On the formation of sulphuric acid - amine clusters in varying atmospheric conditions and its influence on atmospheric new particle formation

by P Paasonen, T Olenius, O Kupiainen, T Kurten, T Petaja, W Birmili, A Hamed, M Hu, L G Huey, C Plass-Duelmer, J N Smith, A Wiedensohler, V Loukonen, M J McGrath, I K Ortega, A Laaksonen, H Vehkamaki, V -M. Kerminen, M Kulmala show all authors
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Sulphuric acid is a key component in atmospheric new particle formation.\nHowever, sulphuric acid alone does not form stable enough clusters to\ninitiate particle formation in atmospheric conditions. Strong bases,\nsuch as amines, have been suggested to stabilize sulphuric acid clusters\nand thus participate in particle formation. We modelled the formation\nrate of clusters with two sulphuric acid and two amine molecules\n(J(A2B2)) at varying atmospherically relevant conditions with respect to\nconcentrations of sulphuric acid ({[}H2SO4]), dimethylamine ({[}DMA])\nand trimethylamine ({[}TMA]), temperature and relative humidity (RH). We\nalso tested how the model results change if we assume that the clusters\nwith two sulphuric acid and two amine molecules would act as seeds for\nheterogeneous nucleation of organic vapours (other than amines) with\nhigher atmospheric concentrations than sulphuric acid. The modelled\nformation rates J(A2B2) were functions of sulphuric acid concentration\nwith close to quadratic dependence, which is in good agreement with\natmospheric observations of the connection between the particle\nformation rate and sulphuric acid concentration. The coefficients K-A2B2\nconnecting the cluster formation rate and sulphuric acid concentrations\nas J(A2B2) = K-A2B2{[}H2SO4](2) turned out to depend also on amine\nconcentrations, temperature and relative humidity. We compared the\nmodelled coefficients K-A2B2 with the corresponding coefficients\ncalculated from the atmospheric observations (K-obs) from environments\nwith varying temperatures and levels of anthropogenic influence. By\ntaking into account the modelled behaviour of J(A2B2) as a function of\n{[}H2SO4], temperature and RH, the atmospheric particle formation rate\nwas reproduced more closely than with the traditional semi-empirical\nformulae based on sulphuric acid concentration only. The formation rates\nof clusters with two sulphuric acid and two amine molecules with\ndifferent amine compositions (DMA or TMA or one of both) had different\nresponses to varying meteorological conditions and concentrations of\nvapours participating in particle formation. The observed inverse\nproportionality of the coefficient K-obs with RH and temperature agreed\nbest with the modelled coefficient K-A2B2 related to formation of a\ncluster with two H2SO4 and one or two TMA molecules, assuming that these\nclusters can grow in collisions with abundant organic vapour molecules.\nIn case this assumption is valid, our results suggest that the formation\nrate of clusters with at least two of both sulphuric acid and amine\nmolecules might be the rate-limiting step for atmospheric particle\nformation. More generally, our analysis elucidates the sensitivity of\nthe atmospheric particle formation rate to meteorological variables and\nconcentrations of vapours participating in particle formation (also\nother than H2SO4).

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