Aerosol- and updraft-limited regimes of cloud droplet formation: influence of particle number, size and hygroscopicity on the activation of cloud condensation nuclei (CCN)

Abstract

Abstract.We have investigated the formation of cloud droplets under pyro-convective conditions using a cloud parcel model with detailed spectral microphysics and with the κ-Köhler model approach for efficient and realistic description of the cloud condensation nucleus (CCN) activity of aerosol particles. Assuming a typical biomass burning aerosol size distribution (accumulation mode centred at 120 nm), we have calculated initial cloud droplet number concentrations (N CD ) for a wide range of updraft velocities (w=0.25-20 m s −1 ) and aerosol particle number concentrations (N CN =200-10 5 cm −3 ) at the cloud base. Depending on the ratio between updraft velocity and particle number concentration (w/N CN ), we found three distinctly different regimes of CCN activation and cloud droplet formation:(1) An aerosol-limited regime that is characterized by high w/N CN ratios (>≈10 −3 m s −1 cm 3 ), high maximum values of water vapour supersaturation (S max >≈0.5%), and high activated fractions of aerosol particles (N CD /N CN >≈90%). In this regime N CD is directly proportional to N CN and practically independent of w.(2) An updraft-limited regime that is characterized by low w/N CN ratios (<≈10 −4 m s −1 cm 3 ), low maximum values of water vapour supersaturation (S max <≈0.2%), and low activated fractions of aerosol particles (N CD /N CN <≈20%). In this regime N CD is directly proportional to w and practically independent of N CN .Correspondence to: H. Su (hsu@mpch-mainz.mpg.de) (3) An aerosol-and updraft-sensitive regime (transitional regime), which is characterized by parameter values in between the two other regimes and covers most of the conditions relevant for pyro-convection. In this regime N CD depends non-linearly on both N CN and w.In sensitivity studies we have tested the influence of aerosol particle size distribution and hygroscopicity on N CD . Within the range of effective hygroscopicity parameters that is characteristic for continental atmospheric aerosols (κ≈0.05-0.6), we found that N CD depends rather weakly on the actual value of κ. A compensation of changes in κ and S max leads to an effective buffering of N CD . Only for aerosols with very low hygroscopicity (κ<0.05) and also in the updraft-limited regime for aerosols with higher than average hygroscopicity (κ>0.3) did the relative sensitivities ∂lnN CD /∂lnκ≈ ( N CD /N CD )/( κ/κ) exceed values of ∼0.2, indicating that a 50% difference in κ would change N CD by more than 10%.The influence of changing size distribution parameters was stronger than that of particle hygroscopicity. Nevertheless, similar regimes of CCN activation were observed in simulations with varying types of size distributions (polluted and pristine continental and marine aerosols with different proportions of nucleation, Aitken, accumulation, and coarse mode particles). In general, the different regimes can be discriminated with regard to the relative sensitivities of N CD against w and N CN (∂lnN CD /∂lnw and ∂lnN CD /∂lnN CN ). We propose to separate the different regimes by rel...

Assessment of cloud supersaturation by size-resolved aerosol particle and cloud condensation nuclei (CCN) measurements

“…Particles with a critical supersaturation equal to or lower than the peak supersaturation are activated and grow into cloud droplets. The peak supersaturation is a major determinant for the cloud droplet number and the regime of CCN activation (aerosol-vs. updraft limited; Reutter et al, 2009;Rosenfeld et al, 2012). Due to inhomogeneities of the atmospheric aerosol load and air flow pattern (turbulence, entrainment), the peak supersaturations of different air parcels in a cloud can be temporally and spatially heterogeneous.…”

“…To fully describe the process of CCN activation and cloud droplet growth in the atmosphere, however, the supersaturation of water vapor in the cloud also needs to be known (e.g., Reutter et al, 2009;Pruppacher and Klett, 2010;Rosenfeld et al, 2012;Renno et al, 2013).…”

The scientific basis for a satellite mission to retrieve CCN concentrations and their impacts on convective clouds

“…With state-of-the-art global atmospheric chemistry and transport models, the average deviation between predicted and measured κ values and CCN concentrations can usually be kept below ∼30 % (e.g., Pringle et al, 2010;Spracklen et al, 2011). An error of 30 % in κ leads to an error of only 10 % in the critical particle diameter of CCN activation (Kreidenweis et al, 2009;Pöschl et al, 2009), and the actual influence of κ on cloud droplet number is even smaller because of compensation effects between updraft velocity, aerosol hygroscopicity and water vapor supersaturation (Reutter et al, 2009). Field measurement data confirm that the prediction of CCN concentration depends much more strongly on the variability of aerosol particle concentration and size than on the variability of κ (e.g., Gunthe et al, 2009;Rose et al, 2010Rose et al, , 2011.…”

“…For this purpose, the CCN(S) spectra obtained from the CHASER measurements of the concentration of activated CCN as a function of the water vapor supersaturation, N a (S), are considered to represent the CCN concentrations as a function of particle size (d) and composition (Reutter et al, 2009). Figure 7 illustrates the relation between S and N a calculated as a function of the updraft speed near the cloud base (w b ), which can be described by Eq.…”

Cloud droplet size distribution broadening during diffusional growth: ripening amplified by deactivation and reactivation

“…Previous studies show that aerosol number concentration and vertical velocity are the two most important factors controlling cloud properties in an adiabatic cloud parcel model (e.g., McFiggans et al, 2006;Reutter et al, 2009;Chen et al, 2018). Two regimes are frequently considered: an aerosol-limited regime exists when there is an ample supply of water, and the cloud droplet number concentration is limited by the aerosol number concentration; and an updraftlimited regime exists when supersaturation is starved, and the cloud droplet number concentration is limited by the updraft velocity.…”