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 one year (12 March 2014 to 3 March 2015). Particle hygroscopicity (κCCN) and mixing state were derived from the size-resolved CCN spectra, and the hygroscopicity of the organic component of the aerosol (κorg) was then calculated from κCCN and concurrent chemical composition measurements. The annual average κCCN 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, κCCN, κorg, and particle composition under background conditions exhibited essentially no diel variations. The constant κorg 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 κorg 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, κorg, and κCCN 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 analysis of AMS spectra, were estimated through multi-variable 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 of κorg with O : C, as observed during this and earlier field studies. This finding helps better understand and reconcile the differences in the relationships between κorg and O : C observed in laboratory and field studies, therefore providing a basis for improved parameterization in global models, especially in a tropical context

Cloud droplet activation of secondary organic aerosol is mainly controlled by molecular weight, not water solubility

“…Whereas the variation of precursor molecule size and fragmentation during oxidation described above explain the general trend of increasing κ org with the oxidation level, laboratory 15 SOA generated from a wide range of precursors with different vapor pressures, molecular weights and structures, exhibit nearly the same relationship between κ org and O:C (Lambe et al, 2011). During the recent GoAmazon 2014/5 field campaign, κ org of various SOA components was derived and the variation of κ org with O:C also follows a similar relationship to that observed in laboratory studies (Thalman et al, 2017). While such a "universal" relationship allows 20 efficient parameterization of κ org based on O:C, it also raises a question why such a "universal" relationship is applicable to SOA generated from a range of precursors and under different conditions.…”

“…Despite the four orders of magnitude difference in C*, the derived variation of κ org with O:C falls within a relatively narrow band. Figure 5 also shows the linear relationship between κ org with O:C derived by Lambe et al (2011) for laboratory generated SOA with a wide range of O:C values, SOA hygroscopicity derived from 20 this study, and hygroscopicity of SOA factors derived from the measurements during recent GoAmazon 2014/5 campaign (Thalman et al, 2017). For β-caryophyllene SOA, κ org shown in Fig.…”

“…Predicted variation of κ org as a function of O:C for organics with mean logC* ranging from -1 to -5. Also shown are κ org of SOA Positive Matrix Factorization (PMF) factors derived from measurements during GoAmazon 2014/5 (Thalman et al, 2017), κ org for SOA formed from isoprene, monoterpene, β-caryophyllene, and toluene in this study, and the relationship between 5 κ org and O:C for SOA derived from a laboratory study (Lambe et al, 2011). The shaded area represents the uncertainty in the derived relationship (Lambe et al, 2011).…”

Aerosol characteristics and particle production in the upper troposphere over the Amazon Basin

“…Our AMS measurements (see Sect. 3.4.4) showed that the UT secondary organic aerosol (SOA) contains a substantial fraction of organics derived from the oxidation of isoprene (IEPOX-SOA) , which has relatively high hygroscopicity (κ ≥ 0.12) (Engelhart et al, 2011;Thalman et al, 2017). Assuming a conservative value of κ org ∼ = 0.1, which had been found previously for the Amazon PBL (Gunthe et al, 2009;Pöhlker et al, 2016), pure SOA particles would have to have diameters of ≥80 nm to act as CCN at 0.52 % supersaturation, whereas for pure ammonium sulfate particles (κ ∼ = 0.6), the critical diameter would be about 45 nm (Petters and Kreidenweis, 2007).…”

Understanding of atmospheric aerosol particles with improved particle identification and quantification by single particle mass spectrometry

“…According to our observation, these particles are expected to show a significant hygroscopicity due to their secondary inorganic contents (Fuzzi et al, 2015), as well as the presence of organosulfates (Thalman et al, 2017). The light absorption of soot particles might be enhanced by mixing with non-absorbing species such as most of the organic compounds that can reflect the light to the soot part (Bond et al, 2013).…”