Aerosol mixing-state, hygroscopic growth and cloud activation efficiency during MIRAGE 2006
1Chemical Sciences Division, Earth System Research Laboratory, National Oceanic and Atmospheric Administration, Boulder, CO, USA
2Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO, USA
3School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA, USA
4Aerodyne Research Inc., Billerica, MA, USA
5Pacific Northwest National Laboratory, Richland, WA, USA
6University of Minnesota, Minneapolis, MN, USA
7National Center for Atmospheric Research, Boulder, CO, USA
8School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, Atlanta, GA, USA
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. κ-Köhler theory is used to evaluate the characteristic water uptake coefficient, κ*, for the CCN active aerosol population using both size-resolved HTDMA and size-resolved CCNc measurements. Organic mass fractions, forg, are evaluated from size-resolved aerosol mass spectrometer (AMS) measurements, from which κAMS is inferred and compared against κ*.
Strong diurnal profiles of aerosol water uptake parameters and aerosol composition are observed. We find that new particle formation (NPF) events are correlated with an increased κ* 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 can surpass by more than a factor of two the concentrations of 100 nm particles acting as CCN, at supersaturations of 0.51% ± 0.06%. We also find that at 06:00–08:00 in the morning 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.
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 κ* 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 κ* versus particle size, which can be attributed to unresolved mixing-state and the presence of refractory material not measured by the AMS. Measured κ* typically ranges from 0.2 to 0.35, and organics typically make up 60–85% of the aerosol mass in the size range studied. Despite some disagreement between κAMS and κCCNc, we show that κAMS is able to describe CCN concentrations reasonably well, 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 and the organic fraction is relatively low. During the early morning rush hour, however, failing to account for the aerosol mixing-state often results in systematic overestimation of CCN concentrations by 50–100%.