Long-term cloud condensation nuclei number concentration, particle number size distribution and chemical composition measurements at regionally representative observatories

Schmale, Julia,Henning, Silvia,Decesari, Stefano,Henzing, Bas,Keskinen, Helmi,Sellegri, Karine,Ovadnevaite, Jurgita,Pö,hlker, Mira L.,Brito, Joel,Bougiatioti, Aikaterini,Kristensson, Adam,Kaliviti Copernicus GmbH 2018 Atmospheric Chemistry and Physics Vol.18 No.4

<P><p><strong>Abstract.</strong> Aerosol-cloud interactions (ACI) constitute the single largest uncertainty in anthropogenic radiative forcing. To reduce the uncertainties and gain more confidence in the simulation of ACI, models need to be evaluated against observations, in particular against measurements of cloud condensation nuclei (CCN). Here we present a data set - ready to be used for model validation - of long-term observations of CCN number concentrations, particle number size distributions and chemical composition from 12 sites on 3 continents. Studied environments include coastal background, rural background, alpine sites, remote forests and an urban surrounding. Expectedly, CCN characteristics are highly variable across site categories. However, they also vary within them, most strongly in the coastal background group, where CCN number concentrations can vary by up to a factor of 30 within one season. In terms of particle activation behaviour, most continental stations exhibit very similar activation ratios (relative to particles<span class='thinspace'></span><span class='inline-formula'>&gt;</span><span class='thinspace'></span>20<span class='thinspace'></span>nm) across the range of 0.1 to 1.0<span class='thinspace'></span>% supersaturation. At the coastal sites the transition from particles being CCN inactive to becoming CCN active occurs over a wider range of the supersaturation spectrum.</p> <p>Several stations show strong seasonal cycles of CCN number concentrations and particle number size distributions, e.g. at Barrow (Arctic haze in spring), at the alpine stations (stronger influence of polluted boundary layer air masses in summer), the rain forest (wet and dry season) or Finokalia (wildfire influence in autumn). The rural background and urban sites exhibit relatively little variability throughout the year, while short-term variability can be high especially at the urban site.</p> <p>The average hygroscopicity parameter, <span class='inline-formula'><i>κ</i></span>, calculated from the chemical composition of submicron particles was highest at the coastal site of Mace Head (0.6) and lowest at the rain forest station ATTO (0.2-0.3). We performed closure studies based on <span class='inline-formula'><i>κ</i></span>-Köhler theory to predict CCN number concentrations. The ratio of predicted to measured CCN concentrations is between 0.87 and 1.4 for five different types of <span class='inline-formula'><i>κ</i></span>. The temporal variability is also well captured, with Pearson correlation coefficients exceeding 0.87.</p> <p>Information on CCN number concentrations at many locations is important to better characterise ACI and their radiative forcing. But long-term comprehensive aerosol particle characterisations are labour intensive and costly. Hence, we recommend operating “migrating-CCNCs” to conduct collocated CCN number concentration and particle number size distribution measurements at individual locations throughout one year at least to derive a seasonally resolved hygroscopicity parameter. This way, CCN number concentrations can only be calculated based on continued particle number size distribution information and greater spatial coverage of long-term measurements can be achieved.</p> </P>

Single-particle characterization of aerosols collected at a remote site in the Amazonian rainforest and an urban site in Manaus, Brazil

Wu, Li,Li, Xue,Kim, HyeKyeong,Geng, Hong,Godoi, Ricardo H. M.,Barbosa, Cybelli G. G.,Godoi, Ana F. L.,Yamamoto, Carlos I.,de Souza, Rodrigo A. F.,Pö,hlker, Christopher,Andreae, Meinrat O.,Ro, Chul Copernicus GmbH 2019 Atmospheric chemistry and physics Vol.19 No.2

<P><p><strong>Abstract.</strong> <span id='page1222'/>In this study, aerosol samples collected at a remote site in the Amazonian rainforest and an urban site in Manaus, Brazil, were investigated on a single-particle basis using a quantitative energy-dispersive electron probe X-ray microanalysis (ED-EPMA). A total of 23 aerosol samples were collected in four size ranges (0.25-0.5, 0.5-1.0, 1.0-2.0, and 2.0-4.0<span class='thinspace'></span><span class='inline-formula'>µ</span>m) during the wet season in 2012 at two Amazon basin sites: 10 samples in Manaus, an urban area; and 13 samples at an 80<span class='thinspace'></span>m high tower, located at the Amazon Tall Tower Observatory (ATTO) site in the middle of the rainforest, 150<span class='thinspace'></span>km northeast of Manaus. The aerosol particles were classified into nine particle types based on the morphology on the secondary electron images (SEIs) together with the elemental concentrations of 3162 individual particles: (i) secondary organic aerosols (SOA); (ii) ammonium sulfate (AS); (iii) SOA and AS mixtures; (iv) aged mineral dust; (v) reacted sea salts; (vi) primary biological aerosol (PBA); (vii) carbon-rich or elemental carbon (EC) particles, such as soot, tarball, and char; (viii) fly ash; and (ix) heavy metal (HM, such as Fe, Zn, Ni, and Ti)-containing particles. In submicron aerosols collected at the ATTO site, SOA and AS mixture particles were predominant (50<span class='thinspace'></span>%-94<span class='thinspace'></span>% in relative abundance) with SOA and ammonium sulfate comprising 73<span class='thinspace'></span>%-100<span class='thinspace'></span>%. In supermicron aerosols at the ATTO site, aged mineral dust and sea salts (37<span class='thinspace'></span>%-70<span class='thinspace'></span>%) as well as SOA and ammonium sulfate (28<span class='thinspace'></span>%-58<span class='thinspace'></span>%) were abundant. PBAs were observed abundantly in the PM<span class='inline-formula'>₂₋₄</span> fraction (46<span class='thinspace'></span>%), and EC and fly ash particles were absent in all size fractions. The analysis of a bulk PM<span class='inline-formula'>_0.25−0.5</span> aerosol sample from the ATTO site using Raman microspectrometry and attenuated total reflection Fourier-transform infrared spectroscopy (ATR-FTIR) showed that ammonium sulfate, organics, and minerals are the major chemical species, which is consistent with the ED-EPMA results. In the submicron aerosols collected in Manaus, either SOA and ammonium sulfate (17<span class='thinspace'></span>%-80<span class='thinspace'></span>%) or EC particles (6<span class='thinspace'></span>%-78<span class='thinspace'></span>%) were dominant depending on the samples. In contrast, aged mineral dust, reacted sea salt, PBA, SOA, ammonium sulfate, and EC particles comprised most of the supermicron aerosols collected in Manaus. The SOA, ammonium sulfate, and PBAs were mostly of a biogenic origin from the rainforest, whereas the EC and HM-containing particles were of an anthropogenic origin. Based on the different contents of SOA, ammonium sulfate, and EC particles among the samples collected in Manaus, a considerable influence of the rainforest over the city was observed. Aged mineral dust and reacted sea-salt particles, including mineral dust mixed with sea salts probably during long-range transatlantic transport, were abundant in the supermicron fractions at both sites. Among the aged mineral dust and reacted sea-salt particles, sulfate-containing ones outnumbered those containing nitrates and sulfate<span class='thinspace'></span><span class='inline-formula'>+</span><span class='thinspace'></span>nitrate in the ATTO samples. In contrast, particles containing sulfate<span class='thinspace'></span><span class='inline-formula'>+</span><span class='thinspace'></span>nitrate were comparable in number to particles containing sulfate only in the Manaus samples, indicating