Low hygroscopic scattering enhancement of boreal aerosol and the implications for a columnar optical closure study

Abstract

Abstract. Ambient aerosol particles can take up water and thus change their optical properties depending on the hygroscopicity and the relative humidity (RH) of the surrounding air. Knowledge of the hygroscopicity effect is of crucial importance for radiative forcing calculations and is also needed for the comparison or validation of remote sensing or model results with in situ measurements. Specifically, particle light scattering depends on RH and can be described by the scattering enhancement factor f (RH), which is defined as the particle light scattering coefficient at defined RH divided by its dry value (RH < 30-40 %).Here, we present results of an intensive field campaign carried out in summer 2013 at the SMEAR II station at Hyytiälä, Finland. Ground-based and airborne measurements of aerosol optical, chemical and microphysical properties were conducted. The f (RH) measured at ground level by a humidified nephelometer is found to be generally lower (e.g. 1.63 ± 0.22 at RH = 85 % and λ = 525 nm) than observed at other European sites. One reason is the high organic mass fraction of the aerosol encountered at Hyytiälä to which f (RH) is clearly anti-correlated (R 2 ≈ 0.8). A simplified parametrization of f (RH) based on the measured chemical mass fraction can therefore be derived for this aerosol type. A trajectory analysis revealed that elevated values of f (RH) and the corresponding elevated inorganic mass fraction are partially caused by transported hygroscopic sea spray particles. An optical closure study shows the consistency of the ground-based in situ measurements.Our measurements allow to determine the ambient particle light extinction coefficient using the measured f (RH). By combining the ground-based measurements with intensive aircraft measurements of the particle number size distribution and ambient RH, columnar values of the particle extinction coefficient are determined and compared to columnar measurements of a co-located AERONET sun photometer. The water uptake is found to be of minor importance for the column-averaged properties due to the low particle hygroscopicity and the low RH during the daytime of the summer months. The in situ derived aerosol optical depths (AOD) clearly correlate with directly measured values of the sun photometer but are substantially lower compared to the directly measured values (factor of ∼ 2-3). The comparison degrades for longer wavelengths. The disagreement between in situ derived and directly measured AOD is hypothesized Published by Copernicus Publications on behalf of the European Geosciences Union. 7248 P. Zieger et al.: Scattering enhancement of boreal aerosol and columnar closure study to originate from losses of coarse and fine mode particles through dry deposition within the canopy and losses in the in situ sampling lines. In addition, elevated aerosol layers (above 3 km) from long-range transport were observed using an aerosol lidar at Kuopio, Finland, about 200 km east-northeast of Hyytiälä. These elevated layers further explain parts of the disagree...

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

“…Understanding the factors controlling the change in extinction or scattering as a function of RH, known as f (RH), is important for evaluating remote sensing measurements (e.g., Brock et al, 2016;Crumeyrolle et al, 2014;Esteve et al, 2012;Ferrare et al, 1998;Hegg et al, 1993;Kotchenruther et al, 1999;van Donkelaar et al, 2015;Voss et al, 2001;Zieger et al, 2011Zieger et al, , 2012Ziemba et al, 2012), aerosol direct radiative forcing calculations (e.g., Attwood et al, 2014;IPCC, 2013;Kahn, 2011;Koloutsou-Vakakis et al, 1998;Nemesure et al, 1995), and atmospheric visibility estimates (e.g., Charlson et al, 1967;Malm et al, 2000). In general, particles composed primarily of organic material and dust are less hygroscopic Petters and Kreidenweis, 2007;Zieger et al, 2015), while those that are predominantly inorganic take up water more readily (e.g., Petters and Kreidenweis, 2007;Quinn et al, 2005). Particles may exhibit sharp phase transitions as they deliquesce, as well as hysteresis as they effloresce, especially for inorganic compositions (e.g., Santarpia et al, 2005;Tang, 1996;Zieger et al, 2014).…”

“…Particles may exhibit sharp phase transitions as they deliquesce, as well as hysteresis as they effloresce, especially for inorganic compositions (e.g., Santarpia et al, 2005;Tang, 1996;Zieger et al, 2014). Particles dominated by organic compounds are more likely to present more gradual hygroscopic growth with increasing RH without evident phase transition behavior (e.g., Carrico et al, 2005;Zieger et al, 2015).…”

Hygroscopic growth study in the framework of EARLINET during the SLOPE I campaign: synergy of remote sensing and in situ instrumentation

“…Although the behavior of the backscatter coefficient at enhanced RH is expected to differ from the scattering coefficient, a qualitative comparison can be performed due to the scarcity of backscatter-related γ values in the literature. For example, using in situ techniques, Zieger et al (2015) reported a low scattering enhancement of boreal aerosol in Hyytiälä (Finland) (γ 525 = 0.25) related to the high contribution of organic aerosols at this site that contribute to decreasing the hygroscopic enhancement. At Cape Cod (USA), Titos et al (2014) reported significantly lower γ values for polluted aerosols (γ 550 = 0.4 ± 0.1) compared with marine aerosols (γ 550 = 0.7 ± 0.1).…”

Pan-Eurasian Experiment (PEEX): towards a holistic understanding of the feedbacks and interactions in the land–atmosphere–ocean–society continuum in the northern Eurasian region

“…The spatial distribution of the carbon budget shows considerable variation, and substantial areas, particularly in permafrost regions and in disturbed forests, display both sink and source behaviour. The already clearly observable greening of the Arctic is going to have large consequences on the carbon sink in the upcoming decades (Myneni et al, 1997;Zhou et al, 2001), although future predictions are uncertain. The net ecosystem carbon budget (NECB) or net biome production (NBP) are usually a sensitive balance between carbon uptake through forest growth, ecosystem heterotrophic respiration, and carbon release during and after disturbances such as fire, insect outbreaks, or weather events, e.g.…”

“…Hollstein and Fisher, 2014), or instruments providing multiple viewing algorithms such as MISR (Nelson et al, 2013) or AATSR (Virtanen et al, 2014). Another complication for aerosols may be the vertical variation of the physical and chemical properties, which renders it difficult to obtain closure between column and groundbased in situ measurements (Zieger et al, 2015, and references cited therein). Nevertheless, good progress has been made in aerosol retrieval, and column-integrated aerosol measurements (aerosol optical depth, AOD) from satellites and ground-based observations compare favourably (e.g.…”