A reassessment of HOx South Pole chemistry based on observations recorded during ISCAT 2000

Abstract

Reported here are modeling results based on ISCAT (Investigation of Sulfur Chemistry of Antarctic Troposphere) 2000 observations recorded at the South Pole (SP) during the Austral Summer of 2000. The observations included a comprehensive set of photochemical parameters, e.g., NO, O 3 , and CO. It is worthy to note that not only were OH and HO 2 observed, but also HO x precursor species CH 2 O, H 2 O 2 , and HONO were measured. Previous studies have suggested that HONO is the major source of OH/HO x in the Arctic; however, observed HONO levels at SP induced dramatic model overprediction of both HO x and NO x when used to constrain the model calculations. In contrast, model predictions constrained by observed values of CH 2 O and H 2 O 2 are consistent with the observations of OH and HO 2 (i.e., within 20%) for more than half of the data. Significant model overpredictions of OH, however, were seen at the NO levels lower than 50 pptv or higher than 150 pptv. An analysis of HO x budget at the median NO level suggests that snow emissions of H 2 O 2 and CH 2 O are the single most important primary source of SP HO x , contributing 46% to the total source. Major sinks for HO x are found to be dry deposition of HO 2 NO 2 and HNO 3 as well as their reactions with OH. Although ISCAT 2000 led to a major progress in our understanding of SP HO x chemistry, critical aspects of this chemistry are still in need of further investigation.

Possible contribution of triboelectricity to snow - air interactions

“…It has been suggested that recorded levels of OH are a product of photochemical transformations at the snow-air interface. Molecules and radicals such as HCHO, [22] NO x , [23] HONO, [24] hydrogen peroxide, [25] higher aldehydes, [26] and HO 2 radicals, [27] produced by other photochemical processes taking place in the snowpack, are considered as sources of OH formation Measured levels of these compounds were used in calculations by a steady-state photochemical box model. [27,28] For data obtained in 2000 at the South Pole, model predictions of OH were accurate only in a certain range of NO levels.…”

“…Molecules and radicals such as HCHO, [22] NO x , [23] HONO, [24] hydrogen peroxide, [25] higher aldehydes, [26] and HO 2 radicals, [27] produced by other photochemical processes taking place in the snowpack, are considered as sources of OH formation Measured levels of these compounds were used in calculations by a steady-state photochemical box model. [27,28] For data obtained in 2000 at the South Pole, model predictions of OH were accurate only in a certain range of NO levels. [4,27] For data obtained at Summit, the discrepancy between model and measured values was found to depend strongly on meteorology: [6] when the wind was light (,6 m s À1 ) the measured value was twice the calculated values, whereas in high wind (.6 m s À1 ) and blowing snow the measured value was an order of magnitude higher than the calculated value.…”

“…[27,28] For data obtained in 2000 at the South Pole, model predictions of OH were accurate only in a certain range of NO levels. [4,27] For data obtained at Summit, the discrepancy between model and measured values was found to depend strongly on meteorology: [6] when the wind was light (,6 m s À1 ) the measured value was twice the calculated values, whereas in high wind (.6 m s À1 ) and blowing snow the measured value was an order of magnitude higher than the calculated value. [6] Thus, the model calculations that have used only photochemical reactions have underestimated the level of OH in all cases.…”

Investigating the photo-oxidative and heterogeneous chemical production of HCHO in the snowpack at the South Pole, Antarctica

“…However, although measurement campaigns at the South Pole have confirmed low to negligible background concentrations of non-methane hydrocarbons (NMHCs), unexpectedly high concentrations of short-lived photochemical species have been observed. For example, elevated mixing ratios of ozone (O 3 ), NO y [1][2][3][4][5] and HO x [6][7][8] have been observed indicating that the South Pole boundary layer can be a highly oxidising environment. [7][8][9][10] NO x is emitted from the snowpack due to the photolysis of NO 3 À contained within the ice.…”

“…For example, elevated mixing ratios of ozone (O 3 ), NO y [1][2][3][4][5] and HO x [6][7][8] have been observed indicating that the South Pole boundary layer can be a highly oxidising environment. [7][8][9][10] NO x is emitted from the snowpack due to the photolysis of NO 3 À contained within the ice. [11][12][13] Other snowpack emissions have also been identified, such as HCHO and H 2 O 2 , within the interstitial air and in the overlying atmosphere across Antarctica and, specifically, at the South Pole.…”