Abstract
Abstract. Large-scale measurements of cloud condensation nuclei (CCN) are difficult to obtain on a routine basis, whereas aerosol optical quantities are more readily available. This study investigates the relationship between CCN and aerosol optical quantities for some distinct aerosol types using extensive observational data collected at multiple Atmospheric Radiation Measurement (ARM) Climate Research Facility (CRF) sites around the world. The influences of relative humidity (RH), aerosol hygroscopicity (fRH) and single scattering albedo (SSA) on the relationship are analyzed. Better relationships are found between aerosol optical depth (AOD) and CCN at the Southern Great Plains (US), Ganges Valley (India) and Black Forest sites (Germany) than those at the Graciosa Island (the Azores) and Niamey (Niger) sites, where sea salt and dust aerosols dominate, respectively. In general, the correlation between AOD and CCN decreases as the wavelength of the AOD measurement increases, suggesting that AOD at a shorter wavelength is a better proxy for CCN. The correlation is significantly improved if aerosol index (AI) is used together with AOD. The highest correlation exists between CCN and aerosol scattering coefficients (σsp) and scattering AI measured in situ. The CCN–AOD (AI) relationship deteriorates with increasing RH. If RH exceeds 75%, the relationship where AOD is used as a proxy for CCN becomes invalid, whereas a tight σsp–CCN relationship exists for dry particles. Aerosol hygroscopicity has a weak impact on the σsp–CCN relationship. Particles with low SSA are generally associated with higher CCN concentrations, suggesting that SSA affects the relationship between CCN concentration and aerosol optical quantities. It may thus be used as a constraint to reduce uncertainties in the relationship. A significant increase in σsp and decrease in CCN with increasing SSA is observed, leading to a significant decrease in their ratio (CCN / σsp) with increasing SSA. Parameterized relationships are developed for estimating CCN, which account for RH, particle size, and SSA.
Highlights
Aerosols play important roles in Earth’s climate and the hydrological cycle via their direct and indirect effects (IPCC, 2007)
cloud condensation nuclei (CCN) generally increases with condensation nuclei (CN) during dust events, but the ratio of CCN to CN tends to decrease sharply with increasing CN, implying that less CCN become available under dusty conditions (Liu et al, 2011)
Based on extensive measurements of aerosol optical quantities and CCN number concentrations made at different Atmospheric Radiation Measurement (ARM) Climate Research Facility sites, the relationships between aerosol optical quantities, including columnar aerosol optical depth (AOD), surface-measured aerosol scattering parameters, and CCN concentrations are assessed
Summary
Aerosols play important roles in Earth’s climate and the hydrological cycle via their direct and indirect effects (IPCC, 2007). They can influence warmand cold-rain processes (Rosenfeld and Woodley, 2000; Andreae et al, 2004; Lin et al, 2006; Bell et al, 2008; Li et al, 2011), the depth of the mixed-phase region in a cloud (Andreae et al, 2004; Koren et al, 2005, 2008, 2010; Niu and Li, 2012) and the occurrence of lightning (Orville et al, 2001; Steiger and Orville, 2003; Yuan et al, 2011; Yang et al, 2013) These are known as aerosol’s indirect effects, which are the largest sources of uncertainty in the forcing of Earth’s climate system. Determining CCN (cloud condensation nuclei) concentrations and their spatial and temporal variations are key challenges in quantifying aerosol’s indirect effects
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