Aerosol optical properties in the southeastern United States in summer – Part 1: Hygroscopic growth

Abstract

Abstract. Aircraft observations of meteorological, trace gas, and aerosol properties were made during MaySeptember 2013 in the southeastern United States (US) under fair-weather, afternoon conditions with well-defined planetary boundary layer structure. Optical extinction at 532 nm was directly measured at relative humidities (RHs) of ∼ 15, ∼ 70, and ∼ 90 % and compared with extinction calculated from measurements of aerosol composition and size distribution using the κ-Köhler approximation for hygroscopic growth. The calculated enhancement in hydrated aerosol extinction with relative humidity, f (RH), calculated by this method agreed well with the observed f (RH) at ∼ 90 % RH. The dominance of organic aerosol, which comprised 65 ± 10 % of particulate matter with aerodynamic diameter < 1 µm in the planetary boundary layer, resulted in relatively low f (RH) values of 1.43 ± 0.67 at 70 % RH and 2.28 ± 1.05 at 90 % RH. The subsaturated κ-Köhler hygroscopicity parameter κ for the organic fraction of the aerosol must have been < 0.10 to be consistent with 75 % of the observations within uncertainties, with a best estimate of κ = 0.05. This subsaturated κ value for the organic aerosol in the southeastern US is broadly consistent with field studies in rural environments. A new, physically based, single-parameter representation was developed that better described f (RH) than did the widely used gamma power-law approximation.

A novel method for deriving the aerosol hygroscopicity parameter based only on measurements from a humidified nephelometer system

“…In this study, we estimated γ values with observed f (RH) curves and to our knowledge, we are the first ones to further examine the relationship between γ and κ f (RH) . Recently, a new physically based single-parameter representation was proposed by Brock et al (2016) to describe f (RH). Their results demonstrated that this proposed parameterization scheme can better describe f (RH) than the widely used gamma power-law approximation (Brock et al, 2016).…”

“…Recently, a new physically based single-parameter representation was proposed by Brock et al (2016) to describe f (RH). Their results demonstrated that this proposed parameterization scheme can better describe f (RH) than the widely used gamma power-law approximation (Brock et al, 2016). The formula of this new scheme is written as…”

“…In addition, hygroscopicity parameter κ also facilitates the intercomparison of aerosol hygroscopicity derived from different techniques and measurements made at different RHs. The hygroscopicity parameter κ is widely used to account for the influence of aerosol hygroscopic growth on aerosol optical properties as well as aerosol liquid water contents (Tao et al, 2014;Kuang et al, 2015;Brock et al, 2016;Bian et al, 2014;Zieger et al, 2013) and to examine the role of aerosol hygroscopicity in CCN Gunthe et al, 2009;Ervens et al, 2010). The κ values derived from field campaigns and laboratory studies will further our understanding of aerosol hygroscopicity and help estimate the influences of aerosol hygroscopic growth on different aspects of atmospheric processes.…”

Modification, calibration, and performance of the Ultra-High Sensitivity Aerosol Spectrometer for particle size distribution and volatility measurements during the Atmospheric Tomography Mission (ATom) airborne campaign

“…The UHSAS uses a high-intensity infrared laser (semiconductor-diode-pumped solid-state neodymiumdoped yttrium lithium fluoride, Nd 3+ :Y LiF 4 , operating at 1054 nm with intra-cavity circulating power of ∼ 1 kW cm −2 ), an inlet jet assembly, and two detection systems: a highly sensitive avalanche photodiode (APD) to detect and size the smallest particles, and a less sensitive secondary PIN photodiode to size larger particles (Fig. 1) (Brock et al, 2016). This geometry contrasts with that reported by Cai et al (2008), who used scattering angles from 22 to 158 • to simulate UHSAS response.…”

“…These sample treatments are not possible with the compact, wing-mounted instrument. The ground-type UH-SAS has been deployed in various airborne-based campaigns (Brock et al, 2011(Brock et al, , 2016Kassianov et al, 2015;Yokelson et al, 2011). However, as reported by Brock et al (2011), modifications to the flow system are required to make them suitable for airborne sampling.…”