Observations of aminium salts in atmospheric nanoparticles and possible climatic implications

Abstract

We present laboratory studies and field observations that explore the role of aminium salt formation in atmospheric nanoparticle growth. These measurements were performed using the Thermal Desorption Chemical Ionization Mass Spectrometer (TDCIMS) and Ultrafine Hygroscopicity Tandem Differential Mobility Analyzers. Laboratory measurements of alkylammonium-carboxylate salt nanoparticles show that these particles exhibit lower volatilities and only slightly lower hygroscopicities than ammonium sulfate nanoparticles. TDCIMS measurements of these aminium salts showed that the protonated amines underwent minimal decomposition during analysis, with detection sensitivities comparable to those of organic and inorganic deprotonated acids. TDCIMS observations made of a new particle formation event in an urban site in Tecamac, Mexico, clearly indicate the presence of protonated amines in 8-10 nm diameter particles accounting for about 47% of detected positive ions; 13 nm particles were hygroscopic with an average 90% RH growth factor of 1.42. Observations of a new particle formation event in a remote forested site in Hyytiälä, Finland, show the presence of aminium ions with deprotonated organic acids; 23% of the detected positive ions during this event are attributed to aminium salts while 10 nm particles had an average 90% RH growth factor of 1.27. Similar TDCIMS observations during events in Atlanta and in the vicinity of Boulder, Colorado, show that aminium salts accounted for 10-35% of detected positive ions. We conclude that aminium salts contribute significantly to nanoparticle growth and must be accounted for in models to accurately predict the impact of new particle formation on climate.

Fig. 1.

Results of laboratory investigations of the volatility and hygroscopicity of aminium salt nanoparticles. MA: methylamine, DMA: dimethylamine, TMA: trimethylamine, AA: acetic acid, PA: propanoic acid. (A) Decrease in particle size versus temperature for 7.5 nm initial particle diameter. (B) Hygroscopic growth factor versus initial particle diameter at 90% RH.

Fig. 2.

Analysis of a new particle formation event commencing at 9:00 on March 21, 2006, in Tecamac, Mexico. (A) Particle size distribution intensity plot. Black lines mark the particle sizes that are analyzed by the TDCIMS. (B) Hygroscopic growth factors obtained at 90% RH for 13 nm ambient particles. (C) TDCIMS positive ion molecular composition for the diameters indicated in (A). Individual bar lengths correspond to the ion molar ratio, defined as the ratio of the average ion abundance for each compound to the total average ion abundance, and their width indicates sampling time. Average uncertainty for each species measurement is ± 25%; refer to Materials and Methods for an uncertainty analysis.

Fig. 3.

Analysis of a new particle formation event commencing at 12:00 on April 9, 2007, in Hyytiälä, Finland. (A) Particle size distribution intensity plot. Black line marks the particle size that is analyzed by the TDCIMS. (B) Hygroscopic growth factors obtained at 90% RH for 10 nm ambient particles. (C) TDCIMS positive ion and (D) negative ion molecular composition for the diameter indicated in (A). Individual bar lengths correspond to the ion molar ratio, defined as the ratio of the average ion abundance for each compound to the total average ion abundance, and their width indicates sampling time. Average uncertainty for each species measurement is ± 25%; refer to Materials and Methods for an uncertainty analysis.

Fig. 4.

Overview of the molar ratio of aminium salt formation during nanoparticle growth events, grouped according the land type, for representative events in which the TDCIMS performed measurements. Error bars indicate standard deviation of the mean.