EFFECT OF ELEVATION ON ACID LOADS IN WINTER SNOWPACK, MOUNT WASHINGTON, NEW HAMPSHIRE

Elevation and orographic barriers are well known to affect precipitation volume; their effect on precipitation chemistry is less well established. Conflicting conclusions have been made concerning an orographic effect on precipitation chemistry, but a key variable appears to be the elevation of the cloud base. The Mount Washington massif, New Hampshire, is enshrouded in clouds for part of approximately 300 days per year. Consequently, ion stripping from clouds/fog is expected to be particularly effective at this site.

Multiple sites were sampled on both the western, windward side and the eastern, downwind side of Mount Washington in early March of 2011 and 2012. Sample sites ranged in elevation from a low of ~620 m (~2000 ft.) on both sides of the mountain to a high of 1180 m (3870 ft.) on the eastern side and 1533 m (5030 ft.) on the western side. All sampling sites are outside of avalanche-hazard regions.

NO₃ and SO₄ loads varied with elevation, sampling year, and western versus eastern location. 2011 loads were higher than 2012 loads, and at the base of the mountain were comparable to values obtained at lower elevations at nearby NADP sites. Loads increased in both sample years by factors of 3 to 6 with increasing elevation. Near-treeline loads reached 3.7 kg/ha for NO₃ and 3.25 kg/ha for SO₄in 2011, and 2.8 kg/ha and 2.5 kg/ha, respectively, in 2012. Comparing similar elevations, the data from both sample years suggest that loads generally are higher on the eastern (downwind) side of the mountain. Ion concentration appears to be roughly correlated with elevation in the 2011 data, but only suggestively so in the 2012 data; thus much of the increase in load with elevation is due to deeper snowpack at elevation.

d¹⁸O and dD values on the full snowpack show a negative shift with increasing elevation. Isotopic values on samples from pits near treeline show large range (-13 to -18.7 per mil for d¹⁸O and -84 to -126 per mil for dD), yet there is no clear vertical trend. This suggests minimal impact of sublimation, evaporation, and melt events up to the early March sampling dates.