Ambient observations of hygroscopic growth factor and f(RH) below 1: Case studies from surface and airborne measurements

Abstract

This study reports a detailed set of ambient observations of optical/physical shrinking of particles from exposure to water vapor with consistency across different instruments and regions. Data have been utilized from (i) a shipboard humidified tandem differential mobility analyzer during the Eastern Pacific Emitted Aerosol Cloud Experiment in 2011, (ii) multiple instruments on the NASA DC‐8 research aircraft during the Studies of Emissions, Atmospheric Composition, Clouds and Climate Coupling by Regional Surveys in 2013, and (iii) the Differential Aerosol Sizing and Hygroscopicity Spectrometer Probe during ambient measurements in Tucson, Arizona, during summer 2014 and winter 2015. Hygroscopic growth factor (ratio of humidified‐to‐dry diameter, GF = Dp,wet/Dp,dry) and f(RH) (ratio of humidified‐to‐dry scattering coefficients) values below 1 were observed across the range of relative humidity (RH) investigated (75–95%). A commonality of observations of GF and f(RH) below 1 in these experiments was the presence of particles enriched with carbonaceous matter, especially from biomass burning. Evidence of externally mixed aerosol, and thus multiple GFs with at least one GF < 1, was observed concurrently with f(RH) < 1 during smoke periods. Possible mechanisms responsible for observed shrinkage are discussed and include particle restructuring, volatilization effects, and refractive index modifications due to aqueous processing resulting in optical size modification. To further investigate ambient observations of GFs and f(RH) values less than 1, it is recommended to add an optional prehumidification bypass module to hygroscopicity instruments, to preemptively collapse particles prior to controlled RH measurements.

In situ measurements of water uptake by black carbon‐containing aerosol in wildfire plumes

“…Figure 3 (bottom) shows a histogram of the observed ratio of wet-to-dry modal scattering when the wet SP2 was sampled at 90% RH (functionally equivalent to f(RH) 90 measured at 1064 nm). The mean ratio is 1.02, indicating little water uptake by wildfire BC in aggregate, in general agreement with Shingler et al [2016b] who report little water uptake for size-selected bulk biomass burning aerosol during SEAC 4 RS. Martin et al [2013] reported collapse of fractal carbon aggregates in fresh emissions from several combustion sources upon humidification, which would result in a reduction in scattering following humidification.…”

“…The mean ratio is 1.02, indicating little water uptake by wildfire BC in aggregate, in general agreement with Shingler et al . [] who report little water uptake for size‐selected bulk biomass burning aerosol during SEAC 4 RS. Martin et al .…”

On the Morphology and Composition of Particulate Matter in an Urban Environment

“…Although many particle shapes exist in the atmosphere, such as isometric particles, platelets, and fibers, the difficulty to measure morphological properties often leads to the assumption of particle sphericity for most applications (Reist, 1993). An added complication is that particles with irregular morphology can collapse into a spherical shape upon humidification (Shingler et al , 2016). Scanning Electron Microscopy (SEM) combined with Energy Dispersive Spectrometry (EDX) provides useful information about the elemental composition, size and shape of PM, and thus is a useful technique in distinguishing particles originating from different sources (Salma et al , 2001).…”

Southwestern U.S. Biomass Burning Smoke Hygroscopicity: The Role of Plant Phenology, Chemical Composition, and Combustion Properties

“…Both properties depend on the chemical composition of the smoke and are in turn a function of fuel composition and combustion phase. Measuring aerosol water uptake is key to understanding its effects on visibility (Day et al, ), cloud condensation nuclei activity (Petters, Carrico, et al, ), and climate significance (Shingler et al, ). Two recent studies review many of the major studies globally examining the hygroscopic response of light scattering coefficient (often abbreviated f (RH)) as it relates to effects on atmospheric radiation (Dumka et al, ; Titos et al, ).…”