While snow is fundamentally composed of frozen water vapor, its journey from cloud to ground is far from sterile. As a snowflake descends, it acts as a filter, collecting various particles and chemical compounds suspended in the atmosphere. This process means that, by the time snow reaches the surface, it carries a complex mixture of environmental materials. The purity of snow, therefore, reflects the air quality through which it travels.
How Snow Cleans the Atmosphere
Snowfall plays a significant role in atmospheric cleansing through a process known as wet deposition or scavenging. This mechanism involves snowflakes capturing airborne contaminants and transporting them to the Earth’s surface. A snowflake begins to form when water vapor freezes around a condensation nucleus, which is often a tiny particle like dust, pollen, or a pollutant. As the developing ice crystal falls through the air column, it sweeps up additional aerosols and gas molecules, functioning like a natural scrubbing brush for the atmosphere. This below-cloud scavenging is highly efficient because the complex, dendritic structure of a snowflake presents a large surface area for collection. Snow is generally a more efficient scavenger of aerosols than rain, meaning it cleans the air more effectively but becomes dirtier in the process.
The Makeup of Dirty Snow: Key Contaminants and Sources
The contents collected by falling snow are diverse and depend heavily on the surrounding environment, resulting in a complex chemical signature. One major category of contaminant is particulate matter, such as black carbon, a form of soot produced by the incomplete combustion of fossil fuels and wood. Black carbon concentrations, often measured in parts per billion (ppb), are significantly higher in urban areas compared to remote locations, reflecting the density of local pollution sources.
Snow also efficiently removes chemical pollutants, including various heavy metals like mercury, zinc, cadmium, and antimony. These metals often originate from industrial emissions, mining activity, and vehicular traffic, becoming integrated into the snow crystal structure. In urban settings, the snow that has settled on the ground quickly incorporates de-icing agents like road salts and petroleum residues from vehicles, further increasing its chemical load.
The concentration of these pollutants is dramatically influenced by the timing of the snowfall, a phenomenon sometimes called the “first flush” effect. The initial flakes of a storm are often the most contaminated because they are the first to scrub the highest concentration of accumulated pollutants from the air. After the atmosphere has been partially cleaned over several hours of continuous precipitation, subsequent snow tends to be less polluted. Furthermore, microorganisms are also incorporated, including bacteria and fungi collected from the air, in addition to debris like animal waste that may contaminate snow once it settles on the ground.
Practical Safety Guidelines for Snow Exposure and Consumption
Given the complex composition of snow, it is advisable to avoid consuming it due to the risk of ingesting atmospheric pollutants and microbes. If consumption is necessary, choosing the freshest, whitest snow minimizes the potential for contamination. The safest option is to collect snow that has fallen a few hours into a continuous storm, after the initial, dirtiest air has been cleansed.
Avoid any snow that is discolored, particularly gray, brown, or yellow, as these colors indicate high concentrations of dirt, soot, or animal waste. Never consume snow that has been disturbed, such as snow near roads, sidewalks, or parking lots, as it will contain concentrated levels of road salts, heavy metals, and oil or gas residue. While eating a small amount of clean, fresh snow is unlikely to cause serious problems, individuals with compromised immune systems should exercise greater caution.