Mixtures of pollution, dust, sea salt, and volcanic aerosol during ACE‐Asia: Radiative properties as a function of relative humidity

Abstract

[1] The Ron Brown cruise during ACE-Asia (March-April 2001) encountered complex aerosol that at times was dominated by marine, polluted, volcanic, and dust aerosols. Average total light scattering coefficients (s sp for D p < 10 mm, relative humidity (RH) = 19%, and l = 550 nm) ranged from 23 (marine) to 181 Mm À1 (dust). Aerosol hygroscopicity ranged from deliquescent with hysteresis (marine frequently and polluted variably) to hygroscopic without hysteresis (volcanic) to nearly hygrophobic (dustdominated). Average deliquescence and crystallization RH were 77 ± 2% and 42 ± 3%, respectively. The ambient aerosol was typically on the upper branch of the hysteresis loop for marine and polluted air masses and the lower branch for dust-dominated aerosols. Average f (RH = ambient), defined as s sp (RH = ambient)/s sp (RH = 19%), ranged from 1.25 (dust) to 2.88 (volcanic). Average h(RH $60%), defined as f (RH) upper branch / f (RH) lower branch , were 1.6, 1.3, 1, and 1.25 for marine, polluted, volcanic, and dust, demonstrating an importance of hysteresis to optical properties. Hemispheric backscatter fraction (b) at ambient RH ranged from 0.077 (marine) to 0.111 (dust), while single scattering albedo (w) at ambient RH ranged from 0.94 (dust and polluted) to 0.99 (marine).

Impact of particulate organic matter on the relative humidity dependence of light scattering: A simplified parameterization

“…[9] Comparisons between f ssp (RH) derived from measurements (TSI scanning RH nephelometer) and Mie light scattering model calculations show agreement for NaCl and (NH 4 ) 2 SO 4 within 10% [Carrico et al, 2003;Kus et al, 2004]. Likewise, RHB measurements of f ssp (RH) at high RH during ACE Asia from TSI and Radiance Research scanning RH nephelometers demonstrated agreement within 10% for all air mass.…”

“…where scanning RH measurements were available, measured f ssp (RH) were fit based on observed curve structure [Carrico et al, 2003]. We express f ssp (RH) in terms of g s as shown in Equation (2) …”

Development of a New Airborne Humidigraph System

“…A red rectangle marks the flight leg over ocean below 400-m ASL, well within the MABL. For this leg, the average γ parameter is 0.68 ± 0.02 and the scattering factor is F = 2.85 ± 0.05, which is at the upper end of reported values for aerosol scattering factors measured over ocean: 1.1 to 3.2 (Hegg et al 1996;McInnes et al 1998;Sheridan et al 2002;Carrico et al 2003). A blue rectangle in Figure 3 marks the descent though the CABL from its top around 700-m ASL.…”

“…The impact of humidity on aerosol light scattering has been traditionally studied with the help of humidigraphs or humidographs (Covert et al 1972;Carrico et al 2003;Hegg et al 2008;Fierz-Schmidhauser et al 2010) where one or several nephelometers combined with a humidity conditioning system measures the dependence of the aerosol scattering coefficient σ sp on relative humidity (RH). Several groups have extensively deployed humidigraphs on airborne platforms in numerous field campaigns; two central groups are noted here: the University of Washington (Hegg et al 2008) and NOAA (Sheridan et al 2012).…”

Aerosol Properties Computed from Aircraft-Based Observations During the ACE-Asia Campaign: 2. A Case Study of Lidar Ratio Closure

“…Markowicz et al (2003) reported single scattering albedos above 0.86 at 55% RH for polluted airmasses. Carrico et al (2003) obtained the single scattering albedo as a function of RH: they used the humidograph nephelometer method to determine dependence of aerosol scattering coefficient on RH, and PSAP measurements of absorption coefficient at 55% RH, and assumed that the change of absorption coefficient with RH is negligible. Their results showed that in the polluted regions during the campaign the single scattering albedo increased from 0.91 to 0.96 for an increase in RH from 40% to 85%.…”