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

Abstract. During the Observations and Modeling of the Green Ocean Amazon (GoAmazon2014/5) campaign, sizeresolved 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 deeperPublished by Copernicus Publications on behalf of the European Geosciences Union. 11780 R. Thalman et al.: CCN activity and organic hygroscopicity of aerosols downwind of an urban region 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 parameterizati...

Urban influence on the concentration and composition of submicron particulate matter in central Amazonia

“…The highest individual correlation was with the ADOA factor loading (Adam Bateman, personal communication, 2017). In addition, Thalman et al (2017), also using the PMF results reported herein, concluded that the larger relative contribution of secondary organic material during the daytime compared with the nighttime was the primary driver of the diel trend of higher particle hygroscopicity during the day compared to the night, as tied to the properties of cloud condensation nuclei.…”

Overview: Precipitation characteristics and sensitivities to environmental conditions during GoAmazon2014/5 and ACRIDICON-CHUVA

“…at the T3 site (main GoAmazon site at Manacapuru; see Martin et al, 2016, for a detailed description). The data were averaged to 5 min intervals covering the period from January 2014 to March 2015 (Thalman et al, 2017). Background or polluted conditions were defined based on the specific CPC distribution for each season by the thresh-old value associated with the 33 and 66 % percentile, respectively, for the dry and wet seasons.…”

Concept for an electrostatic focusing device for continuous ambient pressure aerosol concentration

“…For example, the leading techniques for detailed chem-ical characterization of the organic fraction of ambient particulate matter rely on particle collection-volatilization cycles (Williams et al, 2006;Lopez-Hilfiker, 2014). Other techniques that explore size-dependent properties of ambient (Mei et al, 2013;Thalman et al, 2017) or laboratorygenerated (Ahern et al, 2016;Petters et al, 2006;Vaden et al, 2011) organic aerosol using differential mobility analyzers (DMAs), such as Humidified Tandem DMA (HT-DMA) or cloud condensation nuclei counters (CCNCs), are complicated by the necessity of size selection of polydisperse samples, which work with a small mass fraction of a given aerosol sample's original population. Online (continuousflow) aerosol concentration would improve instrument signal and time response in these types of studies, simplifying data analysis.…”