Measurements of the effect of water uptake on particulate light extinction or
scattering made at two locations during the 2010 Carbonaceous Aerosols and Radiative Effects Study (CARES) study around
Sacramento, CA are reported. The observed influence of water uptake,
characterized through the dimensionless optical hygroscopicity parameter
It is well established that atmospheric particles can have a strong influence on climate through their direct effect: scattering and absorption of solar and terrestrial radiation. Models must incorporate the net counteracting effects of cooling due to light scattering by particles and warming due to light absorption by greenhouse gases and particles to be successful at predicting global mean temperature. Uncertainties associated with climate forcing by particles remain sizable, and the negative forcing may be comparable to the collective positive radiative forcing from greenhouse gases (IPCC, 2013). Refinements of the linkage between the end results of models and measurements – particulate optical effects on the climate system as observed by in situ, ground-based, remote, and satellite measurements – and the presumptive sources of the particles are desirable to allow prediction of the effects of regulatory and other changes in future emissions.
Global climate models cannot currently fully represent the complex mixing state of particles indicated by in situ measurements. Therefore, such models typically utilize compositionally ensemble-averaged particle types with defined size distributions to represent the contributions from various sources. At the other extreme, some detailed models used for regional climate modeling and air quality simulation account more explicitly for particle dynamics, aging and mixing state (e.g., Riemer et al., 2010; Zaveri et al., 2010). In both simple and complex models, the extent of particulate water is determined by the local atmospheric relative humidity (RH) and the particle composition, the latter of which controls the particle hygroscopicity.
Particle composition is variable in space and time. Ambient measurements of
submicron particle (i.e., particles with diameters
One common method used to characterize particle hygroscopicity is through
comparison between the light extinction or scattering coefficients
(
During June of 2010, a variety of aerosol and gas-phase species, as well as meteorological and radiative properties were measured as part of the CARES field intensive campaign in the Sacramento/Central Valley region of California (Zaveri et al., 2012). The CARES study was designed to take advantage of a persistent southwesterly flow pattern that transports pollutants from the Sacramento urban core and nearby Bay Area across the mostly agricultural areas in the Central Valley toward the forested foothills of the Sierra Nevada mountains (Fast et al., 2012). Two heavily instrumented ground sites were used to capture the evolution of the urban plume: one located just to the northeast of Sacramento, denoted T0, and one in the foothills of the Sierra Nevada in Cool, CA, referred to as T1 (Zaveri et al., 2012). Aircraft were also used to directly monitor the transport during periods predicted to have favorable meteorology. The results presented in this work are based on measurements obtained only at the two ground sites.
Much of the campaign was characterized by daytime west-east transport between
the T0 and T1 sites, although there were occasional disruptions to the
generalized flow pattern by shifts to northerly/northwesterly flow (Fast et
al., 2012; Zaveri et al., 2012). The analysis here focuses primarily on
periods with T0
Instruments were housed in dual, air-conditioned construction trailers with common aerosol and gas-phase manifolds. The detailed specifications of the aerosol inlet system are provided in Zaveri et al. (2012). Briefly, a high-throughput pump pulled air into a stainless steel aerosol inlet positioned between and above the trailers. The aerosol flow was split between the trailers and within the trailers into two separate 3/4 inch stainless manifolds connected to high-flow return pumps. Each aerosol instrument station accessed the manifold through a 1/4 inch centerline pick-off using the instrument's pumping system. No intentional size selection was incorporated into the aerosol sampling mast or manifold system, but some of the instruments used size fractionation at their individual sampling points, as noted below.
Light extinction coefficients were measured at T0 at 405 and 532 nm using
the UC Davis two-wavelength cavity ring down-photoacoustic spectrometer
(CRD-PAS) instrument from 16–29 June 2010 (Langridge et al., 2011). Only
data from the 532 nm CRD channels are used here, averaged to 10 min. Four
532 nm channels were operated: low RH (
Light scattering by particles at multiple RH values was measured at the T1
site using a “humidigraph” (Pekour et al., 2012). The humidigraph is
comprised of three nephelometers (Aurora Nephelometer, Model 1000) that
measure
The instruments were operated with one of two distinct configurations. In one
(8–17 June), the three nephelometers were operated in parallel, with the
aerosol stream being split and sampled respectively (i) through a Nafion
dryer (low RH), (ii) without alteration (mid RH), and (iii) through a
water-cooled line (high RH). In the second configuration (21–27 June), the
entire aerosol stream was first humidified, after which 1/3 of the flow was
split to a high-RH nephelometer and the remaining flow was passed through a
Nafion dryer, after which 1/2 of this flow was directed to a mid-RH
nephelometer while the remaining flow was passed through a second Nafion
drier and on to a low-RH nephelometer. The second configuration provided for
more useful ranges of RH (since the original configuration often resulted in
near-coincidence of the low and ambient RH channels) and assured that
salt-like aerosols would be on the high RH branch (efflorescence) of the
hysteresis curve. A time-series of the RH in the high-RH channel is shown in
Fig. S4; the average value during the second configuration was
72 %
The low and highest RH
Mass concentrations of submicron non-refractory particulate matter (NR-PM)
were measured at both T0 and T1 using Aerodyne high resolution time-of-flight
aerosol mass spectrometers (HR-ToF-AMS, henceforth AMS) (Canagaratna et al.,
2007; DeCarlo et al., 2006). NR-PM components measured by the AMS include the
major inorganic species sulfate, nitrate and ammonium (along with some forms
of chlorine), and OA. The AMS measures ensemble-average particle composition
for particles with vacuum aerodynamic diameters (
Further characterization of the OA was obtained via positive matrix factorization (PMF), from which different OA “types” (or factors) were identified (Zhang et al., 2011). During CARES, three major factors were identified at T0 and T1. At T0, there were two less-oxygenated factors and one highly oxygenated factor, while at T1 there was only one less-oxygenated factor but two highly oxygenated factors (Setyan et al., 2012). Since the hygroscopicity of the two less oxygenated OA factors at T0 and the two highly oxygenated factors at T1 are likely similar they have been combined into one factor in each case. Thus, only two OA types are considered at each site, one less oxygenated, referred to as hydrocarbon-like OA (HOA), and one highly oxygenated, referred to as OOA.
Two different types of single particle mass spectrometers were deployed, one
at T0 and one at T1. At T1, the Particle Analysis by Laser Mass Spectrometry
(PALMS) instrument was deployed (Cziczo et al., 2006). PALMS samples
particles through an aerodynamic lens into a vacuum chamber where individual
particles are detected and sized using dual continuous 532 nm lasers after
which 193 nm light is used to ablate and ionize the particles. The resulting
ions are analyzed using a ToF-MS. PALMS detects and characterizes the
composition, including refractory components, of particles in the size range
300 nm
At T0 SPLAT II was deployed (Zelenyuk et al., 2009). SPLAT II works similarly
to the PALMS, with a key difference being that SPLAT II uses a CO
For both the T0 and T1 sites, the single particle measurements were the only instruments deployed that provide information on the composition of supermicron particles. This data limitation has implications as to how the supermicron particle composition is treated in the optical calculations discussed in Sect. 4.
Refractory black carbon (rBC) mass concentrations were measured at both sites
using single particle soot photometers (SP2; DMT, Inc.; Schwarz et al.,
2010). The SP2s were calibrated using mobility size-selected Aquadag
(Acheson, Inc.) graphite-containing particles and the known relationship
between mobility diameter and per-particle mass for this particle type. The
rBC concentrations have been adjusted to account for the higher sensitivity
of the SP2 to Aquadag than to other black carbon types (in particular, to
diesel soot; Laborde et al., 2012a). The CARES SP2 instruments measured
rBC-containing particles with volume equivalent core diameters
(
Submicron dry particle mobility diameter (
Supermicron dry particle aerodynamic diameter (
Overview time-series data of
The SMPS and converted APS size distributions were merged into a single
mobility-diameter size distribution (Fig. 1c and f). The SMPS measurements
were used for particles with diameters
The merged size distributions were ultimately used as input to the Mie theory
calculations (see next section), and thus the assumption regarding the
particle density will have some influence on the calculated scattering. It is
unlikely that the particle density is much larger than 2 g cm
The APS at T0 malfunctioned after 22 June 21:00 PST, limiting the period
over which observations of extensive properties, such as
Time-series of
The
Species properties used in model calculations.
The RH-specific physical growth factors
(GF
Particle composition varies with particle size and between individual
particles in a given size range (Zaveri et al., 2012). Such variations can
lead to size-dependent GFs and VF
The third approach, referred to as the size-dependent composition model, accounts for size-dependent variations in submicron particle composition. Both the AMS (for T1) and the SPLAT II (for T0) measurements indicate that particle composition did vary with particle size and that this variation was time-dependent (e.g., with the time of the day and from day to day). Ideally, highly time-resolved, quantitative size-dependent composition measurements would be used in these calculations. However, given site-to-site differences in measurement/data availability, the analysis here uses the campaign average size-dependent composition for each site. The use of the campaign-average information allows for a first-order assessment of how variations in particle composition with size influence the calculated optical properties. (Again, because of data availability and concerns about variable detection efficiency of larger particles, the supermicron mode was assumed to have a size-independent composition.)
Schematic of the process for determining size-dependent
hygroscopic growth factors and real refractive indices. The top
panels (
The basic framework for the size-dependent submicron calculations,
illustrated schematically in Fig. 2, is as follows: first, normalized
campaign-average mass-weighted size distributions for each particle component
(e.g. OA, ammonium nitrate, and ammonium sulfate) or particle type were
determined. These component-specific distributions are used to determine the
fraction of each component as a function of particle size. The fractions are
used as size-dependent weighting-factors to apportion the measured ensemble
particle composition of each component at each point in time onto the
observed size distribution at that time point. This yields a time-series of
composition-weighted size distributions with an assumption of completely
internally mixed particles for each size bin. These composition-weighted size
distributions are then used to calculate size-dependent GFs and refractive
indices for use in the calculation of
For the T1 site, the normalized campaign-average size-dependent composition
for submicron particles was determined from the AMS particle time-of-flight
measurements (Setyan et al., 2012). It was assumed that the BC, HOA and OOA
components had similar average size distributions, based on the general
similarity of the observed size distributions for the AMS tracer ions
As noted above, for T0 size-dependent submicron composition data from the AMS
were not available. Therefore, the SPLAT II data were used to obtain the
variation in composition with size within the submicron range and to
determine the normalized size-dependent composition. To provide some
consistency between T0 and T1, the SPLAT particle types were mapped onto the
AMS
It is assumed in all cases that the supermicron composition is
size-independent, a simplification that has been made to account for
limitations regarding time-dependent variations in the supermicron particle
composition. Unless otherwise stated, results of calculations in
Sect.
Time series for the dried (RH
The time-series of the observed (gray) and calculated
(orange)
Time-series of the observed optical hygroscopicity parameter,
Time-series of
Scatter plot comparisons between observed and calculated
The calculated average supermicron fractional contribution (
The calculation of wet particle optical properties requires that the
GFs (or equivalently
Average optimal values for
Visualization of the optimization procedure for the
supermicron and OOA hygroscopicities at T0 (left) and T1 (right). The
Time-series of the optimized
The optimal average OOA hygroscopicities are
The consistency of the derived
The optimal campaign-average
Fraction of total sampled number of supermicron particles at
T0, as identified by the SPLAT II instrument, over the period 17 June–25 June.
It should be noted that the sea-salt particle type, which is the most
abundant particle type observed, includes particles with varying amounts of
NaCl, NaNO
The average
Variability in the supermicron composition could result from variations in
sources of primary supermicron PM or from photochemical processing.
Sacramento is located about 90 miles from the San Francisco Bay and Pacific
Ocean; thus, sea-spray particles transported to the T0 site in Sacramento
and the T1 site in the Sierra Foothills will likely have undergone some
photochemical processing along the way. As noted above, sea-salt-containing
particles make up a substantial proportion of supermicron particles sampled
during the measurement period (Fig. 6). The majority of the sea-salt
particles observed were processed to differing extents as indicated in the
single particle mass spectra by the presence of characteristic peaks for NaCl
(
An example of the dependence of
Fractional contributions (left axis) of chloride (purple),
organics (green) and nitrates (blue) in the PALMS single particle negative
ion mass spectra for sea-salt-containing particles, identified by sodium in
the positive ion spectra, and (right axis) the derived supermicron
hygroscopicity parameter,
In addition to chemical processing affecting particle hygroscopicity,
variations in the sources of emitted primary supermicron particles can
influence the observed supermicron hygroscopicity. For example, non-sea-salt-containing particles can be emitted as sea spray in addition to sea-salt
(i.e., sodium-containing) particles (Facchini et al., 2008; Prather et al.,
2013; Quinn et al., 2014), which may have lower hygroscopicity than sea salt.
Further, there are also supermicron dust particles, the hygroscopicity of
which can be quite variable but is typically lower than sea salt (Koehler et
al., 2009; Zhang et al., 2014). Finally, sulfate
Change in absolute model retrieved
The sensitivity of the retrieved values of
For T0, the
At T1, because
Three different models of the submicron particle mixing state were tested in
calculating the particle optical properties: an internal mixture with
size-dependent composition (the base case discussed above), an internal
mixture with size-independent composition and an external mixture with
size-dependent composition. The optimization procedure was repeated for the
two alternative models. The derived optimal
In the Mie calculations presented herein, it was assumed that the particles
were non-absorbing, and thus that
The magnitude of the potential influence of light absorption on the
calculated hygroscopic growth has been assessed through a series of test
calculations. Results are compared between calculations for particles that
were assumed to have an overall dry diameter of 300 nm, but where (i) the particles are
well-mixed and non-absorbing, (ii) the particles are well-mixed and the BC
fraction is absorbing and (iii) the BC exists as an absorbing core with a
non-absorbing coating, i.e., in a core-shell morphology. The dry
particles are assumed to have 5 % by volume BC, 20 % ammonium sulfate
and 75 % OOA, giving a composite
Measurements of light extinction and light scattering by ambient particles
(PM
D. B. Atkinson, J. G. Radney, J. Lum, K. R. Kolesar, C. D. Cappa and Q. Zhang were supported by of the US Department of Energy (DOE) Office of Biological and Environmental Research (OBER), Atmospheric System Research (ASR) Program through Grants No. DE-SC0008937 and DE-FG02-11ER65293. A. Zelenyuk and M. S. Pekour were supported by ASR and the EMSL (Environmental Molecular Sciences Laboratory), a national scientific user facility sponsored by the DOE-OBER and located at Pacific Northwest National Laboratory. The authors thank R. Subramanian for use of the SP2 data. Additional funding for data collection at the ground sites (including of the SP2 data) was provided by the Atmospheric Radiation Measurement (ARM) Program of the DOE-OBER. Edited by: J. Thornton