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 compared with 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.aerosol particles | atmospheric nucleation | atmospheric chemistry | mass spectrometry A erosol particles are important constituents of the Earth's atmosphere. A large fraction of the particles form by nucleation of low-volatility vapors. The newly formed particles first consist only of a few molecules and have diameters between 1 and 2 nm. Under favorable conditions, where loss rates are small and growth rates are sufficiently large, the particles can reach sizes of ∼50 nm where they can act as cloud condensation nuclei (CCN). Model simulations suggest that globally about half the CCN originate from new particle formation (NPF) (1). Therefore, NPF is an important process affecting the climate through the cloud albedo effect (2). Numerous studies have shown that sulfuric acid (SA; H 2 SO 4 ) and water vapor are important compounds participating in the formation of new particles (3, 4). However, atmospheric boundary layer NPF events cannot be explained by the binary nucleation of these two compounds alone (5); therefore, at least one additional substance besides SA and water vapor is required (6). However, the chemical identity of the compounds responsible for the high observed NPF rates remains to be fully elucidated. Recent theoretical (7, 8) and experimental (6, 9-15) studies have shown that amines efficiently stabilize SA Significance A significant fraction of atmospheric aerosols is formed from the condensation of low-volatility vapors. These newly formed particles can grow, become s...

Ideas and perspectives: on the emission of amines from terrestrial vegetation in the context of new atmospheric particle formation

“…Investigating higher SA mixing ratios (approximately 1-250 ppt), Jen et al (2014) found an increase in cluster concentration of roughly 2 orders of magnitude, caused by DMA or TMA compared to the maximum caused by NH 3 alone. From SA and DMA, Kurten et al (2014) detected NPF rates close to the collision rate between both molecules, which represents the maximum possible kinetic limit. They argue that collision-rate-driven NPF might be observed only with a DMA concentration higher than a threshold of approximately 100 times the SA concentration, because of cluster coagulation and fission (see Ortega et al, 2012).…”

“…Estimates of the global secondary aerosol budget are highly uncertain due to uncertainties in gas-phase precursor contributions to NPF and limited mechanistic comprehension (Spracklen et al, 2011). While NPF rates observed in the field do not comply with the classical process understanding involving sulfuric acid (SA) and water (Kirkby et al, 2011;Chen et al, 2012), it has recently become increasingly clear that the ternary system of SA and water, together with neutralising compounds like ammonia (NH 3 ) or amines, is a key system in NPF (Chen et al, 2012;Almeida et al, 2013;Kurten et al, 2014). Amines can also form stable molecular clusters with methanesulfonic acid (Dawson et al, 2012).…”

“…The importance of amines for NPF has recently been established by field (Angelino et al, 2001;Makela et al, 2001;Smith et al, 2010;Zhao et al, 2011) and laboratory experiments (Murphy et al, 2007;Berndt et al, 2010;Erupe et al, 2011;Bzdek et al, 2010;Zhao et al, 2011;Yu et al, 2012;Zollner et al, 2012;Almeida et al, 2013;Jen et al, 2014;Kurten et al, 2014), as well as from modelling studies (Kurten et al, 2008;Barsanti et al, 2009;Bzdek et al, 2010;Ortega et al, 2012;Loukonen et al, 2010;Nadykto et al, 2011;Paasonen et al, 2012;Almeida et al, 2013;DePalma et al, 2014). On a molecular level, Kurten et al (2014) observed how DMA stabilises SA, increasing the stable cluster concentration by up to 6 orders of magnitude. DMA (and the other MAs) is a stronger base than NH 3 , which is one of the main reasons for its stabilisation efficiency .…”

Bisulfate – cluster based atmospheric pressure chemical ionization mass spectrometer for high-sensitivity (< 100 ppqV) detection of atmospheric dimethyl amine: proof-of-concept and first ambient data from boreal forest

“…In the present experiment we did not tune the instrument specifically for the purpose but we used the settings optimized for dimethyl amine-sulfuric acid nucleation experiments presented in Kürten et al (2014). Therefore, the sensitivity of the instrument could still be improved by improving the APi transmission, especially in the mass range of 300-500 Da.…”

“…This nitrate ion based CI-APi-TOF has been used in many recent laboratory and ambient air studies probing atmospheric chemistry and particle formation (e.g. Almeida et al, 2013;Ehn et al, 2014;Kürten et al, 2014). It has been shown to be highly sensitive toward sulfuric acid, sulfuric acid-amine clusters, and highly oxidized low volatility organics.…”