Download The Chemistry of Snow: Processes and Nutrient Cycling

The Chemistry of Snow: Processes and Nutrient Cycling Martyn Tranter and H. Gerald Jones Presented by Kelly Overduijn Chapter Overview 3.1 – IntroducIon: snow chemistry and ecology 3.2 – The chemistry of snowfall 3.3 – Chemistry of cold, dry snow cover 3.4 – Chemistry of wet snow and snow-­‐meltwater systems •  3.5 – Snow cover nutrient fluxes and ecosystem budgets •  3.6 – Summary and future research needs • 
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IntroducIon: Snow Chemistry and Ecology •  Snow plays an important role in biogeochemical processes –  Meltwater supplies nutrients to terrestrial and aquaIc ecosystems –  Major input to the hydrology of catchments Main reasons why the chemistry of snow has been studied 1.  PotenIal impact of snow meltwaters on the quality of surface waters 2.  To interpret the climaIc and polluIon records recorded in cold dry snow. Main types of processes that influence chemical composiIon of snow and meltwaters 1.  Those involving heat and mass fluxes that occur during sublimaIon and melIng –  Physical properIes that determine change in the chemical loading of snow 2.  Those involved with chemical transformaIons –  Occur by chemical reacIons –  Arise from microbiological acIvity What have we learned in recent decades? •  Snow is not a passive reservoir of chemical species. •  If you study the chemical dynamics of snow, you must take into account the physical and biogeochemical characterisIcs of the snow cover environment The Chemistry of Snowfall •  Terrestrial and marine emissions recycled in atmosphere; subjected to oxidaIon and returned to the land and oceans •  This recycling occurs in two ways: 1.  By direct (dry) deposiIon of chemical species to surfaces or… 2.  By precipitaIon (snow and rain) General relaIonships between snow crystal formaIon and the amount of scavenged solute 1.  Highest concentraIons of solute associated with smallest unrimed crystals 2.  In general, solute content of snow crystals increased with the degree of riming. Factors that the chemical composiIon of snowfall depends on 1.  The origin of the air masses scavenged 2.  The alItude at which snow is deposited 3.  The meteorological condiIons during snowfall Main chemical processes of cold, dry snow cover 1.  Surface exchange at the snow-­‐atmosphere interface 2.  Surface and subsurface chemical reacIons 3.  Snow grain metamorphism within the pack 4.  Basal-­‐exchange at the snow-­‐soil interface Dry deposiIon and volaIlizaIon •  Dry deposiIon = the direct deposiIon of chemical species from the atmosphere to the snow surface. •  VolaIlizaIon  contributes some chemical species (parIcularly organic species) directly from snow to the atmosphere because of their relaIvely high vapor pressures Photochemical reacIons in surface snow cover •  Light may play a key role in the chemistry of snow cover through photochemical reacIons •  Unequivocal evidence is yet to be obtained •  These reacIons in snow are complex and difficult to disInguish from other processes (such as dry deposiIon) In-­‐pack processes •  Metamorphism – occurs because of redistribuIon of water vapor between and within crystals as water vapor is transferred from surfaces of high curvature to sites of low curvature. •  Windpumping – the cyclic movement between the atmosphere and snow cover that occurs when wind condiIons are favorable •  Thermal convecIon – if temperature gradients increase to a criIcal threshold level, air movement may be iniIated in the snow cover by thermal convecIon. Basal exchange between soil and snow •  Snow cover usually results in a decrease in the overall rate of microbiological acIvity in the soil. •  Trace gas emissions can sIll represent a significant fracIon of annual emissions •  Extent to which snow cover influences gas emissions from soil varies. •  Can vary from year to year Chemistry of Wet Snow and Snow-­‐
Meltwater Systems •  PercolaIon of meltwaters through the snow cover causes the chemical composiIon to change. •  ConcentraIon and distribuIon of solutes in the snow-­‐meltwater system is controlled by various physical and biological processes. Processes controlling the concentraIon and distribuIon of solutes 1.  Leaching of solute from snow grains and crystals 2.  Meltwater-­‐parIculate interacIons 3.  Microbiological acIvity 4.  Basal processes: soil-­‐meltwater interacIons Leaching of solute from snow grains and crystals 1.  FracIonaIon occurs 2.  Meltwater becomes more concentrated 3.  Deeper snow = higher snowmelt concentraIons 4.  Difficult to model Meltwater-­‐parIculate interacIons 1.  Dust neutralizes meltwaters 2.  Lower region dust more efficient at neutralizing 3.  Organic debris also found along with dust 4.  Leaching removes soluble organics/other chemical species Microbial AcIvity •  Increases with increases in water and solar radiaIon •  Meltwater results in photosyntheIc acIvity of algal populaIons in the snow cover •  Decreases in NH4+ and NO3-­‐ •  Can lead to depleIons of N in systems Basal processes: soil-­‐meltwater interacIons •  Soil may be frozen at the end of winter affecIng the volume and Iming of meltwater runoff and its chemistry •  InfiltraIng meltwater refreezes if the soil is sufficiently cold, thus delaying runoff of some snowmelt. •  As a result, the availability of N to both the soil and stream runoff may be limited during the iniIal runoff period when N concentraIons are highest. Snow cover nutrient fluxes and ecosystem budgets •  Major input arises in snow meltwaters in spring •  Major output occurs when hydrological export by streams is greatest (may be at same Ime as major input) •  Annual input and output generally small with regard to the amount of nutrient that is being recycled within the system itself. •  More study needed to understand factors controlling the hydrological export of nitrogen during melt Take home points •  Snow is not a passive reservoir of the chemical species scavenged from the atmosphere during snowfall •  Snow-­‐atmosphere exchange by dry deposiIon, volaIlizaIon, air movement, and snow metamorphism can increase or decrease certain chemical species within the snow pack •  Microorganisms influence nutrient concentraIons during the melt period •  Strong gradients of CO2 found in snow covers as snow mediates gas emissions from the soil to the atmosphere •  More study needed to elucidate the relaIonships between the chemistry of snow, terrestrial nutrient cycles, and ecology. Basically, don’t this at home. Or proceed with cauIon.