formation rates of particles at 1.7-nm size, which contain about 10 SA molecules, are up to 4 orders of magnitude smaller comparedwith those of the dimer due to coagulation and wall loss of particles before they reach 1.7 nm in diameter. This demonstrates that neither the atmospheric particle formation rate nor its dependence on SA can simply be interpreted in terms of cluster evaporation or the molecular composition of a critical nucleus.

• The paper demonstrates that neutral SA-DMA clusters form at nearly kinetic limits via successive addition of individual molecules, with dimer formation rates showing a power dependence on [H₂SO₄].
• It employs the CLOUD chamber and CI-APi-TOF mass spectrometry to track cluster growth from dimers to ~2 nm particles containing up to 14 SA and 16 DMA molecules.
• The findings reveal significant particle losses through coagulation and wall deposition around 1.7 nm, underlining the need for refined nucleation models in atmospheric chemistry.

Insights into Neutral Molecular Cluster Formation of Sulfuric Acid and Dimethylamine

The paper under discussion presents a detailed investigation into the formation of neutral molecular clusters involving sulfuric acid (SA) and dimethylamine (DMA) under atmospherically relevant conditions. The study addresses a notable gap in the understanding of new particle formation (NPF) by examining sulfuric acid, one of the most important precursors to atmospheric aerosols, and its interaction with amines in the boundary layer, utilizing a state-of-the-art experimental approach.

The authors utilize the Cosmics Leaving Outdoor Droplets (CLOUD) chamber at CERN, equipped with a Chemical Ionization Atmospheric Pressure interface–Time Of Flight (CI-APi-TOF) mass spectrometer, to provide new insights into the formation and growth of neutral clusters beyond the previously explored binary SA–water systems. One of the key advancements reported is the real-time observation of neutral particles comprising up to 14 SA and 16 DMA molecules with a mobility diameter of roughly 2 nm—dimensions verifiable by existing condensation particle counters.

Experimental Insights and Numerical Analysis

The study reveals that the formation of these neutral clusters occurs at or near the kinetic limit, determined by the collision rate between SA molecules, and predominantly through a neutral channel. Clusters are shown to grow systematically by the addition of single SA and DMA molecules, closely following the base stabilization mechanisms previously demonstrated for charged clusters. Numerical modeling suggests that observed formation rates of SA particles at dimer sizes show a remarkable consistency with kinetic limit calculations, exhibiting a power dependence on [H₂SO₄] of approximately two.

While neutral particles are stable against evaporation from the dimer stage onward, the data underscore significant losses at sizes of around 1.7 nm due to coagulation and wall losses, which result in formation rates up to four orders of magnitude lower than the initial dimer formation rates. This discrepancy is critical to note—it indicates that reaching observable particle sizes is profoundly affected by intervening losses rather than cluster volatility alone.

Practical and Theoretical Implications

These findings are significant for advancing our understanding of NPF mechanisms, particularly in polluted atmospheric boundary layers where amine concentrations may facilitate SA nucleation processes more efficiently than ammonia. The results imply that in the presence of sufficient DMA, the new particle formation can approach the kinetic limits with negligible evaporation, suggesting a robustness in the nucleation process under specific conditions. This sheds light on the potential for SA–DMA systems to lead new particle formation even in environments where traditional nucleation pathways might be considered insufficient.

Future Directions

The study opens various paths for future research. One area of interest would be the exploration of whether similar kinetic stabilization occurs in systems involving sulfuric acid and oxidized organics, given their atmospheric prevalence. Additionally, examining the influence of temperature and relative humidity on SA–DMA cluster stability can further elucidate the robustness of these findings under varying atmospheric conditions.

Finally, this research underscores the importance of developing comprehensive nucleation models that incorporate kinetic effects accurately. Bridging the gap between laboratory measurements and atmospheric processes will enhance predictive models of aerosol formation, with implications for understanding cloud condensation nuclei (CCN) and climate dynamics.

Overall, this paper contributes substantially to atmospheric chemistry by elucidating the mechanisms of new particle formation involving sulfuric acid and amines, guiding both theoretical and applied research towards a deeper understanding of climate-affecting aerosol processes.