Aerosol nucleation and its role for clouds and Earth's radiative forcing in the aerosol-climate model ECHAM5-HAM

Abstract

Nucleation from the gas phase is an important source of aerosol particles in the Earth's atmosphere, contributing to the number of cloud condensation nuclei, which form cloud droplets. We have implemented in the aerosol-climate model ECHAM5-HAM a new scheme for neutral and charged nucleation of sulfuric acid and water based on laboratory data, and nucleation of an organic compound and sulfuric acid using a parametrization of cluster activation based on field measurements. We give details of the implementation , compare results with observations, and investigate the role of the individual aerosol nucleation mechanisms for clouds and the Earth's radiative forcing. The results of our simulations are most consistent with observations when neutral and charged nucleation of sulfuric acid proceed throughout the troposphere and nucleation due to cluster activation is limited to the forested boundary layer. The globally averaged annual mean contributions of the individual nucleation processes to total absorbed solar shortwave radiation via the direct, semi-direct, indirect cloud-albedo and cloud-lifetime effects in our simulations are −1.15 W/m 2 for charged H 2 SO 4 /H 2 O nucleation, −0.235 W/m 2 for cluster activation, and −0.05 W/m 2 for neutral H 2 SO 4 /H 2 O nucle-ation. The overall effect of nucleation is −2.55 W/m 2 , which exceeds the sum of the individual terms due to feedbacks and interactions in the model. Aerosol nucleation contributes over the oceans with −2.18 W/m 2 to total absorbed solar shortwave radiation, compared to −0.37 W/m 2 over land. Correspondence to: J. Kazil (jan.kazil@noaa.gov) We explain the higher effect of aerosol nucleation on Earth's radiative forcing over the oceans with the larger area covered by ocean clouds, due to the larger contrast in albedo between clouds and the ocean surface compared to continents, and the larger susceptibility of pristine clouds owing to the saturation of effects. The large effect of charged nucleation in our simulations is not in contradiction with small effects seen in local measurements: over southern Finland, where cluster activation proceeds efficiently, we find that charged nucleation of sulfuric acid and water contributes on average less than 10% to ultrafine aerosol concentrations, in good agreement with observations.

Simulating Particle Size Distributions over California and Impact on Lung Deposition Fraction

“…Also, despite significant ammonia concentrations in SJV, results of Zhang et al (2010) indicate that particle number concentrations in California based on CMAQ simulations using a recent ternary nucleation parameterization (NH 3 −H 2 SO 4 −H 2 O; Merikanto et al 2007Merikanto et al , 2009) are similar to those with the binary schemes just mentioned. For conditions where nucleation has a significant impact on particle number concentrations, the growth of fresh nuclei should be treated either by analytical estimates (Kerminen and Kulmala 2002;Lehtinen et al 2007) or by explicitly simulating a nucleation mode (Sartelet et al 2006;Kazil et al 2010).…”

On the composition of ammonia–sulfuric-acid ion clusters during aerosol particle formation

“…An important source of atmospheric aerosol particles is the formation of molecular clusters from gas-phase precursors (vapors) and their subsequent growth to larger sizes by vapor condensation and other processes. Such new-particle formation gives a potentially large contribution to regional and even global cloud condensation nuclei (CCN) populations (Merikanto et al, 2009;Lee et al, 2013), thereby affecting aerosol-cloud interactions and ultimately climate (Kazil et al, 2010;Makkonen et al, 2012;Ghan et al, 2013). However, the very first steps of the atmospheric new-particle formation process are still poorly understood and a subject of ongoing research .…”

Unexpectedly acidic nanoparticles formed in dimethylamine–ammonia–sulfuric-acid nucleation experiments at CLOUD

“…The mechanisms and chemical species responsible for NPF have been the subject of many recent studies, with the motivation of understanding the impacts of these processes on air quality and climate. NPF is an important source of cloud condensation nuclei (CCN) and therefore may significantly affect the global radiative energy balance (Wang and Penner, 2009;Kazil et al, 2010), but the mechanisms and chemical species driving NPF are still poorly understood (e.g., Riipinen et al, 2012). Sulfuric acid is widely accepted as one of the most important species for particle formation in the atmosphere (e.g., Kirkby et al, 2011;Kuang et al, 2008;Kulmala et al, 2007;Weber et al, 1997).…”