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

Paasonen P, Olenius T, Kupiainen O, Kurten T, Petaja T, Birmili W, Hamed A, Hu M, Huey L, Plass-Duelmer C, Smith J, Wiedensohler A, Loukonen V, McGrath M, Ortega I, Laaksonen A, Vehkamaki H, Kerminen V, Kulmala M...(+19 more)

ATMOSPHERIC CHEMISTRY AND PHYSICS, vol. 12, issue 19 (2012) pp. 9113-9133 Published by COPERNICUS GESELLSCHAFT MBH

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Sulphuric acid is a key component in atmospheric new particle formation.
However, sulphuric acid alone does not form stable enough clusters to
initiate particle formation in atmospheric conditions. Strong bases,
such as amines, have been suggested to stabilize sulphuric acid clusters
and thus participate in particle formation. We modelled the formation
rate of clusters with two sulphuric acid and two amine molecules
(J(A2B2)) at varying atmospherically relevant conditions with respect to
concentrations of sulphuric acid ({[}H2SO4]), dimethylamine ({[}DMA])
and trimethylamine ({[}TMA]), temperature and relative humidity (RH). We
also tested how the model results change if we assume that the clusters
with two sulphuric acid and two amine molecules would act as seeds for
heterogeneous nucleation of organic vapours (other than amines) with
higher atmospheric concentrations than sulphuric acid. The modelled
formation rates J(A2B2) were functions of sulphuric acid concentration
with close to quadratic dependence, which is in good agreement with
atmospheric observations of the connection between the particle
formation rate and sulphuric acid concentration. The coefficients K-A2B2
connecting the cluster formation rate and sulphuric acid concentrations
as J(A2B2) = K-A2B2{[}H2SO4](2) turned out to depend also on amine
concentrations, temperature and relative humidity. We compared the
modelled coefficients K-A2B2 with the corresponding coefficients
calculated from the atmospheric observations (K-obs) from environments
with varying temperatures and levels of anthropogenic influence. By
taking into account the modelled behaviour of J(A2B2) as a function of
{[}H2SO4], temperature and RH, the atmospheric particle formation rate
was reproduced more closely than with the traditional semi-empirical
formulae based on sulphuric acid concentration only. The formation rates
of clusters with two sulphuric acid and two amine molecules with
different amine compositions (DMA or TMA or one of both) had different
responses to varying meteorological conditions and concentrations of
vapours participating in particle formation. The observed inverse
proportionality of the coefficient K-obs with RH and temperature agreed
best with the modelled coefficient K-A2B2 related to formation of a
cluster with two H2SO4 and one or two TMA molecules, assuming that these
clusters can grow in collisions with abundant organic vapour molecules.
In case this assumption is valid, our results suggest that the formation
rate of clusters with at least two of both sulphuric acid and amine
molecules might be the rate-limiting step for atmospheric particle
formation. More generally, our analysis elucidates the sensitivity of
the atmospheric particle formation rate to meteorological variables and
concentrations of vapours participating in particle formation (also
other than H2SO4).

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