Snowpack acid

One of the major effects of acidic deposition is felt by aquatic ecosystems in mountainous terrain, where considerable precipitation occurs due to orographic lifting. The maximum effect is felt where there is little buffering of the acid by soil or rock structures and where steep lakeshore slopes allow little time for precipitation to remain on the ground surface before entering the lake. Maximum fish kills occur in the early spring due to the "acid shock" of the first meltwater, which releases the pollution accumulated in the winter snowpack. This first melt may be 5-10 times more acidic than rainfall. 

In areas with large snowpacks (e.g., much of the Northeast and all of the mountainous West), ions have been shown to drain from the pack in the early stages of snowmelt. This process leads to concentrations that are much higher than the average concentration of the snowpack itself (82). D ifferential elution of acid anions (like N03") during the initial stages of snowmelt has been shown to be responsible for the elevated N03" concentrations observed in parts of Scandinavia (81), Canada (82), the Adirondack Mountains (181), the Midwest (182), and the Sierra Nevada Mountains (180). Ammonium deposited to the snowpack (either wet or dry deposition) can subsequently... 

The increase of atmospheric CO2 may have decreased the pH of precipitation very slightly, but PcOj in soils is far more important for the acid-base status of surface waters. Variation in forest soil Pco is related to the temperature and moisture content of soils as well as the release of excess soil CO2 to the atmosphere. Warmer conditions increase the rate of microbial and root respiration in the soil, thereby increasing soil Pco above the long-term average value and producing short-term increases in runoff ANC, and vice versa. Norton et al. (2001) found that intra-seasonal variations in Pco caused by variable snowpack thickness could induce variation in ANC in runoff of 10-15 peqL Such variability is comparable to variability in ANC caused by a 15-20 p.eqL change in SO4 in runoff. Decline in soil Pco, despite increased temperature and possibly increased soil respiration could result from a lower soil moisture content and a greater efflux of soil CO2. 

After the water falls to the Earth, further concentration enhancement can take place if it freezes as snow. When snow melts the dissolved ions are lost preferentially, as they tend to accumulate on the outside of ice grains which make up snowpacks. This means that at the earliest stages of melting it is the dissolved H2S04 that comes out. Concentration factors of as much as 20-fold are possible. This has serious consequences for aquatic organisms, and especially their young, in the spring as the first snows thaw. It is not just acid rain, but acid rain amplified. 

Narukawa M., Kawamura K., Li S.-M. and Bottenheim J. W. (2002) Dicarboxylic acids in the arctic aerosols and snowpacks collected during ALERT2(XX). Atmos. Environ. 36, 2491-2499. 

The primary purpose of the acid rain NOx emission reduction program is to reduce the adverse effects of acidic deposition on natural resources, ecosystems, visibility, materials, and public health by substantially reducing annual emissions of NOx- NOx emissions are a principal acidic deposition precursor. Although sulfate deposition is considered to be the major contributor to long-term aquatic acidification, nitric acidic deposition plays a dominant role in the "acid pulses" associated with the fish kills observed during the springtime meltdown of the snowpack in sensitive watersheds. 

Forms of precipitation other than rainfall may contain excess acidity. Acidic fog can be especially damaging because it is very penetrating. In early December 1982, Los Angeles experienced a severe, 2-day episode of acid fog. This fog consisted of a heavy concentration of acidic mist particles at ground level that reduced visibility and were very irritating to breathe. The pH of the water in these particles was 1.7, much lower than ever before recorded for acid precipitation. Another source of precipitation heavy in the ammonium, sulfate, and nitrate ions associated with atmospheric acid is acid rime. Rime is frozen cloudwater that may condense on snowflakes or exposed surfaces. Rime constitutes up to 60% of the snowpack in some mountainous areas, and the deposition of acidic constituents with rime may be a significant vector for the transfer of acidic atmospheric constituents to Earth s surface in some cases. 

---