Aerosol mixing state, hygroscopic growth and cloud activation efficiency during MIRAGE 2006

Abstract

Observations of aerosol hygroscopic growth and CCN activation spectra for submicron particles are reported for the T1 ground site outside of Mexico City during the MIRAGE 2006 campaign. <i>κ</i>-Köhler theory is used to evaluate the characteristic hygroscopicity parameter, &kappa;*, for the CCN active aerosol population using both size-resolved HTMDA and size-resolved CCNc measurements. Organic mass fractions (<i>f</i>_org) are evaluated from size-resolved aerosol mass spectrometer (AMS) measurements, from which predictions of the hygroscopicity parameter are compared against &kappa;*. <br></br> Strong diurnal changes in aerosol water uptake parameters and aerosol composition are observed. We find that new particle formation (NPF) events are correlated with an increased &kappa;* and CCN-active fraction during the daytime, with greater impact on smaller particles. During NPF events, the number concentration of 40 nm particles acting as CCN at 0.51% ± 0.06% supersaturation can surpass by more than a factor of two the corresponding concentrations of 100 nm particles. We also find that at 06:00–08:00 LT throughout the campaign, fresh traffic emissions result in substantial changes to the chemical distribution of the aerosol, with on average 65% externally mixed fraction for 40 nm particles and 30% externally mixed fraction for 100 nm particles, whereas at midday nearly all particles of both sizes can be described as "internally mixed". <br></br> Average activation spectra and growth factor distributions are analyzed for different time periods characterizing the daytime (with and without NPF events), the early morning "rush hour" and the entire campaign. We show that &kappa;* derived from CCNc measurements decreases as a function of size during all time periods, while the CCN-active fraction increases as a function of size. Size-resolved AMS measurements do not predict the observed trend for &kappa;* versus particle size, which can be attributed to unresolved mixing state and the presence of refractory material not measured by the AMS. Measured &kappa;* typically ranges from 0.2 to 0.35, and organics typically make up 60–85 % of the aerosol mass in the size range studied. We show that &kappa;_AMS is able to describe CCN concentrations reasonably well, provided mixing-state information is available, especially at the highest CCN concentrations. This is consistent with other CCN studies carried out in urban environments, and is partly due to the fact that the highest CCN concentrations occur during the daytime when the aerosol is internally mixed. During the early morning rush hour, however, failing to account for the aerosol mixing state results in systematic overestimation of CCN concentrations by as much as 50–100% on average

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

“…The particle hygroscopicity parameter, κ , was derived from the activation spectrum (i.e., activated fraction as a function of supersaturation S) at the individual particle sizes using approaches detailed in the literature (Bougiatioti et al, 2011;Cerully et al, 2011;Lance et al, 2013;Mei et al, 2013a;Rose et al, 2008). The activation spectrum of size-selected particles was first corrected for the influence of multiply charged particles, which is estimated using the size distribution measured by the SMPS in MAOS and the activation spectrum measured at the sizes of the doubly and triply charged particles (see Sect.…”

Measured and modelled cloud condensation nuclei (CCN) concentration in São Paulo, Brazil: the importance of aerosol size-resolved chemical composition on CCN concentration prediction

“…In recent years a number of studies were performed aiming to characterize the CCN properties of the aerosol particles in urban environments and their effects on regional air quality and climate (e.g. Matsumoto et al, 1997;Yum et al, , 2007Broekhuizen et al, 2006;Kuwata et al, 2007Kuwata et al, , 2008Kuwata et al, , 2009Wiedensohler et al, 2009;Rose et al, 2010Kuhn et al, 2010;Gunthe et al, 2011;Lance et al, 2013;Mei et al, 2013).…”

“…Lance et al (2006) has provided the correction function in supersaturation for a given ambient pressure. Taking in account that the system was originally calibrated in Boulder, Colorado (820 mbar), and deployed in São Paulo (928 mbar), the correction factor is roughly 13 %.…”

“…A single-column continuous-flow stream-wise thermal gradient CCN chamber (DMT CCNC-100; Roberts and Nenes, 2005;Lance et al, 2006) was used to measure the total polydisperse CCN number concentration as a function of time and supersaturation (SS). The effective water vapour supersaturation was regulated by the temperature gradient applied between the upper and lower wetted ends of the CCNC flow column, where the activation takes place.…”

Assessment of cloud supersaturation by size-resolved aerosol particle and cloud condensation nuclei (CCN) measurements

“…As a result of long-term efforts and recent advances in atmospheric research, the CCN activity of atmospheric aerosol particles can be fairly well constrained by measurements and models (e.g., Kreidenweis, 2007, 2008;Gunthe et al, 2009;Kinne, 2009;Moore and Nenes, 2009;Juranyi et al, 2010;Pringle et al, 2010;Rose et al, 2010;Wex et al, 2010;Gunthe et al, 2011;Kim et al, 2011;Kulmala et al, 2011;Rose et al, 2011;Spracklen et al, 2011;Anttila et al, 2012;Christensen and Petters, 2012;Engelhart et al, 2012;Padró et al, 2012;Jurányi et al, 2013;Lance et al, 2013;Mikhailov et al, 2013;Petters and Kreidenweis, 2013). To fully describe the process of CCN activation and cloud droplet growth in the atmosphere, however, the supersaturation of water vapor in the cloud also needs to be known (e.g., Reutter et al, 2009;Pruppacher and Klett, 2010;Rosenfeld et al, 2012;Renno et al, 2013).…”