Towards the ambient aerosol extinction from dried aerosol in situ observations.

Accurate predictions of the extinction and scattering properties of the atmosphere are important for climate research and interpreting satellite data. This study introduces a model (called the H-model) that calculates the scattering coefficients and scattering enhancement factors based on in situ measurements of the dried ambient aerosol. A disadvantage of using dried aerosol measurements is that they do not correspond with the ambient conditions, as they are measured at a relative humidity below 40% and thus the particles are assumed to contain no water. Measurements of aerosol chemical composition do not contain water mass concentrations and measurements of the particle size distribution do not include water. To solve this problem, the H-model uses ISORROPIA, a thermodynamic equilibrium model, to estimate the expected amount of aerosol water content and growth factor g(RH) of aerosol particles for any given temperature and relative humidity (RH). With this information, the conversion between dry and enhanced relative humidity can be made. The chemical composition measurements can be complemented with the estimated aerosol water concentrations and the particle size distribution can be recalculated based on the growth factor for any given RH. In addition, the growth factor is also calculated by using k-Kohler theory and compared to the results of ISORROPIA. The findings of this sub-study show that the growth factors calculated by both approaches (ISORROPIA and k-Kohler theory) are similar as they significantly correlate. ISORROPIA, however, is more sensitive to small chemical changes which makes it more appropriate for the H-model. The calculated growth factors are used in the H-model to estimate changes in the chemical composition and particle size distribution of the aerosol particles at enhanced relative humidity. Subsequently, the H-model uses MIE theory to estimate the scattering properties of the particles at a specific relative humidity. By doing so, the scattering properties can be calculated at dry and enhanced RH, making it possible to calculate scattering enhancement factors. Finally, the H-model is validated by comparing the calculated scattering properties to measured scattering properties of a (humidified) nephelometer. To do so, in situ measurements from the CINDI campaign in 2009 and the TROLIX campaign in 2019 at Cabauw are used. The findings of this validation show that the results from the H-model do not yet accurately match the measurements. That being said, a strong correlation is observed between the calculated and the measured scattering properties. This shows that the H-model is able to capture changes in the particle size distribution and chemical composition while calculating the enhancement factors. It can be concluded that the results from the H-model are promising but need further work to close the gap between the calculations and measurements. The H-model makes multiple simplifications and assumptions which could be improved upon, thereby increasing the precision of the results as well. Furthermore, to fully conclude the findings of this study, the measurements of the SMPS and the nephelometers should be calibrated. A better statement can then be made about the accuracy of the comparison between the scattering properties calculated by the H-model and measured by the nephelometers.