Bryan R. Bzdek

Nanoparticle Chemical Composition During New Particle Formation

The nano aerosol mass spectrometer (NAMS) was deployed at a coastal site in Lewes, Delaware, to measure the composition of 21 nm mass normalized (18 nm mobility) diameter nanoparticles during new particle formation (NPF) events. NAMS provides a quantitative measure of the atomic composition of individual nanoparticles. NAMS analysis of ambient particles showed only a small change in particle composition during NPF events in Lewes compared with off-event (before and/or after the event). The N mole fraction increased 15% on-event, whereas the C mole fraction decreased 25%, suggesting an enhanced inorganic component to the aerosol during NPF. The measured changes in atomic composition constrain the possible changes in molecular composition. To explore these constraints, an apportionment algorithm was applied to the atomic composition data. This algorithm partitions the atomic composition into sulfate, nitrate, and ammonium on the basis of the atomic abundance of S, N, and O and into organic matter on the basis of C and residual O after removing contributions to inorganic species. Particles were fully neutralized both on- and off-event. The nitrate to sulfate ratio during NPF ranged from 0.7 to 1.4, suggesting that ammonium nitrate is important to particle growth in this environment. Nonetheless, nanoparticles had a significant organic fraction, and upper limits for cationic amine content were determined. The relatively small changes in total particle composition on-event versus off-event suggest that observed changes in particle hygroscopicity and volatility during NPF at other locations may be linked to subtle changes in particle composition or to shifts in the character of the organic content.

Reactivity of methanesulfonic acid salt clusters relevant to marine air

[1]xa0Aliphatic amines and methanesulfonic acid (MSA) are important emissions in the marine environment. Although some studies of marine aerosols have shown that particles containing MSA can be internally mixed with ammonium, others have indicated a significant amine component. The present work employs Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS) to determine the kinetics and thermodynamics of exchange of dimethylamine (DMA) for ammonia in ammonium methanesulfonate clusters. DMA displaces ammonia with near unit reaction efficiency. The Gibbs free energy for exchange (ΔG°) was <−23.1 kJ mol−1, strongly suggesting that dimethylaminium methanesulfonate salts are preferred over ammonium methanesulfonate salts. These results are consistent with previous studies of amine exchange in ammonium bisulfate and ammonium nitrate clusters. Additionally, cluster growth, characterized by the addition of a base to neutralize remaining acid in the cluster, was examined. Methanesulfonate clusters grew more effectively by addition of a DMA molecule than by addition of an ammonia molecule, indicating that in some environments, particle growth by amines may compete favorably with growth by ammonia at small particle size. These results also suggest that in diverse environments the presence of aminium salts is preferred over the presence of ammonium salts in sub-3 nm diameter clusters.

Nanoparticle chemical composition and diurnal dependence at the CalNex Los Angeles ground site

[1]xa0The Nano Aerosol Mass Spectrometer (NAMS) was deployed to the California Nexus Los Angeles ground site in Pasadena, California during May–June 2010 to study nanoparticles in the 20–25 nm size range. NAMS gives a quantitative measure of the elemental composition of individual particles, and molecular apportionment of the elemental data allows the O/C mole ratio of carbonaceous matter in each particle to be determined. Abrupt increases in nanoparticle number concentration were observed in the afternoon on sunny days, and coincided with a shift in the wind direction from the southeast to the southwest. Nanoparticles analyzed during these time periods were found to contain enhanced levels of sulfur and silicon relative to particles analyzed earlier in the day, and the O/C ratios of carbonaceous matter changed from a distribution dominated by primary motor vehicle emissions (O/C ratio < 0.25) to one dominated by “fresh” secondary organic aerosol (O/C ratio between 0.25 and 0.65). The wind direction and chemical composition dependencies suggest that the afternoon increase in number concentration originated from motor vehicle emissions in the downtown Los Angeles area that were photochemically processed during transport to the measurement site. It is likely that photochemical processing led to both a change in the composition of preexisting particles and to the formation of new particles.

Molecular constraints on particle growth during new particle formation

Atmospheric new particle formation (NPF) produces large numbers of nanoparticles which can ultimately impact climate. A firm understanding of the identity and contribution of the inorganic and carbonaceous species to nanoparticle growth is required to assess the climatic importance of NPF. Here, we combine elemental and molecular nanoparticle composition measurements to better define the composition and contribution of carbonaceous matter to nanoparticle growth in a rural/coastal environment. We show that carbonaceous matter can account for more than half of the mass growth of nanoparticles and its composition is consistent with that expected for extremely low volatility organic compounds. An important novel finding is that the carbonaceous matter must contain a substantial amount of nitrogen, whose molecular identity is not fully understood. The results advance our quantitative understanding of the composition and contribution of carbonaceous matter to nanoparticle growth, which is essential to more accurately predict the climatic impacts of NPF.