Irina Dmitrievna has spent forty years studying Moscow’s rain composition. What substances appear in the drops?
Any natural water, including rain, shares a common makeup. The most frequent components are cations such as calcium, sodium, potassium, magnesium, and ammonium, along with anions like sulfates, chlorides, nitrates, and bicarbonates. Fluorides and phosphates occur too, but in very small amounts. And not every rain exhibits these elements in the same quantities—dirty rain shows higher levels.
Overall, rain is among the purest waters on Earth, especially when rainfall is frequent and heavy. When forecasts say, “It will rain for a month,” that rain tends to be exceptionally clean. The same elements appear in all waters but at levels near the detection limit of standard tests. Rainwater has historically been collected for drinking because it is soft and contains little salt.
In dirty rain, the same components show up in larger amounts. If rain is infrequent and light, it can become quite dirty. The summer of 2010 in Moscow is recalled when a fog cleared and then light rain fell in small amounts, resulting in unusually dirty conditions.
How common are dirty rain events in a year?
The mineralization of all components in rain water—both anions and cations—exceeding 100 mg per liter marks a dirty rain. Some years see five or six such events, others none, and the average typically does not exceed three percent. Across forty years of observations at the meteorological observatory of Moscow State University, the average mineralization of rainwater settled around 17 mg per liter.
Are there acceptable limits for the content of the various components in the sediments of rain?
No universal rules govern rain composition. For drinking water, there are maximum allowable concentrations, but there is no formal MPC for rain. Tap water often contains sulfates, chlorides, nitrates, calcium, and magnesium, but usually in amounts higher than those in typical rainfall.
Do rainfalls contain nitrates?
Yes. Emissions from businesses and vehicles contribute to nitrate levels. Interactions between air droplets and atmospheric chemistry create nitrates that can be found in many locations.
Was snow included in the study as well?
Yes. Snow samples were collected yearly from the Leninsky district and other parts of Moscow. Seasonal snow provides a snapshot of the entire winter’s snowfall prior to melt. A snow tube captured a representative vertical column of snow from November through March, avoiding sidewalks and edges to sample the park where snow remained undisturbed.
There was a claim that snow is cleaner in the north and dirtier in the south. Is that still accurate?
Earlier observations supported that view. Today, a city’s pollution influences results more than latitude alone. For a large city like Moscow, the impact of nearby pollution sources can dominate results at the sampling site.
How far must one live from an industrial facility for its rain to reflect local pollution?
That depends on the wind patterns. If the wind tends to carry pollutants toward a person, rain may be dirtier on the downwind side while cleaner on the leeward side. The timing of emissions matters too. A plant emitting pollutants an hour earlier may not cause immediate rain; emissions can react in the air and the rain can collect substances that later react within the droplet path, altering the final composition.
In practice, clean weather from the north and dirty weather from the south cannot be assumed every time. Many factors interact.
There was a fear in Soviet times about acid rain, with old warnings about baldness and permeable raincoats. Is acid rain real, and is it dangerous?
The concentration of hydrogen ions governs both mineral content and acidity. The pH index translates the hydrogen ion activity into a readable form. A pH of 7 is neutral. The scale runs from 1 to 14, with values below seven acidic and values above seven alkaline. Tap water is usually near neutral.
In the cleanest areas, away from pollution sources and highways, rain and snow never reach a neutral pH of 7 due to atmospheric carbon dioxide forming carbonic acid, which lowers pH. This means even the most pristine rain is slightly acidic.
Did the concept of acid rain disappear?
No. When the research began in the 1980s, the World Meteorological Organization identified several reference points with very clean air, such as Amsterdam Island in the Indian Ocean. Even there, precipitation registered below pH 5.0. Since then, researchers have accounted for multiple natural acid-forming factors. Rain with pH below 5.0 is considered acid rain, while pH values between 5.0 and 7.0 are seen as a pH balance, with 7.0 and above implying alkalinity. Across forty years of study, the most acidic rain recorded had a pH of 3.2 in July 1987, while the most alkaline reached 9.65 in November 1983.
How often does rain with extreme pH occur? On average, not more than a few times per year. From 1980 to 1998 the rate hovered around 1.8 percent; from 1999 to 2004 the appearance of acid rain diminished as industry slowed. By 2005, acids returned and have continued to decline since then.
Are acid rains more common in summer?
Yes. The peak often appears in July. Summer precipitation tends to produce the lowest pH values, while winter values are higher. Typically from May through September the average pH remains acidic.
Is the acidity tied to temperature?
It depends on many factors—temperature, pollution levels, humidity, and emissions. The exact cause remains uncertain.
Could acid rain or alkaline rain be harmful?
Relative effects matter. If one compares the acidity of juice or wine, their pH is lower than rain. For example, wine sits around pH 3.5. Yet pouring wine on the head does not cause baldness. Acid rain does not occur frequently, and alkaline rain with pH around 9 is extremely rare, observed only a handful of times in decades.
Is rainwater fit to drink now as it once was for grandmothers?
Not recommended, especially in Moscow. The air carries dust, soot, emissions, and bacteria. Toxic metals may be present in microgram levels, and while their harm can be significant, rainwater remains among the cleanest natural waters but is not guaranteed drinkable.
[Citations: Moscow Meteorological Observatory records, city pollution reports]