As one aspect of the NETwork on
Climate and Aerosols: addressing key
uncertainties in Remote Canadian
Environments (NETCARE), measurements of the cloud condensation
nucleation properties of 50 and 100 nm aerosol particles were conducted
at Ucluelet on the west coast of Vancouver Island in August 2013. The
overall hygroscopicity parameter of the aerosol (
Atmospheric aerosol particles directly affect the Earth's radiative budget by scattering and absorbing incoming solar radiation (Charlson et al., 1992). Additionally, these particles can indirectly influence the radiative budget of the Earth by acting as cloud condensation nuclei (CCN) in the formation of warm clouds (Albrecht, 1989). Warm clouds can trap the Earth's outgoing infrared radiation, reflect incoming solar radiation, and influence the hydrological cycle and thus impact its climate (Albrecht, 1989; Twomey, 1977b). More specifically, warm clouds in marine regions are significant contributors to the Earth's radiative flux due to their extensive coverage and large albedo in relation to the ocean surface (Hartmann et al., 1992). It is well known that the indirect effects of these particles on climate constitute one of the largest uncertainties in understanding the present-day climate sensitivity (IPCC, 2013). This large degree of uncertainty arises partially from an incomplete understanding of aerosol particles' abilities to act as CCN. Consequently, because of the resulting sensitivity of the Earth's radiative budget to low-lying clouds, it is crucial to ensure that their CCN properties are well characterized.
Marine aerosol consists of two distinct sources: (1) primary sea-spray aerosol directly produced by breaking waves, consisting of inorganic salts and biogenic material such as surface-active microorganisms and exopolymer secretions; and (2) secondary aerosol formed by gas-to-particle conversion processes, mainly consisting of non-sea-salt (nss) sulfate and organic species (O'Dowd et al., 1997; Twomey, 1977a). However, in both coastal and marine locations it is also common for aerosol of anthropogenic and continental biogenic origin to be present, for instance, as a result of emissions by forests, populated areas, and shipping traffic. These sources can in turn result in high levels of substances of relevance to cloud formation, such as secondary organic aerosol (SOA) and sulfates, that are carried into the marine boundary layer by turbulence and convective mixing (Chang et al., 2010; Charlson et al., 1992; Coggon et al., 2012; Shantz et al., 2010).
While an enormous amount of effort has been applied to understanding and characterizing the aerosol hygroscopicity and CCN properties of continental biogenic, anthropogenic, and primary sea-spray aerosol (e.g. Andreae and Rosenfeld, 2008; Bigg, 2007; Chang et al., 2010; Coggon et al., 2012; Fuentes et al., 2011; Hegg et al., 2009; Kanakidou et al., 2005; Langley et al., 2010; Leck and Bigg, 2005a, b; Mei et al., 2013a, b; Moore et al., 2011; Orellana et al., 2011; Ovadnevaite et al., 2011; Prather et al., 2013; Roberts et al., 2006; Shantz et al., 2010; Sun and Ariya, 2006), this has not been the case for coastal or marine aerosol (herein referred to as coastal/marine aerosol) that has been influenced by marine organics. In particular, past studies have resulted in a wide range in the CCN properties of measured coastal/marine aerosol, making the direct comparison of results a challenge (Aalto and Kulmala, 2000; Allen et al., 2011; Ayers and Gras, 1991; Ayers et al., 1997; Bougiatioti et al., 2009; Good et al., 2010; Hegg et al., 1991; Hudson, 2007; Hudson et al., 2011; Kleinman et al., 2012; Lohmann and Leck, 2005; Meng et al., 2014; Moore et al., 2012; Ovadnevaite et al., 2011; Roberts et al., 2006, 2010; Shantz et al., 2008; Shinozuka et al., 2009; Sun and Ariya, 2006; Wang et al., 2008).
In addition to the poor characterization of ambient coastal/marine aerosol, only a handful of studies have estimated the CCN properties of organics in such a setting, which have been reported to possess similar CCN abilities to organics in continental regions (Bougiatioti et al., 2009; Cavalli et al., 2004; Martin et al., 2011; Matsumoto et al., 1997; Meng et al., 2014; Novakov and Penner, 1993). However, the CCN properties of marine organics have not been consistently reported using a standardized method, which makes the relative ranking of their water droplet formation abilities a challenge.
Determination of the effective hygroscopicity parameter,
In this study we use the hygroscopicity parameter approach to report the CCN
activity of 50 and 100 nm ambient aerosol particles that were present on
the west coast of Vancouver Island (Ucluelet, British Columbia) in August
2013. These experiments were conducted as part of a campaign to examine
cloud formation properties of marine aerosol, as one component of the NETCARE
(the NETwork on Climate and
Aerosols: addressing key uncertainties in
Remote Canadian Environments) project
(
The field campaign took place in August 2013 at a coastal field site which
was situated roughly 100 m from shore and 2 km from the small town of
Ucluelet (population 1800) on Vancouver Island, BC, Canada (48.92
Schematic of the main components of the experimental set-up, which
included SO
Time series (PDT) for 7–28 August of SMPS total number
concentrations (top) particle size distribution (bottom). The colour bar
indicates d
The focus of this work is on data that were collected using a CCN counter (DMT 100), which was used to calculate the
After size selection, the particle flow was split for measurement by the CCN
counter as well as a Condensation Particle Counter (CPC, TSI 3010), which
was used to measure the aerosol number concentrations. The number
concentrations measured by the CCN and CPC (
Time series (PDT) for 9–25 August of SMPS total volumetric concentrations (top) and wind speed (bottom). The wind's direction is classified as type a, b, c and d, according to the air mass classification scheme described in Sect. 3.1.
A scanning mobility particle sizer (SMPS, TSI 3081, 3782) measured dry aerosol size distributions of mobility diameters within the range of 19–914 nm every 2 minutes from 9 to 25 August, with some interruptions (Figs. 2 and 3, top) due to instrument maintenance. A 0.071 cm diameter impactor was attached to the inlet of the SMPS, which sampled from the same main inlet as the CCN counter and CPC.
A micro-orifice uniform deposition impactor (MOUDI, MSP 110R) sampled
directly from a separate main inlet, with a sample flow of 24 L min
Time series (PDT) for 7–24 August of sea surface temperature (red
circles), NO
Time series (PDT) for 13–31 August of ACSM organics (green),
NO
Time series (PDT) for 7–24 August of CCN-derived
An aerosol chemical speciation monitor (ACSM, Aerodyne, Ng et al., 2011) was
located in a second trailer 14 m from the primary trailer, and sampled from
its own main inlet. The trailer's temperature was controlled at 22
The NOAA Air Resources Laboratory HYSPLIT model was used to generate 72 h
back trajectories. To indicate their diversity, back trajectories are shown
for 12:00 (Pacific Daylight Savings Time, PDT) on each day of the study as
Fig. S1. Throughout the campaign, these
trajectories indicate that air reaching the sampling site (red star) was the
result of on-shore winds and was within the marine boundary layer (below
A characteristic clean marine ratio of MSA to nss-SO
Figure 4 shows a time series (PDT) of the daily sea-surface temperature,
SO
The average sulfate mass concentration as measured by the ACSM (Fig. 5) was
quite low, 0.68
The experimentally determined supersaturation required for 50 % of the
ambient particles to be activated as CCN (
As shown in Fig. 6, the time series of
Through the use of the air mass classification scheme described in Sect. 3.1,
four periods of air masses and
Using this air mass classification, the highest
The large range of
To interpret our observations, it is valuable to consider the composition of
the particles in the size range that typically activate as CCN. In
particular, according to Fig. 7, the ionic composition of 42–75 and 78–141
The observation that sulfate species dominate the composition of particles
of the size ranges considered in this study agrees well with previous
studies of the chemical composition of marine aerosol
(Good
et al., 2010; Hawkins et al., 2008; Moore et al., 2012; O'Dowd et al., 2004;
Phinney et al., 2006; Prather et al., 2013). We note that the molar ratio of
NH
By inspection of Fig. 8 (bottom), where
Average 2-day molar concentrations of ionic species present in
48–75
We note, as well, that there is a relationship between
CCN-derived
The asymptotic value of Fig. 8 (bottom) at high values of organic to
SO
A few previous studies have inferred values for
A cloud condensation nucleus counter was used to investigate the CCN
activity of ambient aerosol particles on the west coast of Canada (Ucluelet,
BC) in August 2013 as one aspect of NETCARE. These results were used in
conjunction with the ambient aerosol's 2-day average size-resolved ionic
composition as well as PM
The hygroscopicity parameter of the ambient marine aerosol exhibits a large
degree of variability, ranging from 0.14
The PM
The authors thank R. Zhao (University of Toronto), A. Lee (University of Toronto), Emma Mungall (University of Toronto), Robert Christensen (University of Toronto), R. Y.-W. Chang (Dalhousie University), Meng Si (University of British Columbia) and Yuri Jixiao Li (University of Denver) for their help during the campaign and constructive comments regarding this project. NETCARE is funded by NSERC (Canada) through the Climate Change and Atmospheric Research Program. J. A. Huffman acknowledges internal faculty support from the University of Denver. The Marine Boundary Layer site at Ucluelet is located at the coast guard site and we would like to thank the Department of Fisheries and Oceans and all the staff at the site for their help. The MBL site is jointly supported and maintained by Environment Canada, BC Ministry of Environment and MetroVancouver. Edited by: M. Petters