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Solar Absorption by Aerosol Bound Nitrophenols Compared to Aqueous and Gaseous Nitrophenols Ryan Zahn Hinrichs, Pawel Buczek, and Jal Trivedi Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b00302 • Publication Date (Web): 13 May 2016 Downloaded from http://pubs.acs.org on May 19, 2016
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Solar Absorption by Aerosol Bound Nitrophenols
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Compared to Aqueous and Gaseous Nitrophenols.
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Ryan Z. Hinrichs*, Pawel Buczek, Jal Trivedi,
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Department of Chemistry, Drew University, Madison, NJ 07940
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*
[email protected], phone:973-408-3853, fax:973-408-3572
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ABSTRACT. Nitrophenols are well-know absorbers of near-UV/blue radiation and are
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considered to be a component of solar absorbing organic aerosol material commonly labeled
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brown carbon. Nitrophenols have been identified in a variety of phases in earth’s atmosphere,
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including the gaseous, aqueous and aerosol bound, and these different environments alter their
10
UV-visible absorption spectra, most dramatically when deprotonated forming nitrophenolates.
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We quantify the impact of these different absorption profiles by calculating the solar power
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absorbed per molecule for several nitrophenols. For instance, aqueous 2,4-dinitrophenol
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absorption varies dramatically over the pH range of cloud droplets with pH = 5.5 solutions
14
absorbing three times the solar power compared to pH = 3.5 solutions. We also measured the
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UV-visible spectra of 2-nitrophenol adsorbed on several aerosol substrates representative of
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mineral dust, inorganic salts and organic aerosol, and compare these spectra to gaseous and
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aqueous 2-nitrophenol. 2-Nitrophenol adsorbed on mineral and chloride aerosol substrates
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exhibit a red-shifted absorption band (~450-650 nm) consistent with 2-nitrophenolate and absorb
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twice the solar power per molecule compared to gaseous, aqueous and organic aerosol bound 2-
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nitrophenol. We also discuss how different nitrophenol absorption profiles alter important
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atmospheric photolysis rate constants [e.g., J(NO2) and J(O3)] by attenuating solar flux. 1 ACS Paragon Plus Environment
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KEYWORDS: UV-visible, absorption cross sections, 2-nitrophenol, 4-nitrophenol
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TOC Graphic
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INTRODUCTION
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The absorption and scattering of solar radiation by atmospheric aerosols exerts a direct effect on
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earth’s climate.1 Whether this radiative effect due to aerosol-radiation interactions (REari) results
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in net warming or cooling depends on the optical properties of aerosols, which in turn depends
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on their size, morphology and chemical composition. Most types of tropospheric aerosols,
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including sea salt, sulfates and nitrates, predominately scatter incoming solar radiation causing a
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net cooling of the lower atmosphere (i.e., negative REari).1-2 Black carbon, in contrast, absorbs
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solar radiation which results in a net heating (i.e., positive REari).1, 3 Organics comprise a
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significant fraction of aerosol mass, and recent attention has focused on identifying the
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composition and origin of “brown carbon,” which is the light absorbing fraction of aerosol
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organics.4-9 Recent modeling suggests that brown carbon may account for up to 19% of the solar
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absorption by anthropogenic aerosol.10 This absorbing fraction of organic aerosol causes the net
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direct radiative forcing of organic aerosol to change from net cooling (-0.08 W m-2) to net
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warming (+0.025 W m-2).10
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Field studies have characterized UV-visible absorbing organic material as humic-like substances
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(HULIS)4, 11-12 and have identified biomass combustion as a major source of brown carbon.5-6, 13-
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15
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aqueous and gaseous chemistry of biogenic and anthropogenic precursors are strong absorbers of
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UV-visible radiation.16-20 Attempts to identify the specific chromophores responsible for UV-
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visible absorption by organic aerosol are complicated by the complex and diverse composition of
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the compounds and photochemical aging of atmospheric aerosol. Systems observed to generate
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brown carbon in the laboratory include highly conjugated and oligomeric products of secondary
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organic aerosol (SOA),21-25 NH3/NH4+ mediated chemistry of SOA carbonyl products,16-19, 26 and
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nitrated aromatics.27-28 In reality, brown carbon is likely a complex mixture which varies with
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source region and aging, and recent work by Philips and Smith also suggests that charge transfer
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complexes enhance optical absorption in organic aerosol indicating that net absorption is greater
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than the sum of individual chromophore cross sections.29-30
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Nitrophenols, which are the focus of the present study, were identified early as strong absorbers
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in atmospheric systems,27 affecting both the radiative balance and decreasing photolysis rate
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constants of atmospheric photochemical reactions (e.g., NO2 and O3 photolysis in the boundary
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layer) by attenuating photon flux at lower altitudes.27, 31 Nitrophenols have been detected in
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many different phases in earth’s atmosphere, including the gas and aqueous phases32-34 and in
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biomass burning aerosol.28, 35 Mohr and colleagues measured the composition and optical
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properties of winter time wood burning aerosols finding several nitrophenolic compounds to
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account for 4% of the absorption at 370 nm.28 Field and laboratory studies have shown that
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phenols and methoxyphenols, major lignin pyrolysis products found in biomass combustion
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emissions, readily undergo atmospheric processing via reactions with NOx forming nitroaromatic
Laboratory studies have also demonstrated that some secondary organic material formed from
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compounds.12, 24-25, 36-38 Photo-oxidation of toluene, a major anthropogenic VOC, under high-
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NOx conditions also produces several nitroaromatic compounds that account for 40-60% of the
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total light absorption by toluene-SOA.39
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The primary goal of this study is to compare the UV-visible absorption spectra for
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atmospherically relevant nitrophenols in a variety of phases to quantify how changes in their
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absorption spectra affect their radiative forcing and alter photolysis rate constants by attenuating
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the solar flux. We first consider the impact of droplet pH on the absorption cross sections of six
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nitrophenols because it is well known that deprotonation of nitrophenols forming nitrophenolates
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significantly alters their absorption profiles.40-41 To quantify this effect, we compare their
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absorption cross sections with the solar flux to calculate the solar power absorbed on a per
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molecule basis. We then used diffuse reflectance UV-visible spectroscopy to record absorption
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spectra for 2-nitrophenol adsorbed on a range of aerosol substrates, including mineral dust,
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inorganic salts and an organic aerosol surrogate, to compare the solar absorption of particulate
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bound nitrophenols to gaseous and aqueous phases.
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METHODS
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Aqueous Phase Spectroscopy Aqueous UV-visible spectra were recorded using a Thermo
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Scientific Evolution 300 spectrophotometer with 1 nm resolution. All chemicals were supplied
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by Sigma-Aldrich and used without further purification: 2-nitrophenol (≥99%), 4-nitrophenol
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(spectrophotometric grade), 2,4-dinitrophenol (Fluka, 99.9%), 4-nitrocatechol (97%) and 4-
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nitroguaiacol (99%). Solutions were made with deionized water (> 18 MΩ-cm), and the pH was
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adjusted by the addition of dilute hydrochloric acid, sodium hydroxide or acetate buffers.
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Gas Phase Spectroscopy A 190 cm pathlength flow cell recorded gas phase UV-visible spectra
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for 2-nitrophenol (2NP), the only compound with high enough vapor pressure to study with this
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setup. Fiber optic cables connected an Ocean Optics high resolution spectrometer (HR2000) and
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a deuterium-halogen light source (DH4000) to either end of the flow cell. Gaseous 2-nitrophenol
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was generated by flowing UHP nitrogen (150 sccm) through a bubbler containing 2NP powder,
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which was located upstream of the flow cell. After establishing a stable absorption signal, the
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gaseous 2NP exiting the flow cell was directed through a bubbler containing 10 mL of 1M
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sodium hydroxide, which trapped 2NP as 2-nitrophenolate (2NP-) with a trapping efficiency of
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>99% as determined by analysis of two sequential NaOH traps. The trapped solution was then
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analyzed using aqueous UV-visible spectroscopy to quantify the amount of trapped 2NP– by
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comparison to a 2NP– standard calibration curve. The 2NP gas phase concentration was then
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calculated by combining the flow rate with the concentration of 2NP trapped during 10 minute
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intervals.
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Surface Adsorbed Spectroscopy Diffuse reflectance UV-visible spectroscopy monitored the
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absorption of 2NP adsorbed on several aerosol substrates. A nitrogen carrier gas flow introduced
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gaseous 2NP to the diffuse reflectance reaction chamber (Harrick Scientific) while spectra were
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recorded with the Evolution 300 spectrophotometer. Spectra for surface adsorbed 2NP are
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referenced to the diffuse reflectance of the aerosol substrate prior to exposure to 2NP, which
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eliminates complications due to scattering effects since diffuse reflectance scattering is not
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significantly altered by surface adsorbed 2NP. All aerosol surrogates were used as powders and
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include: kaolinite (Aldrich), α-alumina (Alfa Aesar 99.98%), γ-alumina (Alfa Aesar 99.997%)
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and rutile (Aldrich) to model mineral dust aerosol; sodium chloride (Aldrich), potassium chloride
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(Aldrich) and ammonium sulfate (Aldrich) as representative salt substrates; and azelaic acid 5 ACS Paragon Plus Environment
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(Aldrich 98%) as a model organic aerosol substrate. Particle BET surface areas ranged from 10-
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20 m2 g-1 for mineral samples to 0.1-0.2 m2 g-1 for salt and azelaic acid samples. The effect of
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relative humidity was also investigated by adjusting the relative flow rates of a dry carrier gas
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flow, which contained the gaseous 2NP, and a “wet” carrier gas flow generated by passing
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nitrogen gas through a fritted water bubbler held at constant temperature. A Vaisala gauge
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measured relative humidity.
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RESULTS AND DISCUSSION
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pH Affects UV-Visible Absorption of Aqueous Nitrophenols It is well known that the UV-
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visible absorption spectra of nitrophenols exhibit a red-shift when deprotonated at pH’s above
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their pKa’s. Figure 1 demonstrates this effect for 2,4-dinitrophenol (DNP), which has a pKa of
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4.04. At pH < 3 the absorbance peaks at 325 nm corresponding to a π→π* transition, while at pH
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> 6 dinitrophenolate (DNP-) exhibits an absorbance maximum at 425 nm associated with an
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n→π* transition. Absorption cross sections, base-e, were calculated from absorption spectra,
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log(Io/I), using the solution concentration, C, and cuvet pathlength, l.
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σ (λ ) =
Abs ln(10) C ⋅l
(E1)
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Cross section spectra for all nitrophenols were measured under acidic (i.e., protonated) and basic
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(deprotonated) conditions and were fit to a multi-peak Gaussian function:
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(λ − λ max,i ) 2 σ (λ ) = ∑ Ai exp − wi2 i
(E2)
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where Ai, λmax,i and wi represent the amplitude, center location and width of each Gaussian peak,
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respectively. Table 1 reports pKa’s and Gaussian peak-fit parameters for six protonated and
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deprotonated nitrophenols. Given that the pH of atmospheric droplets ranges from 3.5 to 5.5,42 of
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all the nitrophenols collated in Table 1 only DNP solutions might contain significant
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concentrations of nitrophenolates under atmospheric conditions. Figure S1 shows the relative
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population of DNP and DNP–, along with other nitrophenols, as a function of pH.
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The amount of solar energy absorbed by nitrophenols increases at high pH due to the red-shifted
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absorption maxima associated with nitrophenolates. Even though the nitrophenolate absorption
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maxima correspond to lower energy wavelengths, the increased solar flux at earth’s surface (Fig.
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1, dashed line) in this wavelength region results in the absorption of more photons. To quantify
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this effect, we calculated the molecular absorbed power (MAP) by integrating the product of the
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molecular absorption cross section, σ(λ), with the solar flux, F(λ,θ,z), which depends on
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wavelength, zenith angle and altitude.
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MAP =
∫ σ ( λ ) F ( λ , θ , z ) dλ
(E3)
λ > 290
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To calculate the impact of aqueous aerosol pH on the solar power absorbed by DNP, we
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calculated average solution cross sections as a linear combination of DNP and DNP– weighted by
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their relative population at different pH values (see Supporting Information Fig. S2 and Table
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S1). Figure 2 reports the solar power absorbed on a per molecule basis by aqueous DNP
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solutions (red data) as a function of pH, ranging from 3.5 to 5.5 to model atmospheric conditions,
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and zenith angle. The impact of pH is dramatic; at 0° zenith angle aqueous aerosols at pH = 5.5
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absorb 2.9 times more solar power compared to pH = 3.5 droplets. Although the total power
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absorbed decreases with increasing zenith angle, the impact of pH becomes greater with pH =
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5.5 solutions absorbing 3.3 times more solar power relative to pH = 3.5 solutions at a zenith
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Figure 1. Absorption cross sections for 2,4-dinitrophenol (blue) and 2,4-dinitrophenolate (red). Solar flux at earth’s surface and 0° zenith angle (dashed black). Table 1. UV-visible absorption peaks for aqueous nitrophenols. Absorption cross section fitting parameters Peak I
Peak II
Peak III
Compound
pKa
λmaxa
Ab
w
λmaxa
Ab
w
2-nitrophenol (2NP)
7.14
277.3
2.10
26.3
351.7
1.06
42.9
280.2
1.93
31.7
415.2
2.23
55.2
317.5
3.40
40.7
396.5
7.04
43.1
258.4
3.91
22.4
296.2
2.23
250.6
2.84
39.9
355.0
297.6
1.15
35.2
279.3
1.48
298.9 370.0
2-nitrophenolate 4-nitrophenol (4NP)
7.23
4-nitrophenolate 2,4-dinitrophenol (DNP)
4.04
2,4-dinitrophenolate 4-nitrocatechol (4NC)
6.87
4-nitrocatecholate 4-nitroguaiacol (4NG) 4-nitroguaiacolate
7.05
λmaxa
Ab
w
23.8
326.6
1.73
38.4
4.83
35.4
407.3
3.12
34.9
351.6
1.94
41.0
20.7
384.0
2.05
45.1
512.0
3.79
70.8
1.09
25.1
347.6
2.55
41.5
0.76
73.4
432.1
5.81
45.1
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2-nitro-1-naphthol (2N1N) 2-nitro-1-naphtholate 152 153
6.13
265.9
6.83
25.5
318.0
0.85
18.4
402.1
0.67
55.1
280.3
1.86
38.2
322.5
0.23
13.8
449.4
1.20
50.9
a
Peak wavelength in nm determined by multi-peak Gassian fit. b Cross section × 10-17 (cm2 molecule-1) for individual Gaussian peak determined by multi-peak fit.
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Figure 2. Power absorbed per molecule (MAP) for various nitrophenol aqueous solutions as a function of pH and zenith angle.
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angle of 78°. Over the 3.5-5.5 range of atmospherically relevant pH’s, the solar power absorbed
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by aqueous 2-nitrophenol (2NP), 4-nitrophenol (4NP), 4-nitrocatechol (4NC), 4-nitroguaiacol
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(4NG) and 2-nitro-1-naphthol (2N1N) do not vary significantly due to their comparatively high
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pKa’s (Fig. 2).
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UV-Visible Absorption by Gaseous 2-Nitrophenol Gaseous 2-nitrophenol (2NP) measured in
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the present study exhibited an absorption maximum at 332 nm with an absorption cross section
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of σ = 1.41 × 10-17 cm2 molecule-1 (supporting information Fig. S2). Our spectrum is in good
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agreement with the work of Chen and colleagues,31 in which 2NP absorption peaked at 336 nm
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with σ = 1.681 × 10-17 cm2 molecule-1. In comparison, UV-visible absorption by aqueous 2NP 9 ACS Paragon Plus Environment
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peaks at 350 nm with σ = 1.04 × 10-17 cm2 molecule-1. Although gaseous and aqueous 2NP
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display slightly different absorption maxima, it is interesting to note that the integrated cross
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section associated with this lowest energy transition is similar for both gaseous (8.0 × 10-16 cm2
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molecule-1 nm) and aqueous (7.9 × 10-16 cm2 molecule-1 nm) environments.
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UV-Visible Absorption by Surface Adsorbed Nitrophenols Diffuse reflectance UV-visible
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spectroscopy recorded the absorption spectra of 2NP adsorbed on several aerosol substrates,
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including mineral dust (kaolinite, α-alumina, γ-alumina and rutile), inorganic salt substrates
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(sodium chloride, potassium chloride and ammonium sulfate), and a representative organic
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aerosol (azelaic acid). Since the pathlength associated with diffuse reflectance spectroscopy is
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unknown, it is impossible to directly calculate the absorbance cross sections using E1. Therefore,
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we converted the surface adsorbed 2NP spectra to cross sections assuming that the integrated
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absorption cross section for the lowest energy transitions of surface adsorbed 2NP was similar to
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that measured for gaseous and aqueous 2NP, as discussed above. Figure 3 displays the surface
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adsorbed cross sections as mass absorption coefficients (MAC), calculated using equation E4, for
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2NP adsorbed on eight aerosol substrates at 30% RH.
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MAC(λ ) =
σ (λ ) ⋅ N A MW2 NP
(E4)
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NA is Avagadro’s number and MW2NP is the molecular weight of 2NP. For comparison, the MAC
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of aqueous 2NP is plotted (dashed light blue line) in each panel along with literature values for
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moderately and strongly absorbing brown carbon (squares and circles, respectively).10 Varying
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relative humidity from 0 to 30% had negligible impact on the absorption spectra for all substrates
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except NaCl and KCl, which are discussed below. All surface adsorbed cross sections were peak
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fit using equation E2 and the Gaussian peak fitting parameters for all substrates are reported in
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Supporting Information Table S1.
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Figure 3. Mass absorption coefficients for 2-nitrophenol adsorbed on kaolinite (red), α-alumina (brown), γ-alumina (orange), rutile (purple), NaCl (dark blue), KCl (blue-gray), NH4SO4 (green), and azelaic acid (brown). For comparison, each panel also includes the MAC for aqueous 2NP (dashed, light blue), moderately absorbing brown carbon (squares) and strongly absorbing brown carbon (circles).
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As seen in Fig. 3, many of the surface adsorbed spectra exhibit absorption profiles that extend
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well into the visible range, in some cases as far out as 650 nm. 2NP adsorbed on all substrates
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exhibited an absorption band in the range of 345 – 367 nm, which we attribute to 2NP
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physisorbed on each aerosol substrate based on its similarity to the 350 nm peak observed for
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aqueous 2NP. It is also evident that all mineral aerosol surrogates display a second red-shifted
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peak at, for example, 443 nm and 512 nm for kaolinite and γ-alumina, respectively. Such lower
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energy transitions are near wavelengths similar to the 415 nm peak observed for aqueous 2-
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nitrophenolate. We therefore propose that these absorption bands result from surface adsorbed
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species with electronic structure similar to 2-nitrophenolate, possibly formed via interactions
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with surface hydroxyl groups on the mineral substrates, which have been shown to be displaced
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by gaseous organics such as formic acid producing formate and methanol forming methoxide.43-
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44
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aqueous solutions, we estimate that 20-30% of the surface adsorbed molecules are deprotonated
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on the various mineral substrates.
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Red-shifted absorption bands were also observed for chloride salts at 30% RH but not when the
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salts were baked dry. Laskin and colleagues recently reported the deprotonation of weak organic
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acids on chloride salt aerosols under decreasing relative humidity conditions driven by the
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release of gaseous hydrochloric acid.45-46 Such chemistry is consistent with the observation of
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2NP– absorption bands for NaCl and KCl at 30% RH:
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Assuming that the integrated cross sections for surface adsorbed 2NP and 2NP– are similar to
2NP(ads) + Cl–(substrate) → 2NP–(ads) + HCl(g)
(R1)
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2NP adsorbed on ammonium sulfate and azelaic acid exhibited UV-visible absorption spectra
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similar to aqueous 2NP indicating that no adsorbed molecules were deprotonated. The results for
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2NP adsorbed on azelaic acid, a model for organic aerosol, are consistent with a photolysis
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investigation by Lignell et al. where DNP embedded in an α-pinene ozonolysis secondary
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organic material matrix were not deprotonated.47
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Figure 4 presents the solar power absorbed per molecule for surface adsorbed 2NP calculated
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using equation E3. As was noted for aqueous nitrophenols, the appearance of the red-shifted
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peak associated with nitrophenolates results in higher molecular absorbed power, with mineral
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and NaCl adsorbed 2NP absorbing roughly twice the solar energy compared to gaseous and
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aqueous 2NP as well as 2NP adsorbed on NH4SO4 and azelaic acid. 12 ACS Paragon Plus Environment
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Figure 4. Power absorbed per molecule (MAP) for 2-nitrophenol as a function of zenith angle for gaseous, aqueous and adsorbed on mineral, salt and organic aerosol substrates.
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Atmospheric Implications Adsorption of 2NP on mineral and chloride aerosol surrogates
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results in the deprotonation of a significant fraction of surface adsorbed molecules, and this
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process causes the UV-visible absorption cross section to red-shift as far out as 650 nm. The
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effective deprotonation of aerosol bound 2NP is notable given that aqueous 2NP is not
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deprotonated to a significant extent in atmospheric droplets due to its relatively high pKa of 7.14.
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Although we were not able to measure surface-adsorbed absorption spectra for other
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nitrophenols due to their low vapor pressures, we expect that similar interactions will also cause
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red-shifted absorption spectra when adsorbed on mineral and chloride aerosol. It is important to
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note that although many nitrophenols are not that volatile, they may be generated on atmospheric
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aerosols via heterogeneous chemistry of NOx with surface bound aromatics.24-25, 38 These results
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indicate that nitrophenols, which are thought to be strong absorbers in the near-UV around 370
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nm, may also absorb visible radiation depending on their micro-environment in the atmosphere.
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In fact, several MACs shown in Fig. 3 (kaolinite, alumina and NaCl) fall midway between the 13 ACS Paragon Plus Environment
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MAC values for moderate and strongly absorbing brown carbon out to 650 nm. Nitrophenols are
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known constituents of biomass burning aerosol, which contain complex mixtures of soot,
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organics, and potassium salts. Whether nitrophenols associated with biomass burning aerosol are
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deprotonated will depend on the specific matrix. While nitrophenols embedded in soot and
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organic regions will likely remain protonated, the potassium salts, initially as chloride and
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converting to nitrates with aging, may deprotonate the nitrophenols through the release of
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gaseous acids. Hand and colleagues also observed extensive internal mixing of biomass burning
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aerosol with mineral dust, providing a scenario where nitrophenol absorption spectra may shift
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out into the visible region.48 In such cases, absorption of solar radiation by nitrophenols will
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increase the radiative forcing of these aerosols. It should also be noted that absorption of UV-
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visible radiation by nitrophenols can also result in their photodegradation. The quantum yields
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for nitrophenol photolysis are on the order of 10-4,49-50 which means photolysis will not affect the
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radiative forcing discussed above, but will impact the atmospheric lifetime of nitrophenols.
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The absorption of solar UV radiation by atmospheric nitrophenols can also alter the oxidative
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capacity of the troposphere by attenuating solar flux in wavelength regions responsible for NO2
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and O3 photolysis:27, 31
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NO2 + hυ(λ
