Article pubs.acs.org/ac
Optical Properties of Dispersed Aerosols in the Near Ultraviolet (355 nm): Measurement Approach and Initial Data Lulu Ma and Jonathan E. Thompson* Department of Chemistry & Biochemistry, MS1061, Texas Tech University, Lubbock, Texas 79409, United States S Supporting Information *
ABSTRACT: An aerosol albedometer combining cavity ring-down spectroscopy (CRDS) with integrating sphere nephelometry was developed for use at λ = 355 nm. The instrument measures extinction and scattering coefficients of dispersed particulate matter in the near ultraviolet (UV) spectral region. Several samples have been analyzed, including: ammonium sulfate, secondary organic aerosols (SOA) resulting from the ozonolysis of α-pinene and photooxidation of toluene, redispersed soil dust samples, biomass burning aerosols, and ambient aerosols. When particle size and number density were experimentally controlled, extinction coefficients and scattering coefficients were found to have a linear relationship with particle number concentration, in good agreement with light scattering theory. For ammonium sulfate and pinene samples, extinction cross sections for sizeselected (Dp = 300 nm) samples were within the range of 1.65−2.60 × 10−9 cm2 with the largest value corresponding to ammonium sulfate and the lowest value for pinene SOA. The scattering cross sections of pinene and ammonium sulfate aerosols were indistinguishable from the extinction cross sections, indicating that these particle types had minimal light absorption at 355 nm. However, soil dusts and biomass burning aerosols showed significant absorption with single scatter albedo (SSA) between 0.74 and 0.84. Ambient aerosols also had transient absorption at 355 nm that correlated well with a particle-soot absorption photometer (PSAP) measuring visible light absorption.
F
between scattering and absorption can be described through the aerosol single scatter albedo (SSA, unitless) defined as:
ine solid and liquid particles suspended in the atmosphere (aerosols) scatter and absorb solar radiation, and this causes a climate effect that is often quantitatively estimated by the aerosol’s “direct radiative forcing” (RF).1,2 Compared to well-mixed greenhouse gases that absorb long-wave radiation, aerosols have influence mainly on short-wave radiation with a total direct RF of approximately −0.5 W·m−2.1−3 A positive RF indicates a heating influence, while a negative RF suggests a cooling influence. Light scattering, absorption, and extinction coefficients (bscat, babs, bext; m−1) are the quantitative variables often used to describe these differing effects. The extinction coefficient (bext) is the sum of the effects of absorption and scattering (see eq 1) and is defined through eq 2. bext = bscat + babs
(1)
Iz = I0e−(bextz)
(2)
ω=
(3)
For those aerosols that have no absorption, extinction of light is only composed of scattering, and SSA equals one. If light is absorbed, SSA will fall below one. In this work, the optical properties of aerosols are studied by combining cavity ring-down spectroscopy (CRDS)4−8 and integrating sphere nephelometry. The innovative aspect of this project is the use of a pulsed Nd:YAG laser at λ = 355 nm as a light source to access the near ultraviolet (UV) region. The literature record consists of many papers that study the optical properties of aerosols in the visible spectral range; however, there are far fewer studies in the near UV. Of available data, many studies involve column integrated inversions of direct and
In eq 2, Iz, and I0 are irradiances at point z meters and at z = 0 m, respectively. Light scattering leads to a cooling of climate, while strong absorption can lead to a warming. The interplay © 2012 American Chemical Society
bscat bext
Received: March 1, 2012 Accepted: June 8, 2012 Published: June 8, 2012 5611
dx.doi.org/10.1021/ac3005814 | Anal. Chem. 2012, 84, 5611−5617
Analytical Chemistry
Article
Figure 1. Schematic of the measurement apparatus. The electrostatic classifier was used to select 300 nm diameter particles for (NH4)2SO4 and SOA experiments but removed for experiments on dust, ambient aerosol, and biomass burning aerosols. The 3rd harmonic of a Nd:YAG laser was used to produce 5 ns pulses at λ = 355 nm.
diffuse sky radiances from radiometer and/or sun-photometer data. A recent example of this is found in Corr et al.9 These authors estimate near UV SSA in the vicinity of 0.8 for the Mexico City metropolitan area and also present a summary of additional inversions presented in the literature for a variety of global locations with resulting SSA values ranging from 0.65 to nearly 1.0 for the 300−400 nm spectral window. Unfortunately, extracting aerosol optical data through these inversion methods can be prone to difficulties due to variable light absorption by atmospheric gases aloft and poorly constrained surface reflectivity estimates in the UV. The latter alters actinic flux and consequently the observed radiances. In addition, the process is time-consuming, and measurements can only be made during the day and in the absence of cloud cover. UV light is known to play a very important role in tropospheric photochemistry, and significant recent evidence suggests that a class of material present in atmospheric particulate matter (commonly called “brown-carbon”) absorbs light strongly in the near UV spectral window.10−14 The potential impact of this material on atmospheric radiative transport and photochemistry makes the development of techniques for single-point measurement of light scattering and absorption between 290 and 400 nm a research priority for the atmospheric chemistry community. In addition, active, single point measurements of aerosol optics offer the ability to manipulate the aerosol sample chemically or physically prior to measurement. For instance, thermo-denuders can be used to remove volatile material, or the effect of relative humidity on optical signatures can be studied. This simply cannot be
accomplished using columnar inversions. At present, it appears that only photoacoustic spectroscopy has been employed to make point measurements of aerosol absorption in the UV spectral window.15,16 This manuscript documents an alternative measurement instrument and reports initial optical data for several types of common aerosols for the near UV (355 nm). It is envisioned that the measurement method and resulting data will assist in improving models of radiative transport in atmosphere.
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OVERVIEW OF EXPERIMENTAL METHODS Figure 1 is a schematic of the instrument setup. The apparatus used is very similar to that reported in Thompson et al.17,18 with the major exception of the different measurement wavelength. To summarize the experimental work, the instrument sensitivity and precision has been evaluated through a series of experiments using filtered air and R-134a gas (tetraflouroethane). We find limits of detection for bscat, bext, and babs of 12, 11, and 16 Mm−1, respectively, for a 52 s integration time. In addition, optical measurements were made on size-selected 300 nm diameter (Dp) ammonium sulfate and pinene aerosols. Additional measurements were made on redispersed soil-dust proxies, toluene secondary organic aerosols (SOA), and biomass burning samples. Measurements on ambient aerosols have also been made with the device, and these were compared to measurements made simultaneously with commercial products. Details of particle generation, dust size distribution, data treatment, and optical measurements are reported in the Supporting Information. 5612
dx.doi.org/10.1021/ac3005814 | Anal. Chem. 2012, 84, 5611−5617
Analytical Chemistry
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Article
our previous instrument at λ = 532 nm was able to achieve.17 The cause of this is the poorer mirror reflectivity of UV optics compared to the visible. This decreases the effective sample path length for the experiment and reduces ring-down times, which increases relative error given a fixed imprecision in determining the ring-down constant, τ. Absorption is measured by the difference between bext and bscat. As such, we can propagate standard deviation to babs using the standard approach,19 and the result is 5.2 Mm−1. This is in good agreement with the standard deviation of babs observations for zero air in Figure 2A. The limit of detection for babs (3σ) for single point measurements is therefore 16 Mm−1 at 355 nm. Figure 2B,C also illustrates how the variability in measured single scatter albedo (SSA) scales with the magnitude of extinction/scatter coefficient. Figure 2B is a plot of reported SSA in time as air is switched to R-134a (same X axis as in Figure 2A). As observed, when bext is near zero, SSA is highly variable but when bext increases, the precision of the SSA measurement improves significantly. This is a consequence of the SSA being a ratio of two measured values, each with its own imprecision. Propagation of relative uncertainty to SSA from the std. dev. of replicate bscat and bext measurements yields the plot shown in Figure 2C, which suggests bext ≥ 60 Mm−1 is required to achieve
