Seasonal behavior of non-methane hydrocarbons in the firn air at Summit, Greenland

- Most extensive in snowpack measurements of non-methane hydrocarbons accomplished thus far.
- NMHC in the snowpack closely track above-surface measurements.
- Alkanes, ethyne, and benzene do not show significant differences between ambient and snowpack measurements.
- Ethene and propene were found at elevated levels in the snowpack during the summer.
- These measurements support reconstruction of atmospheric non-methane hydrocarbon histories from deep firn air extraction.

Non-methane hydrocarbons (NMHC) were measured in the ambient air and in the snowpack interstitial firn air at ∼1 m depth continuously for nearly two years at Summit, Greenland, from fall 2008 through summer 2010. Additionally, five firn air depth profiles were conducted to a depth of 3 m spanning winter, spring, and summer seasons. Here we report measurements of ethane, ethene, ethyne, propane, propene, i-butane, n-butane, i-pentane, n-pentane, and benzene and discuss the seasonal behavior of these species in the ambient and firn air. The alkanes, ethyne, and benzene in the firn air closely reflect the ambient air concentrations during all the seasons of the year. In spring and summer seasons, ethene and propene were enhanced in the near-surface firn over that in the ambient air, indicating a photochemical production mechanism for these species within the snowpack interstitial air. Evaluation of the NMHC ratios of i-butane/n-butane, i-pentane/n-pentane, and benzene/ethyne in both ambient and firn air does not provide evidence for chlorine or bromine radical chemistry significantly affecting these gases, except in a few summer samples, where individual data points may suggest bromine oxidation influence.