Abstract
Abstract. Vertical profiles of submicron aerosol from in situ aircraft-based measurements were used to construct aggregate profiles of chemical, microphysical, and optical properties. These vertical profiles were collected over the southeastern United States (SEUS) during the summer of 2013 as part of two separate field studies: the Southeast Nexus (SENEX) study and the Study of Emissions and Atmospheric Composition, Clouds, and Climate Coupling by Regional Surveys (SEAC4RS). Shallow cumulus convection was observed during many profiles. These conditions enhance vertical transport of trace gases and aerosol and create a cloudy transition layer on top of the sub-cloud mixed layer. The trace gas and aerosol concentrations in the transition layer were modeled as a mixture with contributions from the mixed layer below and the free troposphere above. The amount of vertical mixing, or entrainment of air from the free troposphere, was quantified using the observed mixing ratio of carbon monoxide (CO). Although the median aerosol mass, extinction, and volume decreased with altitude in the transition layer, they were ~10 % larger than expected from vertical mixing alone. This enhancement was likely due to secondary aerosol formation in the transition layer. Although the transition layer enhancements of the particulate sulfate and organic aerosol (OA) were both similar in magnitude, only the enhancement of sulfate was statistically significant. The column integrated extinction, or aerosol optical depth (AOD), was calculated for each individual profile, and the transition layer enhancement of extinction typically contributed less than 10 % to the total AOD. Our measurements and analysis were motivated by two recent studies that have hypothesized an enhanced layer of secondary aerosol aloft to explain the summertime enhancement of AOD (2–3 times greater than winter) over the southeastern United States. The first study attributes the layer aloft to secondary organic aerosol (SOA) while the second study speculates that the layer aloft could be SOA or secondary particulate sulfate. In contrast to these hypotheses, the modest enhancement we observed in the transition layer was not dominated by OA and was not a large fraction of the summertime AOD.
Highlights
Shallow cumulus convection is common over the southeastern United States (SEUS) during the summer
For cumulus convection the height of the planetary boundary layer is defined as the cloud base or the top of the mixed layer; we find the term planetary boundary layer confusing in the context of shallow cumulus convection and have avoided using it
In addition to vertical transport and redistribution of aerosol, we observed a modest enhancement of aerosol loading in the transition layer and conclude that secondary aerosol formation in the transition layer is the likely source of the enhancement
Summary
Shallow cumulus convection is common over the southeastern United States (SEUS) during the summer. It enhances the vertical transport of trace gases and aerosol and creates a transition layer between the mixed layer and free troposphere (Siebesma, 1998). Based on the seasonality of the surface-aerosol–aerosoloptical-depth (AOD) relationship in the SEUS and the spatial similarity of biogenic emissions and enhanced AOD, Goldstein et al (2009) and Ford and Heald (2013) have hypothesized the existence of a layer of enhanced secondary aerosol aloft in the summer which contributes to AOD but not to surface measurements of aerosol mass. Neither study speculates about meteorological mechanisms that would lead to the formation of this layer, aerosol production in the transition layer of shallow cumulus convection is a plausible mechanism that could produce the hypothesized layer. The vertical distribution of aerosol and aerosol formation are integral to understanding the relationship between aerosol mass (PM2.5) at the surface and AOD (Hoff and Christopher, 2009)
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