Comparison of particle number size distribution trends in ground measurements and climate models
Authors: Leinonen Ville , Kokkola Harri , Yli-Juuti Taina , Mielonen Tero , Kühn Thomas , Nieminen Tuomo , Heikkinen Simo , Miinalainen Tuuli , Bergman Tommi , Carslaw Ken , Decesari Stefano , Fiebig Markus , Hussein Tareq , Kivekäs Niku , Krejci Radovan , Kulmala Markku , Leskinen Ari , Massling Andreas , Mihalopoulos Nikos , Mulcahy Jane P. , Noe Steffen M. , van Noije Twan , O'Connor Fiona M. , O'Dowd Colin , Olivie Dirk , Pernov Jakob B. , Petäjä Tuukka , Seland Øyvind , Schulz Michael , Scott Catherine E. , Skov Henrik , Swietlicki Erik , Tuch Thomas , Wiedensohler Alfred , Virtanen Annele , Mikkonen Santtu 
Published: October 2022

Abstract. Despite a large number of studies, out of all drivers of radiative forcing,
the effect of aerosols has the largest uncertainty in global climate model
radiative forcing estimates. There have been studies of aerosol optical
properties in climate models, but the effects of particle number size
distribution need a more thorough inspection. We investigated the trends and
seasonality of particle number concentrations in nucleation, Aitken, and
accumulation modes at 21 measurement sites in Europe and the Arctic. For 13
of those sites, with longer measurement time series, we compared the field
observations with the results from five climate models, namely EC-Earth3,
ECHAM-M7, ECHAM-SALSA, NorESM1.2, and UKESM1. This is the first extensive
comparison of detailed aerosol size distribution trends between in situ
observations from Europe and five earth system models (ESMs). We found that
the trends of particle number concentrations were mostly consistent and
decreasing in both measurements and models. However, for many sites,
climate models showed weaker decreasing trends than the measurements.
Seasonal variability in measured number concentrations, quantified by the
ratio between maximum and minimum monthly number concentration, was
typically stronger at northern measurement sites compared to other
locations. Models had large differences in their seasonal representation,
and they can be roughly divided into two categories: for EC-Earth and
NorESM, the seasonal cycle was relatively similar for all sites, and for
other models the pattern of seasonality varied between northern and southern
sites. In addition, the variability in concentrations across sites varied
between models, some having relatively similar concentrations for all sites,
whereas others showed clear differences in concentrations between remote and
urban sites. To conclude, although all of the model simulations had
identical input data to describe anthropogenic mass emissions, trends in
differently sized particles vary among the models due to assumptions in
emission sizes and differences in how models treat size-dependent aerosol
processes. The inter-model variability was largest in the accumulation mode,
i.e. sizes which have implications for aerosol–cloud interactions. Our
analysis also indicates that between models there is a large variation in
efficiency of long-range transportation of aerosols to remote locations. The differences in model results are most likely due to the more complex effect of different processes instead of one specific feature (e.g. the representation of aerosol or emission size distributions). Hence, a more detailed characterization of microphysical processes and deposition processes affecting the long-range transport is needed to understand the model variability.