Peroxyacetyl nitrate photochemistry and interactions with the Arctic surface

Abstract

[1] Peroxyacetyl nitrate (PAN) is a NO x reservoir compound that has the ability to transport NO x to remote environments, allowing for NO x photochemistry and/or deposition of nitrogen to these clean locations. Measurements of PAN have been made at Alert, Nunavut, and Summit, Greenland, aimed at understanding the impact of PAN chemistry on atmospheric nitrogen in the Arctic. These measurements show concentrations of PAN that are only slowly varying, even during ozone depletion events at polar sunrise, when free radical photochemistry is relatively active. We used a zerodimensional photochemical model incorporating known gas-phase chemistry to simulate the observed behavior of PAN at Alert, Nunavut, and Summit, Greenland. The model simulations suggest a substantial net production rate for PAN over sunlit surfaces, which is inconsistent with the measured gas-phase concentrations. These observations thus indicate a fundamental problem with our understanding of PAN chemistry in low-temperature, snow-covered environments. We explore the possibility that we are missing an important sink for atmospheric PAN above snow-covered surfaces. If the loss is caused by snowpack deposition, the data result in calculated deposition velocities ranging from 0.05 to 1 cm s À1 , which would represent a significant fraction of the unidentified total nitrate input to the snowpack and glacial ice at Summit, Greenland.

Seasonal variation of peroxyacetylnitrate (PAN) in coastal Antarctica measured with a new instrument for the detection of sub-part per trillion mixing ratios of PAN

“…The levels seen at Halley are somewhat lower than PAN levels typically observed in the MBL in the Southern Ocean and South Atlantic in early March (Jacobi and Schrems, 1999) though it is recognised that our data was obtained earlier in the season than those obtained in the South Atlantic. They are also much lower than the mixing ratios observed at Ny-Ålesund, Svalbard (78.9 • N, 11.9 • E) in the comparable arctic seasons (Beine and Krognes 2000) and those seen at Alert, Nunavut (82.5 • N, 62.5 • W) and Summit,Greenland (72.5 • N, 38.4 • W) (Dassau et al, 2004). This is not surprising given the much greater distances from sources of pollution in Antarctica compared to the Arctic and the lower levels of PAN precursors in the southern hemisphere in general.…”

“…Whilst the observed median PAN concentrations are highest in winter and lowest in summer, and although the median PAN and ozone values follow a very similar pattern, the much greater day to day variation in PAN mixing ratios means that any seasonal cycle inferred from these data alone are tentative. The alkene data presented here for the July to January period show no clear seasonal pattern, but the full cycle peaks in April-May with a sharp decline into July (Read et al, 2007) For the measurement period the daily local gross production rate of PAN was estimated, in the same manner as Dassau et al (2004), using the following simplified reaction scheme and assuming acetaldehyde was in steady state and was the only source of the PA radical.…”

“…While there have been many studies of PAN in remote northern hemisphere locations, with several of the arctic (Bottenheim and Gallant, 1989;Beine and Krognes, 2000;Dassau et al, 2004), there are fewer published studies of the southern hemisphere and only currently two of high southern latitudes.…”

Measurements and modeling of Δ17O of nitrate in snowpits from Summit, Greenland

“…If PAN decomposes to NO x and is oxidized to nitrate in the atmosphere near Summit, the nitrate deposited would reflect local oxidant concentrations. It has also been suggested that PAN may decompose to nitrate on snowgrains [ Ford et al , 2002; Dassau et al , 2004]. The chemical mechanism and role of local oxidants in this process remain unknown.…”

“…However, more recent ground‐based measurements at Summit [ Ford et al , 2002] suggest that PAN concentrations are up to an order of magnitude lower than those used in the calculation of [ Munger et al , 1999]. While several studies suggest that PAN decomposition on snowgrains may also be important for snowpack nitrate concentrations [ Bartels‐Rausch et al , 2002; Ford et al , 2002; Dassau et al , 2004], attempts to observe and quantify in situ decomposition of PAN on snow crystals have been inconclusive [ Ford et al , 2002]. Because the chemical mechanism for thermal decomposition of PAN on snowgrains is unknown, it is also unclear whether or not this chemistry would contribute to a local oxidation signal in Δ 17 O(NO 3 − ).…”