Light penetration in the snowpack at Summit, Greenland: Part 1: Nitrite and hydrogen peroxide photolysis

doi:10.1016/j.atmosenv.2006.04.072

Atmospheric Environment

Volume 41, Issue 24, August 2007, Pages 5077-5090

https://doi.org/10.1016/j.atmosenv.2006.04.072 Get rights and content

Abstract

Photochemical rate constants (j values) are crucial indicators for evaluating the importance of photochemical reactions in environmental systems. While measurement of aqueous j values via chemical actinometry is relatively straightforward under most conditions, problems arise with ambient conditions below freezing, such as at very high latitudes or altitudes. To address this problem, we have developed a new method for low temperature actinometry using solutions of acetonitrile (ACN) and water, which have freezing points down to $- 44^{\circ} C$ . In this method we measure the formation of phenol from the photolysis of an ${}^{•}{OH}$ -generating chromophore in the presence of benzene. Using results from laboratory tests we correct our phenol field results in $ACN / H_{2} O$ to rate constants for chromophore photolysis expected for water–ice (i.e., in the quasi-liquid layer of snow grains) under the same conditions.

In part 1 of this study, we use this method at Summit, Greenland on the surface snow and to depths of $\sim 30 cm$ using hydrogen peroxide (HOOH) and nitrite $({NO}_{2}^{-})$ as the chromophores. While the method works well for determining the rate constant for HOOH photolysis (j(HOOH)), we encountered problems using the technique with nitrite. However, measured PhOH formation rate constants for nitrite in acetonitrile, $j_{{NO}_{2}^{-} \to PhOH}^{ACN}$ , still provide an excellent means for calculating snowpack $e$ -folding depths for ${NO}_{2}^{-}$ photolysis (i.e., the depth over which the rate constant decreases by a factor of $e$ ). Values of j(HOOH) and $j ({NO}_{2}^{-})$ determined from measurements of actinic flux (above the snow) and irradiance (in snow) suggest that the value of j(HOOH) on the surface snow at midday was $8.6 \times 10^{- 7} s^{- 1}$ in mid-March and increased by 300% by the start of May, while $j_{{NO}_{2}^{-} \to PhOH}^{ACN}$ midday surface values were consistently $(1 - 3) \times 10^{- 7} s^{- 1}$ throughout the season. Within the snowpack, average $e$ -folding depths determined from chemical actinometry were $13.3 (\pm 0.88) cm$ for j(HOOH) and $16.3 (\pm 4.2) cm$ for $j_{{NO}_{2}^{-} \to PhOH}^{ACN}$ ; $e$ -folding depths determined from in-snow spectral radiometer measurements of irradiance were similar. The larger $e$ -folding depth for nitrite is because this chromophore absorbs at longer wavelengths where there is less light extinction in the snow.

Introduction

Snowpack photochemical reactions can influence the chemical composition of snow grains, the pore space (firn air) between grains, and the overlying atmosphere. For example, field studies in the Arctic and Antarctic indicate that the photolysis of snowpack nitrate releases ${NO}_{x}$ and HONO to the overlying air, resulting in significant effects on the ${HO}_{x}$ budget in the firn and atmospheric boundary layer (Dominé and Shepson, 2002, Honrath et al., 1999, Honrath et al., 2000). To calculate the rates of photochemical reactions in the snowpack, and the resulting fluxes of volatile species out of the snowpack, we must have an accurate understanding of snowpack photochemical rate constants, i.e., j values. In turn, determining these rate constants requires that we understand the depth dependence of actinic flux within the snowpack, which is controlled by the actinic flux incident upon the snow surface as well as the radiative properties of the snowpack (Lee-Taylor and Madronich, 2002, Simpson et al., 2002).

One approach to understanding the actinic flux and photolysis rate constants in snowpacks is the use of chemical actinometry. In this technique a solution of light absorbing chemical (chromophore) is put inside a series of UV-transparent tubes that are buried within the snowpack at known depths. Sunlight illumination of these chromophores initiates a chemical reaction and the extent of reaction is determined by measuring the amount of stable product formed after a given exposure time. The advantage of chemical actinometry is that it is a more direct measure of the j value, while photochemical calculations of chromophore reactions could be affected by uncertainties in molar absorptivities or quantum yields.

While a number of past researchers have used solution actinometers based on the production and trapping of ${}^{•}{OH}$ (e.g., Jankowski et al., 1999, Jankowski et al., 2000), there has been only one past report of chemical actinometry measurements in snow. This work by Qiu et al. (2002) examined the photolysis of nitrate at Summit using an ${}^{•}{OH}$ -trapping mechanism established by Vaughan and Blough (1998) except that 99% acetonitrile/1% water was used as the solvent. One disadvantage of their technique is that it involves three reaction steps after ${}^{•}{OH}$ formation and requires degassing of solutions and flame sealing of actinometry tubes to prevent contamination from oxygen, which interferes with their ${}^{•}{OH}$ -trapping method. Finally, their method does not consider that reaction yields in acetonitrile might be temperature dependent and therefore require corrections (Qiu et al., 2002).

Our goal in this work is to develop an alternative method that works in cold weather conditions, but that is simpler and that quantitatively accounts for the use of acetonitrile as solvent as a function of exposure temperature. While the method we have developed can be used with any ${}^{•}{OH}$ -generating chromophore, here we describe its use with hydrogen peroxide and nitrite at Summit, Greenland. In the accompanying paper, we use the actinometry method with nitrate (Galbavy et al., 2007). In both papers we determine j values for photolysis of these chromophores on snow grains and characterize the attenuation of light in the snowpack.

Section snippets

Actinometry overview

With our actinometry system we measure the rate of ${}^{•}{OH}$ formation from photolysis of a chromophore $(i)$ in aqueous acetonitrile (ACN) then convert this rate to the corresponding rate constant in water–ice, $j_{i \to OH}^{ICE}$ or $j (i)$ , using laboratory-determined conversion factors. We use high concentrations of benzene to trap the photoformed ${}^{•}{OH}$ and convert it to phenol (Anastasio and McGregor, 2001): $i + h ν \to {}^{•}{OH},$ ${}^{•}{OH} + Benzene (C_{6} H_{6}) \to Phenol (PhOH) + other products .$

Using this method we conducted field experiments

$j$ (HOOH) at the surface and in the snowpack

We performed HOOH actinometry in nine separate pits on four different days and measured surface values of j(HOOH) during an additional seven days. Fig. 2(a) displays the rates of phenol (PhOH) formation we measured in HOOH actinometry tubes in snowpits on 24 April 2004. The HOOH actinometry is very sensitive, able to measure photolysis at depths below 25 cm in the snowpack throughout the day. The results also demonstrate the good exponential decays we observed for all chromophores tested.

Conclusions

The primary goal in this study was to create a chemical actinometry method that can determine photolysis rate constants on snow grains in order to examine chromophore photolysis and light attenuation in the snowpack. Based on our spring field testing at Summit, Greenland, our method works well for HOOH but is less successful with nitrite. However, measurements of the rate constant for PhOH formation from ${NO}_{2}^{-}$ photolysis in acetonitrile, $j_{{NO}_{2}^{-} \to PhOH}^{ACN}$ , can still be useful as long as the snowpack

Acknowledgments

First and foremost, we thank the Arctic System Science Program in the NSF Office of Polar Programs for funding (Grant #0221052) and the Greenland Home Rule Ministry of Environment and Nature for granting us permission to do research at Summit. Also, special thanks are given to the people of VECO Polar for superb logistical support, Liang Chu and Shauna McKellar for lab assistance, and members of the Summit field team (especially Jack Dibb for leading the campaign, and Mary Albert and Zoe

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However, most liquid-phase actinometer systems utilize water as the solvent and are unsuitable for use in arctic environments where the temperature routinely drops well below 0 °C. Liquid-phase actinometer systems suitable for arctic use have been used for measuring the photodissociation rate constant of hydrogen peroxide (Galbavy et al., 2007a) or nitrate (Qiu et al., 2002; Galbavy et al., 2007b) in Greenland snowpack using acetonitrile (Tf = − 43.8 °C) as the solvent. In these measurements, flame-sealed glass tubes containing an actinometric solution were buried in snowpack and exposed to ambient light for up to 24 h before excavation.
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Sumner and Shepson (1999) first reported very active photochemistry in the polar snowpack, leading to oxidation of organics and the formation of products such as HCHO. This photochemistry also generates oxides of nitrogen including HONO (Jones et al., 2000; Jones et al., 2001; Zhou et al., 2001; Beine et al., 2002; Wolff et al., 2002; Chu and Anastasio, 2003; Cotter et al., 2003; Chu and Anastasio, 2007; George and Anastasio, 2007; Grannas et al., 2007; Wang et al., 2007; Galbavy et al., 2007a; Galbavy et al., 2007b; Davis et al., 2008) and it is clear that OH(g) radicals play a significant role in the snowpack (Beyersdorf et al., 2007). However, the present study shows that although the reaction probability for the OH(g) oxidation of bromide at the surface is expected to be large, this interface reaction is overwhelmed by the very fast bulk aqueous-phase chemistry.
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We measured the photoformation of hydroxyl radical ( $^{\cdot} OH$ ) on snow grains at Summit, Greenland during the spring and summer. Midday rates of $^{\cdot} OH$ formation in the snow phase in the summer range from 130 to $610 nmol L^{- 1} h^{- 1}$ , expressed relative to the liquid equivalent volume of snow. Calculated formation rates of snow-grain $^{\cdot} OH$ based on the photolysis of hydrogen peroxide and nitrate agree well with our measured rates during summer, indicating that there are probably not other major sources of $^{\cdot} OH$ under these conditions. Throughout both the spring and summer, HOOH is by far the dominant source of snow-grain $^{\cdot} OH$ ; on average, HOOH produces approximately 100 times more $^{\cdot} OH$ than does ${NO}_{3}^{-}$ . Rates of $^{\cdot} OH$ photoformation have a strong seasonal dependence and increase by approximately a factor of 10 between early spring and summer at midday. The rate of $^{\cdot} OH$ photoformation on snow grains decreases rapidly with depth in the snowpack, with approximately 90% of photoformation occurring within the top 10 cm, although $^{\cdot} OH$ formation occurs to depths below 20 cm. The formation of $^{\cdot} OH$ on snow grains likely initiates a suite of reactions in the snowpack, including the transformation of organic carbon (OC) and oxidation of halides. The reaction of $^{\cdot} OH$ with OC probably forms a number of volatile organic compounds (VOCs) that are potentially emitted into the atmospheric boundary layer. Indeed, our measured rates of $^{\cdot} OH$ photoformation on snow grains are large enough that they could account for previously reported fluxes of VOCs from the snowpack at Summit, although the relative importance of thermal desorption and photochemical production for most of these VOCs still needs to be determined.

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Light penetration in the snowpack at Summit, Greenland: Part 1: Nitrite and hydrogen peroxide photolysis

Abstract

Introduction

Section snippets

Actinometry overview

j(HOOH) at the surface and in the snowpack

Conclusions

Acknowledgments

Atmospheric Environment

Atmospheric Environment

Atmospheric Environment

Atmospheric Environment

Quantum yields of hydroxyl radical and nitrogen dioxide from the photolysis of nitrate on ice

Journal of Physical Chemistry A

Quantum yields for the formation of hydroxyl radical from the photolysis of hydrogen peroxide on ice

Journal of Physical Chemistry A

Direct measurements of ozone and nitrogen-dioxide photolysis rates in the troposphere

Journal of Geophysical Research—Oceans and Atmospheres

Air–snow interactions and atmospheric chemistry

Science

Photodecomposition of nitrite and undissociated nitrous acid in aqueous solution

Journal of Physical Chemistry

$j$ (HOOH) at the surface and in the snowpack