Aerosol hygroscopicity over the South-East Atlantic Ocean during the biomass burning season: Part I – From the perspective of scattering enhancement

Abstract

Abstract. Aerosol hygroscopicity plays a vital role in aerosol radiative forcing. One key parameter describing hygroscopicity is the scattering enhancement factor, f (RH), defined as the ratio of the scattering coefficient at humidified relative humidity (RH) to its dry value. Here, we utilize the f (80 %) from ORACLES 2016 and 2018 airborne measurements to investigate the hygroscopicity of aerosols, its vertical distribution, its relationship with chemical composition, and its sensitivity to organic aerosol (OA) hygroscopicity over the South-East Atlantic (SEA) Ocean during the biomass burning (BB) season. We found that aerosol hygroscopicity remains steady above 2 km, with a mean f (80 %) of 1.40±0.17. Below 2 km, aerosol hygroscopicity increases with decreasing altitude, with a mean f (80 %) of 1.51±0.22, consistent with higher values of BB hygroscopicity found in the literature. The hygroscopicity parameter of OA (κOA) is retrieved from the Mie model with a mean value of 0.11±0.08, which is in the middle to upper range compared to literature. Higher OA hygroscopicity is related to aerosols that are more aged, oxidized, and present at lower altitudes. The enhanced BBA hygroscopicity at lower altitudes is mainly due to a lower OA fraction, increased sulphate fraction, and greater κOA at lower altitudes. We propose a parameterization that quantifies f (RH) with chemical composition and κOA based on Mie simulation of internally mixed OA-(NH4)2SO4-BC mixture. The good agreement between the predictions and the ORACLES measurements implies that the aerosols in the SEA during the BB season can be largely represented by the OA-(NH4)2SO4-BC internal mixture with respect to the f (RH) prediction. The sensitivity of f (RH) to κOA indicates that applying a constant κOA is only suitable when the OA fraction is low and κOA shows limited variation. However, in situations deviating these two criteria, κOA can notably impact scattering coefficients and aerosol radiative effect; therefore, accounting for κOA variability is recommended.

Prominent role of organics in aerosol liquid water content over the south-eastern Atlantic during biomass burning season

“…This good linear relationship between κOA and ALWCOA/ALWC reveals the significant impact of OA hygroscopicity on ALWC over the SEA region during BB episode. The similar PDF distribution of κOA in both campaigns (Zhang et al, 2023) also explains the similar ALWCOA contribution (Fig. 2).…”

“…It can be considered as the scattering coefficient weighted average κ (Kuang et al, 2021). The detailed procedure of κf(RH) retrieval can be found in Zhang et al (2023). In Section 3.3 of Kuang et al (2020b), the authors meticulously demonstrated that κf(RH) can accurately represent the κchem of PM1 (particulate matter with an aerodynamic diameter less than 1 μm).…”

“…The volumes of OA and BC are converted from the mass of OA from AMS and that of BC from SP2. The densities of the inorganics, OA, and BC are given in Table S1 in Zhang et al (2023). ρw is water density, taken as 1 g cm -3 .…”

“…where κOA represents the hygroscopicity parameter of OA. Its calculation is referred to Zhang et al (2023). It is worthnoting that the κOA is the bulk κ of OA in PM1.…”