Estimation of cloud condensation nuclei concentration from aerosol optical quantities: influential factors and uncertainties

摘要

Large-scale measurements of cloud condensation nuclei (CCN) are difficult toobtain on a routine basis, whereas aerosol optical quantities are morereadily available. This study investigates the relationship between CCN andaerosol optical quantities for some distinct aerosol types using extensiveobservational data collected at multiple Atmospheric Radiation Measurement(ARM) Climate Research Facility (CRF) sites around the world. The influencesof relative humidity (RH), aerosol hygroscopicity () and singlescattering albedo (SSA) on the relationship are analyzed. Betterrelationships are found between aerosol optical depth (AOD) and CCN at theSouthern 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 AODmeasurement increases, suggesting that AOD at a shorter wavelength is abetter proxy for CCN. The correlation is significantly improved if aerosolindex (AI) is used together with AOD. The highest correlation exists betweenCCN and aerosol scattering coefficients (σ) and scattering AImeasured in situ. The CCN–AOD (AI) relationship deteriorates with increasingRH. If RH exceeds 75%, the relationship where AOD is used as a proxy forCCN becomes invalid, whereas a tight σ–CCN relationshipexists for dry particles. Aerosol hygroscopicity has a weak impact on theσ–CCN relationship. Particles with low SSA are generallyassociated with higher CCN concentrations, suggesting that SSA affects therelationship between CCN concentration and aerosol optical quantities. Itmay thus be used as a constraint to reduce uncertainties in therelationship. A significant increase in σ and decrease in CCNwith increasing SSA is observed, leading to a significant decrease in theirratio (CCN / σ) with increasing SSA. Parameterizedrelationships are developed for estimating CCN, which account for RH,particle size, and SSA.