Measurements of the Imaginary Component of the Refractive Index of Weakly Absorbing Single Aerosol Particles

Abstract

The interaction of atmospheric aerosols with radiation remains a significant source of uncertainty in modeling radiative forcing. Laboratory measurements of the microphysical properties of atmospherically relevant particles is one approach to reduce this uncertainty. We report a new method to investigate light absorption by a single aerosol particle, inferring changes in the imaginary part of the refractive index with a change in environmental conditions (e.g., relative humidity) and inferring the size dependence of the optical extinction cross section. More specifically, we present measurements of the response of single aerosol particles to near-infrared (NIR) laser-induced heating at a wavelength of 1520 nm. Particles were composed of aqueous NaCl or (NH)SO and were studied over ranges in relative humidity (40-85%), particle radius (1-2.2 μm), and NIR laser power. The ensuing size change and real component of the refractive index were extracted from measurements of the angular variation in elastically scattered light. From the heating-induced size change at varying NIR beam intensities, we retrieved the change in the imaginary component of the refractive index. In addition, cavity ring-down spectroscopy measurements monitored the change in extinction cross section with modulation of the heating laser power.

Assessing relative humidity dependent photoacoustics to retrieve mass accommodation coefficients of single optically trapped aerosol particles

“…29 On the other hand, Willoughby et al used laser-induced heating to determine the RH-dependence of the imaginary parts of the refractive index (RI) of single particles of NaCl and (NH 4 ) 2 SO 4 . 30 Their study suggests that for particles 1-2.2 mm in radius the absorption decreases with increasing RH, which they attribute to the presence of impurities.…”

Accuracy Required in Measurements of Refractive Index and Hygroscopic Response to Reduce Uncertainties in Estimates of Aerosol Radiative Forcing Efficiency

“…This new experimental approach allows retrieval of accurate σ ext values, made on a single aerosol particle, the size of which is monitored over an extensive period of time as it responds to changes in RH. To achieve such measurements, a combination of a Bessel laser beam trap and cavity ring down spectrometer (BB‐CRDS) has been used to retrieve accurate σ ext values for a single particle of well‐defined composition (Cotterell, Mason, et al, ; Cotterell, Preston, et al, ; Cotterell et al, ; Mason et al, ; Walker et al, ; Willoughby et al, ). The extinction efficiency for a single particle, Q ext , is where σ geo is the geometric cross section for a spherical particle with radius, r , inferred independently from the angularly scattered light profile.…”

“…For fine mode droplets, an accuracy of better than ±0.2% is typical. A detailed explanation of the BB‐CRDS technique and the theory underlying single‐particle measurements can be found in recent papers (Cotterell, Mason, et al, ; Cotterell, Preston, et al, ; Cotterell et al, ; Mason et al, ; Walker et al, ; Willoughby et al, ). Given this improved level of accuracy in optical constant measurements, it is appropriate to consider how this could translate into reductions in uncertainties in RF.…”

Particle Shape Impact on the Radiative Forcing Efficiency Estimated from Single Levitated (NH4)2SO4 Particles

“…Approaches are still being developed to identify and quantify their unique chemical and physical attributes. To overcome this inconvenience, one strategy is to use methods of manipulation of single aerosol particles [26][27][28][29][30][31][32]. These approaches are based on optical levitation methods [33] which facilitate the confinement, levitation and monitoring of single particles over time and generally have focused on trapping transparent particles.…”