Abstract. The concentration of cloud condensation nuclei (CCN) is an essential
parameter affecting aerosol–cloud interactions within warm clouds. Long-term
CCN number concentration (NCCN) data are scarce; there are a lot more
data on aerosol optical properties (AOPs). It is therefore valuable to
derive parameterizations for estimating NCCN from AOP measurements. Such
parameterizations have already been made, and in the present work a new parameterization
is presented. The relationships between NCCN, AOPs, and size
distributions were investigated based on in situ measurement data from six
stations in very different environments around the world. The relationships
were used for deriving a parameterization that depends on the scattering
Ångström exponent (SAE), backscatter fraction (BSF), and total
scattering coefficient (σsp) of PM10 particles. The analysis
first showed that the dependence of NCCN on supersaturation (SS) can be
described by a logarithmic fit in the range SS <1.1 %, without
any theoretical reasoning. The relationship between NCCN and AOPs was
parameterized as NCCN≈((286±46)SAE ln(SS/(0.093±0.006))(BSF − BSFmin) + (5.2±3.3))σsp, where BSFmin is the minimum BSF, in practice
the 1st percentile of BSF data at a site to be analyzed. At the lowest
supersaturations of each site (SS ≈0.1 %), the average bias,
defined as the ratio of the AOP-derived and measured NCCN, varied from
∼0.7 to ∼1.9 at most sites except at a
Himalayan site where the bias was >4. At SS >0.4 % the average bias ranged from ∼0.7 to ∼1.3 at most sites. For the marine-aerosol-dominated site Ascension Island the bias was
higher, ∼1.4–1.9. In other words, at SS >0.4 % NCCN was estimated with an average uncertainty of approximately
30 % by using nephelometer data. The biases were mainly due to the biases
in the parameterization related to the scattering Ångström exponent
(SAE). The squared correlation coefficients between the AOP-derived and
measured NCCN varied from ∼0.5 to ∼0.8. To
study the physical explanation of the relationships between NCCN and
AOPs, lognormal unimodal particle size distributions were generated and
NCCN and AOPs were calculated. The simulation showed that the
relationships of NCCN and AOPs are affected by the geometric mean
diameter and width of the size distribution and the activation diameter. The
relationships of NCCN and AOPs were similar to those of the observed
ones.
Supplementary material to "Aerosol optical properties calculated from size distributions, filter samples and absorption photometer data at Dome C, Antarctica and their relationships between seasonal cycles of sources"
