Correlation between cloud condensation nuclei concentration and aerosol optical thickness in remote and polluted regions

Abstract

Abstract.A large number of published and unpublished measurements of cloud condensation nuclei (CCN) concentrations and aerosol optical thickness (AOT) measurements have been analyzed. AOT measurements were obtained mostly from the AERONET network, and selected to be collocated as closely as possible to the CCN investigations. In remote marine regions, CCN 0.4 (CCN at a supersaturation of 0.4%) are around 110 cm −3 and the mean AOT 500 (AOT at 500 nm) is 0.057. Over remote continental areas, CCN are almost twice as abundant, while the mean AOT 500 is ca. 0.075. (Sites dominated by desert dust plumes were excluded from this analysis.) Some, or maybe even most of this difference must be because even remote continental sites are in closer proximity to pollution sources than remote marine sites. This suggests that the difference between marine and continental levels must have been smaller before the advent of anthropogenic pollution.Over polluted marine and continental regions, the CCN concentrations are about one order of magnitude higher than over their remote counterparts, while AOT is about five times higher over polluted than over clean regions. The average CCN concentrations from all studies show a remarkable correlation to the corresponding AOT values, which can be expressed as a power law. This can be very useful for the parameterization of CCN concentrations in modeling studies, as it provides an easily measured proxy for this variable, which is difficult to measure directly. It also implies that, at least at large scales, the radiative and microphysical effects of aerosols on cloud physics are correlated and not free to vary fully independently. While the observed strong empirical correlation is remarkable, it must still be noted that thereis about a factor-of-four range of CCN concentrations at a given AOT, and that there remains considerable room for improvement in remote sensing techniques for CCN abundance.

Internally Mixed Sea Salt, Soot, and Sulfates at Macao, a Coastal City in South China

“…[1][2][3] These changes can affect the role of aerosols in the regional climate through the scattering and absorption of solar radiation (direct radiative forcing) or by acting as cloud condensation nuclei (CCN) (indirect radiative forcing). [4][5][6] In order to accurately account for the effects of aerosols on these processes, their detailed species and corresponding chemical and physical properties must be investigated. It is well known that concentrations and properties of aerosols are quite different in different regions corresponding to IMPLICATIONS This study of compositional changes in mixed marine aerosols and anthropogenic pollutants underscores the importance of aerosol mixing states with regard to their role in visibility, climate, as well as health inhalation.…”

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

“…from satellite observations where aerosol optical properties are compared to cloud radiative properties. This practice is inaccurate, mainly because of the poor ability to convert the aerosol optical depth (AOD) into concentrations of cloud condensation nuclei (CCN) that actually interact with the clouds (Andreae, 2009). The inaccuracies are caused by the fact that aerosols are not measured at the same height as the cloud base, cloud contamination (i.e., clouds enhancing the aerosol signals), and aerosol swelling in the moist environment near the clouds (Koren et al, 2007).…”

“…This difference (Table 2) is widely attributed to the prevalence of aerosol sources over land (by smoke from fire, air pollution, emissions from vegetation, and lofting of mineral dust and by volcanic eruptions), and the weaker aerosol sources over the sea. The general contrast in CCN concentrations between polluted and clean regions is an order of magnitude (Andreae, 2009). However, the land/ocean contrast in cloud base height is as certain as the contrast between mostly polluted land and clean ocean in CCN (Williams and Satori, 2004;Williams et al, 2005), while the cloud base height is entirely independent of CCN concentrations.…”

What do we learn from long-term cloud condensation nuclei number concentration, particle number size distribution, and chemical composition measurements at regionally representative observatories?

“…Furthermore, these correlations become less reliable when sea salt and mineral dust constitute an important fraction of the particle number, a situation which can be relevant over the ocean or deserts (Liu and Li, 2014). This makes in-situ measurements indispensable 30 and therefore numerous studies of CCN activity have been carried out in a variety of environments, ranging from remote marine over continental background to urban locations, and in the laboratory (e.g., Andreae, 2009a;Asmi et al, 2012; Atmos. Chem.…”

“…A recent study shows that a combination of liquid-liquid phase separation, surfactants and specific particle size distributions could, however, increase the CCN number 5 concentration by a factor of ten compared to climate model predictions (Ovadnevaite et al, 2017). More generally, the importance of a detailed knowledge of the particle chemical composition for CCN activity depends on the distance from the source as more aged particles tend to assume similar particle number size distributions and hygroscopic characteristics (e.g., Andreae, 2009b;Ervens et al, 2010). …”