CCN activity and organic hygroscopicity of aerosols downwind of an urban region in central Amazonia: seasonal and diel variations and impact of anthropogenic emissions

Abstract. During the Observations and Modeling of the Green Ocean Amazon (GoAmazon2014/5) campaign, size-resolved cloud condensation nuclei (CCN) spectra were characterized at a research site (T3) 60 km downwind of the city of Manaus, Brazil, in central Amazonia for 1 year (12 March 2014 to 3 March 2015). Particle hygroscopicity (κ_CCN) and mixing state were derived from the size-resolved CCN spectra, and the hygroscopicity of the organic component of the aerosol (κ_org) was then calculated from κ_CCN and concurrent chemical composition measurements. The annual average κ_CCN increased from 0.13 at 75 nm to 0.17 at 171 nm, and the increase was largely due to an increase in sulfate volume fraction. During both wet and dry seasons, κ_CCN, κ_org, and particle composition under background conditions exhibited essentially no diel variations. The constant κ_org of ∼ 0. 15 is consistent with the largely uniform and high O : C value (∼ 0. 8), indicating that the aerosols under background conditions are dominated by the aged regional aerosol particles consisting of highly oxygenated organic compounds. For air masses strongly influenced by urban pollution and/or local biomass burning, lower values of κ_org and organic O : C atomic ratio were observed during night, due to accumulation of freshly emitted particles, dominated by primary organic aerosol (POA) with low hygroscopicity, within a shallow nocturnal boundary layer. The O : C, κ_org, and κ_CCN increased from the early morning hours and peaked around noon, driven by the formation and aging of secondary organic aerosol (SOA) and dilution of POA emissions into a deeper boundary layer, while the development of the boundary layer, which leads to mixing with aged particles from the residual layer aloft, likely also contributed to the increases. The hygroscopicities associated with individual organic factors, derived from PMF (positive matrix factorization) analysis of AMS (aerosol mass spectrometry) spectra, were estimated through multivariable linear regression. For the SOA factors, the variation of the κ value with O : C agrees well with the linear relationship reported from earlier laboratory studies of SOA hygroscopicity. On the other hand, the variation in O : C of ambient aerosol organics is largely driven by the variation in the volume fractions of POA and SOA factors, which have very different O : C values. As POA factors have hygroscopicity values well below the linear relationship between SOA hygroscopicity and O : C, mixtures with different POA and SOA fractions exhibit a steeper slope for the increase in κ_org with O : C, as observed during this and earlier field studies. This finding helps better understand and reconcile the differences in the relationships between κ_org and O : C observed in laboratory and field studies, therefore providing a basis for improved parameterization in global models, especially in a tropical context.