Gravity is the fundamental force that causes heavier particulate matter suspended in the air to settle onto surfaces. While other factors like precipitation (wet deposition) can assist in removing particles, gravity is the primary mechanism for dry deposition, pulling particles downward once their terminal settling velocity is reached.

An aerosol is defined as a suspension of fine solid particles or liquid droplets in a gas, typically air. Once these particles settle out of the air onto a solid or liquid surface, they are no longer suspended in the gas phase and are therefore no longer classified as an aerosol. The process of settling is known as deposition, which removes the particles from the atmospheric aerosol state.

Atmospheric aerosols interact with radiation by scattering and absorbing both solar and terrestrial energy. While they interact with short-wave solar radiation, they are particularly noted for their role in absorbing and emitting long-wave (infrared) radiation, which significantly influences the Earth's thermal balance and the greenhouse effect.

Particulate matter (PM) consists of a complex mixture of solid particles and liquid droplets suspended in the air. These particles vary significantly in size, ranging from very fine particles (less than 0.1 micrometers) to larger coarse particles (up to 100 micrometers). This range is critical for understanding their atmospheric residence time and health impacts.

Acidic fog can reach very low pH levels, sometimes as low as 1.7, due to the concentration of sulfuric and nitric acids within water droplets. This high acidity is significantly more concentrated than typical acid rain, posing severe risks to vegetation and infrastructure. The low pH value indicates a highly corrosive environment formed by the interaction of pollutants with atmospheric moisture.

Acid rain or acid deposition increases the acidity of the soil. This low pH environment causes the leaching of essential plant nutrients like calcium, magnesium, and potassium, while simultaneously mobilizing toxic metals like aluminum, which can damage plant roots and disrupt the nutrient cycle of the ecosystem.

Acid rain occurs when sulfur dioxide (SO2) and nitrogen oxides (NOx) are emitted into the atmosphere and undergo chemical reactions with water vapor, oxygen, and other chemicals. This process forms sulfuric and nitric acids, which then fall to the ground as precipitation, causing environmental damage.

When sulfur dioxide is absorbed by paper, it can react with moisture and impurities (often metallic catalysts) to form sulfuric acid (H2SO4). This acid causes the paper to become brittle and yellow over time, a process known as acid-induced degradation.

Acidic conditions, often resulting from acid rain or soil acidification, can damage plant foliage and disrupt metabolic functions. Specifically, acidity can interfere with chlorophyll synthesis and stomatal function, thereby inhibiting the efficiency of photosynthesis, which is essential for the plant's energy production and overall survival.

While acid rain is generally considered detrimental to ecosystems, some studies suggest that in specific nutrient-poor soils, the deposition of nitrogen compounds from acid rain can act as a fertilizer, potentially stimulating crop growth. This is a complex interaction where the negative impacts of acidity are temporarily offset by the addition of essential plant nutrients.