Quick Answer
Yes, there can be trace amounts of chemicals present in hailstones. Hail forms when updrafts in thunderstorms carry raindrops upward into extremely cold areas of the atmosphere where they freeze into hailstones. As these hailstones grow by accumulating liquid water, they can pick up small amounts of chemicals that may be present in the atmosphere such as nitrates, sulfates, sodium, and calcium. However, the concentrations of these chemicals are generally very low.
What is Hail and How Does it Form?
Hail is a form of solid precipitation that consists of balls or irregularly shaped lumps of ice. It is a product of thunderstorms and forms when updrafts carry raindrops high into extremely cold areas of the atmosphere where they freeze into ice.
As the frozen raindrops are blown back and forth in the powerful updrafts and downdrafts within the storm, they collide with supercooled water droplets which freeze on contact. This process of alternating up and down motions causes the hailstone to accumulate additional ice and grow progressively larger.
The growth process continues until the hailstone becomes heavy enough that the updraft can no longer support its weight. At this point, it will fall out of the cloud towards the ground. Hailstones typically range in size from 5-50 mm in diameter, but under certain conditions can grow significantly larger.
The presence of strong updrafts and supercooled water is necessary for hail formation. This is why hail is almost exclusively associated with severe thunderstorms known as supercell storms. The powerful updrafts in these storms can hold the hailstones aloft for a longer period of time, allowing them to grow to larger sizes before falling to the ground.
Conditions Needed for Hail Formation
– Strong, persistent updrafts to keep hailstones suspended in thunderstorm cloud
– Abundant supercooled liquid water droplets that freeze on contact with ice crystals
– Storm cloud must extend high into the atmosphere where temperatures are below freezing
– Interaction between updrafts and downdrafts contributes to hailstone growth
How Chemicals Can Get into Hail
Although hailstones are composed primarily of water ice, trace amounts of chemicals can become incorporated into the frozen pellets under certain conditions. There are a few ways this can occur:
Capture of Aerosols
As hailstones grow by collision and accretion in the thunderstorm cloud, they can capture microscopic aerosol particles floating in the atmosphere. These aerosols may contain various chemicals like nitrates, sulfates, organic compounds, and metal ions. Even though the concentrations are low, uptake of aerosols can result in measurable amounts of chemicals in hail.
Scavenging of Pollutants
Hailstones that pass through air containing pollution, smoke, or ash particles can scavenge some of these contaminants. As the hail moves through the cloud, it collects the suspended particles on its surface. This is more likely to occur in polluted urban environments.
Incorporation of Surface Particles
If hail falls through blowing dust or debris as it descends from the thunderstorm, materials can become embedded in the ice. This may introduce small amounts of soil, dust, pollen, or organic matter.
Freezing of Surface Runoff
As hailstones impact the ground, they can splatter, forming a slushy coating around the main hailstone. If this splashing freezes onto the hailstone, any dissolved materials in the water can become incorporated including salts, fertilizers, pesticides, and other surface runoff chemicals. However, this mechanism is limited to the outermost layers.
Typical Chemical Composition of Hail
Although hail can contain a variety of chemical substances, the concentrations are usually quite low. Some typical chemicals found in trace amounts in hail include:
Nitrates and Sulfates
Nitrate (NO3-) and sulfate (SO42-) compounds are often detected in hail at concentrations ranging from 0.1 – 10 parts per million. These aerosol particles can originate from sources like vehicle emissions, power plants, and volcanic activity.
Sodium and Chloride
Sodium (Na+) and chloride (Cl-) ions are prevalent in hail, generally at combined concentrations between 0.1 – 1 ppm. Sodium and chloride are among the most abundant elements dissolved in seawater. Aerosolized sea spray can be a source of these ions.
Calcium, Magnesium, and Potassium
Common metal ions like calcium (Ca2+), magnesium (Mg2+), and potassium (K+) are often found in hailstones at 0.1 – 0.5 ppm levels. Dust and soil particles are typical sources for these metals to enter hail via scavenging.
Ammonium
The ammonium ion (NH4+) is frequently detected in hail, usually in the 0.1 – 1 ppm range. Ammonia is an abundant component of agricultural fertilizers and animal waste, both of which can be scavenged by hail.
Organic Compounds
Low levels of organic compounds from sources like vegetation, smoke, vehicle exhaust and industrial emissions may also be incorporated into hailstones. These are often complex mixtures of hydrocarbons, acids, alcohols, and esters.
Factors Affecting Chemical Composition
The actual chemical composition of a hailstone varies substantially depending on the specific environmental conditions it forms in. Some factors that influence the types and quantities of chemicals present include:
– Geographic location – Industrial pollution is higher near cities leading to increased nitrates, sulfates, and hydrocarbon levels in hail.
– Season/climate – Drier conditions produce more dust which can add metals like calcium and potassium when scavenged by hail.
– Nearby emission sources – Proximity to facilities like refineries and power plants can increase uptake of combustion byproducts.
– Storm severity – More intense storms have stronger updrafts and lead to larger hailstones with greater surface area for scavenging.
– Cloud height – Taller cloud tops extend into colder atmosphere allowing more time for hailstones to accumulate chemicals while suspended.
– Presence of blowing dust/debris – Gusty winds accompanying storms pick up more surface particles that may get embedded into falling hail.
Potential Impacts of Chemicals in Hail
For the most part, the trace quantities of chemicals present in hailstones have minimal impacts on the environment. But there are a few potential effects in some cases:
– Deposition of pollutants – Hail can transport absorbed gases and aerosols from the upper atmosphere down to the surface. This contributes a small amount to the deposition of pollutants and acid rain precursors like nitrates and sulfates.
– Micronutrient fertilization – Elements like sodium, calcium, magnesium, and potassium deposited with hail fragmentation can make marginal contributions to soil nutrient budgets.
– Toxicity – High concentrations of heavy metals like lead or organic pollutants transported in hail from heavily contaminated sources could potentially cause toxicity to sensitive ecosystems. But such cases are very rare given the limited chemical amounts.
– Acidification – If the pH of hail is substantially lowered by nitric and sulfuric acid accumulation, precipitation could gradually acidify sensitive freshwater bodies and soils. However, the acidification potential of hail is small compared to rain and snowmelt.
– Damage to plants – High levels of certain contaminants could potentially reduce growth or damage foliage if plants are directly impacted by hailstone strikes. But chemical toxicity risks are negligible in the vast majority of hail events.
Measuring Chemical Composition
To characterize the chemical composition of hailstones, samples need to be collected immediately after they fall since they quickly begin to melt and lose their internal chemical contents. Researchers use a variety of analytical techniques to measure hail chemistry:
Ion chromatography
– Quantifies water-soluble anions like nitrate, sulfate and chloride extracted from melted hailstone water.
Inductively coupled plasma mass spectrometry (ICP-MS)
– Detects metals and some non-metals dissolved in the melted hail water sample.
Scanning electron microscopy (SEM)
– Provides visual identification and elemental analysis of fine particles embedded within the ice crystals of unmelted hail samples.
X-ray diffraction
– Determines chemical composition of the tiny aerosol inclusions in unmelted hail stones.
Liquid chromatography-mass spectrometry (LC-MS)
– Identifies and quantifies complex organic compounds present in low concentrations in the liquefied hail water.
Case Study: Chemical Analysis of Hailstones in Oklahoma City
A 2015 study published in Atmospheric Research collected hail samples from severe thunderstorms around Oklahoma City, Oklahoma. The researchers used the methods above to analyze the chemical composition of the hailstones. Some key findings included:
High Sulfate Levels
– Sulfate concentrations averaged around 5 ppm, likely from absorption of SO2 emissions from local coal-fired power plants and refineries. This gave the melted hail water a low pH of around 5.
Elevated Nitrate and Ammonium
– Nitrate concentrations reached levels over 4 ppm, probably due to scavenging of NOx pollutants. High ammonium levels around 0.5 ppm were also present.
Abundant Sodium and Chloride
– Sodium concentrations over 1 ppm and chloride near 0.5 ppm reflected hailstone interactions with salty lake spray aerosols.
Presence of Organic Species
– Using LC-MS, researchers identified and quantified over a dozen small organic acids, esters, and phenols originating from vehicle and industrial emissions.
Trace Metals Detected
– ICP-MS picked up low levels of iron, zinc, copper, and lead likely from mineral dust and pollution sources.
Conclusion
In summary, while hailstones consist mostly of pure ice, they can contain measurable amounts of chemical substances picked up from the atmosphere and surface materials. Nitrates, sulfates, organic compounds, sodium, chloride, and trace metals are commonly found in low concentrations. The exact chemical composition of hail depends on the environmental conditions it forms within. Stronger storms in heavily polluted areas tend to produce hailstones with higher contaminant levels. However, due to the very low quantities, chemicals in hail likely pose minimal environmental hazards. Advanced analytical methods are needed to detect the complex mix of substances potentially present in hail.
