Earth, Wind and Fire ... and Water

In early June this year, Marylanders and many mid-Atlantic residents walked outside into a hazy world. The air smelled cloyingly sweet or of burnt plastic, and distant skylines blurred and disappeared behind veils of smog. A low-pressure weather system was pushing soot and gases into the Chesapeake airshed; soon, the smoke from Canadian wildfires prompted air quality alerts and warnings about human health impacts.

Just as watersheds circulate minerals, nutrients and pollutants, airsheds in the atmosphere do the same. The Chesapeake region's airshed is nine times larger than the Bay's watershed, stretching from Kentucky to southern Ontario in Canada—near where these wildfires were raging. Northerly winds picked up the fine mist of charred wood, vegetation and volatile compounds and blew it southward.

The mist of particles did not remain in the atmosphere, though. These pollutants and nutrients eventually fell from the skies through a process called atmospheric deposition. Where and how particles like these fall depends on factors like wind speed, a particle's size, and if it gets caught in water vapor to precipitate as rain or snow or simply drops from the sky like a dust mote. But one thing is certain: It will not land where its journey started.

Everything in Moderation

The flow of minerals and nutrients throughout and between ecosystems is crucial to those habitats maintaining long-term stability. In this sense, atmospheric deposition can play a beneficial role. While nutrient-rich areas can become overwhelmed by short, intense additions, environments strapped for building blocks like nitrogen, phosphorus and iron undergo bursts of life following such deposits.

For example, the Amazon rain forest's plants and trees rely, in part, on dust carried from the arid basins of the Sahara. Phosphorus particles, on the order of thousands of tons worth, cross the Atlantic Ocean each year, replacing nutrients lost to South America's rivers. Regions of the southwestern Atlantic have churned with plankton blooms following volcanic eruptions in Chile, with iron deposits setting off explosive growth in the lower rungs of the food web. In 2020, a similar plankton bloom in the southeastern Pacific occurred after the Australian wildfires, lasting for four months as available nutrients spiked. Despite the destructive force of these events, we are starting to recognize their more subtle impacts and contributions across the world.

A Polluted Past

Over 50 years ago, atmospheric deposition was causing a different problem in the mid-Atlantic region. This time, instead of carrying wildfire soot and ash, the airshed was transporting sulfur compounds from an anthropogenic (human-caused) source. Gaseous sulfur dioxide and nitrogen oxides, stemming from fossil fuels burned in the industrial Rust Belt of the U.S., mixed with water droplets in the air to produce acid rain.

Over the course of a few decades, researchers turned their attention toward what was rapidly becoming an issue not only in industrial regions, but in airsheds across the globe. Since international borders mean nothing to moving air masses, acid rain also became a political issue. Tensions between the U.S. and Canada escalated, specifically because the environmental damage seen in southern Canada was linked to emissions originating in the industrial sectors of the U.S.

The Push for Cleaner Air

Federal legislation around air pollution had existed since the mid-1950s in the U.S. and paved the way for the Clean Air Act of 1963. The landmark legislation was a major step forward, and in the following decades, continual public concern led to Congress amending the Act to set air quality standards, launch monitoring programs and expand enforcement. By the 1980s, political and societal pressure regarding the issue of acid rain surged, and further amendments to the Clean Air Act in 1990 directly addressed sulfur dioxide pollution from manufacturing, requiring emissions be cut by over 50%. The Environmental Protection Agency already had authority to regulate vehicle emissions; further tightening of fuel regulations and personal vehicle emissions led to a continual decline in nitrogen oxides and sulfur dioxide in urban areas. (The agency continues to make progress in improving air quality and as of April 2023 proposed new regulations on vehicle emissions.) Under the combination of these new regulations, both sulfur and nitrogen emissions in the airshed plummeted from their record-high levels. Ecosystems, and government relations, began the long road to recovery—although the U.S.-Canadian relationship mended itself much faster than the acidified habitats.

Since laws and regulations were improved to reduce pollution, forests and lakes throughout southern Canada and the mid-Atlantic have rebounded significantly. Challenges remain, though; although low sulfur fuels have made acid rain less of an environmental threat in the region, emissions from other sources remain a concern. The Chesapeake Bay is surrounded by dense, urban areas and large-scale agriculture, which continue to emit nitrogen oxides from vehicles and fertilizers. Even with tougher standards, an estimated 25% of nitrogen-based pollution in the watershed is entering it through the atmosphere.

Looking Into the Future

The rate of atmospheric deposition today can be categorized as excessive. In the last century, human-caused sources have contributed staggering amounts of nutrients and chemical pollutants to ocean and freshwater environments, outpacing those from natural sources. Increasingly extreme wildfire events like those in Australia, Canada and the U.S. exemplify this. The fossil fuels emitting sulfur dioxide and nitrogen oxides are also unleashing greenhouse gases like carbon dioxide and methane. These contribute to rising temperatures and climatic shifts, which in turn have the potential to alter rainfall amounts and worsen droughts—factors that literally help fuel wildfires.

The cascading effects of climate change will require close monitoring, strict regulations and the use of sustainable, renewable fuel sources. The gradual recovery of forests, rivers and streams following acid rain mitigation is evidence that this multi-pronged approach is effective. By adequately scaling back human-caused pollutants, those from natural sources can continue providing their seasonal and manageable doses. After all, the process of atmospheric deposition will continue, regardless of what nutrients, minerals or chemicals are carried by the wind—another link connecting the terrestrial and aquatic worlds.

Satellite image of the 2019 Australian wildfires is courtesy of the NASA Earth Observatory.