On the composition of ammonia–sulfuric-acid ion clusters during aerosol particle formation

The formation of particles from precursor vapors is an important source of atmospheric aerosol. Research at the Cosmics Leaving OUtdoor Droplets (CLOUD) facility at CERN tries to elucidate which vapors are responsible for this new-particle formation, and how in detail it proceeds. Initial measurement campaigns at the CLOUD stainless-steel aerosol chamber focused on investigating particle formation from ammonia $(NH_3)$ and sulfuric acid $(H-2SO_4)$. Experiments were conducted in the presence of water, ozone and sulfur dioxide. Contaminant trace gases were suppressed at the technological limit. For this study, we mapped out the compositions of small $NH_3–H_2SO_4$ clusters over a wide range of atmospherically relevant environmental conditions. We covered [NH3] in the range from < 2 to 1400 pptv, $[H_2SO_4]$ from 3.3 × $10^6 to 1.4 × 10^9 cm^{−3}$ (0.1 to 56 pptv), and a temperature range from −25 to +20 °C. Negatively and positively charged clusters were directly measured by an atmospheric pressure interface time-of-flight (APi-TOF) mass spectrometer, as they initially formed from gas-phase $NH_3$ and $H_2SO_4$, and then grew to larger clusters containing more than 50 molecules of $NH_3$ and $H_2SO_4$, corresponding to mobility-equivalent diameters greater than 2 nm. Water molecules evaporate from these clusters during sampling and are not observed. We found that the composition of the $NH_3–H_2SO_4$ clusters is primarily determined by the ratio of gas-phase concentrations $[NH_3]$ / $[H_2SO_4]$, as well as by temperature. Pure binary $H_2O–H_2SO_4$ clusters (observed as clusters of only $H_2SO_4$) only form at $[NH_3]$ / $[H_2SO_4]$ < 0.1 to 1. For larger values of $[NH_3]$ / $[H_2SO_4]$, the composition of $NH_3–H_2SO_4$ clusters was characterized by the number of $NH_3$ molecules m added for each added $H_2SO_4$ molecule n (Δm/Δ n), where n is in the range 4–18 (negatively charged clusters) or 1–17 (positively charged clusters). For negatively charged clusters, Δ m/Δn saturated between 1 and 1.4 for $[NH_3]$ / $[H_2SO_4]$ > 10. Positively charged clusters grew on average by Δm/Δn = 1.05 and were only observed at sufficiently high $[NH_3]$ / $[H_2SO_4]$. The $H_2SO_4$ molecules of these clusters are partially neutralized by $NH_3$, in close resemblance to the acid–base bindings of ammonium bisulfate. Supported by model simulations, we substantiate previous evidence for acid–base reactions being the essential mechanism behind the formation of these clusters under atmospheric conditions and up to sizes of at least 2 nm. Our results also suggest that electrically neutral $NH_3–H_2SO_4$ clusters, unobservable in this study, have generally the same composition as ionic clusters for $[NH_3]$ / $[H_2SO_4]$ > 10. We expect that NH3–H2SO4 clusters form and grow also mostly by Δm/Δn > 1 in the atmosphere's boundary layer, as $[NH_3]$ / $[H_2SO_4]$ is mostly larger than 10. We compared our results from CLOUD with APi-TOF measurements of $NH_3–H_2SO_4$ anion clusters during new-particle formation in the Finnish boreal forest. However, the exact role of $NH_3–H_2SO_4$ clusters in boundary layer particle formation remains to be resolved.