Environmental, green and energy chemistry

Aerosols, acid rain

How atmospheric particles form and affect clouds and climate, and how sulfur and nitrogen emissions acidify precipitation and ecosystems.

1 Lower secondary 2 Upper secondary 3 Undergraduate 4 Advanced 5 Research

IntuitionParticles, rain and a connected atmosphere

Air contains suspended particles ranging from newly formed molecular clusters to windblown dust. They scatter or absorb sunlight and can seed cloud droplets; dissolved sulfur and nitrogen compounds can return to the surface as acidic deposition.

The size, chemical composition, water uptake and lifetime of a particle jointly determine its environmental effect. A tiny sulfate particle and a coarse mineral grain are not interchangeable simply because both count as particulate matter.

Rotate the particle cluster and switch between the full particle, coating only and core only views; identify the inorganic core (sulfate/ammonium) and the organic/nitrate coating.

SchoolSchool level: acids and deposition

Definition: Acid deposition

Acid deposition includes wet deposition (rain, snow, fog) and dry deposition of acidic gases and particles. Sulfur dioxide and nitrogen oxides are transformed in air into sulfuric and nitric acids or their salts.

\ce{SO2 ->[OH] SO3 ->[H2O] H2SO4}\qquad \ce{NO2 + OH + M -> HNO3 + M}

A useful reference pH for rain equilibrated only with present-day atmospheric carbon dioxide is about 5.6. It is not a universal boundary between “clean” and “acid rain”: natural acids, sea spray, dust and ammonia also modify rainwater.

[\ce{CO2^*}]=k_Hp_{\ce{CO2}}\quad;\quad K_{a1}=\dfrac{[\ce{H+}][\ce{HCO3-}]}{[\ce{CO2^*}]}

Example: Carbon dioxide and rain pH

Estimate the ideal pH at 25 °C using $k_H=3.4\times10^{-2}$ mol L⁻¹ atm⁻¹, $pCO2=420$ ppm, and $K_{a1}=4.3\times10^{-7}$ . Neglect the second dissociation.

Solution

$[CO2^*]=0.034(420\times10^{-6})=1.43\times10^{-5}$ M. For weak carbonic acid, $[H+]=\sqrt{K_{a1}[CO2^*]}=2.48\times10^{-6}$ M, giving pH ≈ 5.61. Activity and additional dissolved species are omitted.

Major atmospheric precursors and products
Precursor	Transformation	Environmental outcome
SO₂	Oxidation to sulfuric acid / sulfate	Acid deposition; sulfate aerosol
NOx	Oxidation to nitric acid / nitrate	Acid deposition; nitrate PM
NH₃	Neutralizes acids; forms ammonium salts	Secondary inorganic aerosol

UndergraduateUniversity: atmospheric processing

Gas-phase oxidation of SO₂ begins with OH abstraction to form HOSO₂, followed by O₂ chemistry yielding SO₃; hydration makes H₂SO₄, which has very low volatility. In cloud and aerosol water, dissolved S(IV) (SO₂·H₂O, HSO₃⁻, SO₃²⁻) is oxidized to S(VI), mainly by H₂O₂ and O₃, with O₂ plus transition-metal catalysis also contributing.

\ce{SO2(aq) <=> HSO3- <=> SO3^2-}\qquad \ce{S(IV) + oxidant -> S(VI)}

Hydrogen peroxide oxidation of bisulfite is often an efficient cloud-water route, while ozone oxidation becomes more important at higher pH because sulfite abundance rises. NO₂ reacts with OH to form HNO₃ by day; at night, N₂O₅ uptake and hydrolysis on wet particles or droplets provide nitrate.

\ce{SO2 + OH ->[M] HOSO2 ->[O2] HO2 + SO3 ->[H2O] H2SO4}\quad \ce{NO2 + OH + M -> HNO3 + M}

AdvancedAdvanced: particles, clouds and ecosystems

Acidification is controlled not only by acid input but by buffering capacity. Carbonate minerals and base cations neutralize acidity; thin soils on silicate bedrock and lakes with low alkalinity can acidify more readily. Acidification can mobilize toxic aluminum and remove calcium and other nutrients.

Typical particle size modes
Mode	Approximate diameter	Common examples / fate
Nucleation	< 10 nm	Fresh clusters; grow or evaporate
Aitken	≈ 10–100 nm	Small particles; coagulation and growth
Accumulation	≈ 0.1–1 µm	Sulfate / SOA; efficient light scattering and CCN
Coarse	> 1–2.5 µm	Sea salt / dust; faster settling

PM₂.₅ and PM₁₀ refer to mass collected below specified aerodynamic-diameter cutoffs (2.5 and 10 µm), not two chemical substances. The accumulation mode often persists for days to about a week and includes many cloud-condensation nuclei; coarse particles are usually removed faster by settling and precipitation.

\ln S = \dfrac{A}{D} - \kappa\,\dfrac{D_d^3}{D^3}\quad;\quad A=\dfrac{4\sigma_w M_w}{RT\rho_w}\quad;\quad s_c\simeq\left(\dfrac{4A^3}{27\kappa D_d^3}\right)^{1/2}

Definition: Köhler theory and hygroscopicity

Köhler theory balances the Kelvin curvature effect, which raises equilibrium vapour pressure over a curved droplet, against the solute (Raoult) effect, which lowers it. The κ-Köhler parameterization summarizes solute hygroscopicity and predicts a critical supersaturation $s_c$ for activation as a cloud droplet.

Example: Activation of an ammonium-sulfate particle

For a dry diameter $D_d=100$ nm ammonium-sulfate particle, take hygroscopicity $\kappa=0.6$ and water Kelvin length $A≈2.1$ nm. Estimate critical supersaturation.

Solution

$s_c≈[4(2.1)^3/(27(0.6)(100)^3)]^{1/2}=1.53\times10^{-3}$ , or about 0.15%. This approximation assumes a soluble spherical particle and idealized equilibrium; ambient mixtures and surface-active organics can change activation.

Secondary organic aerosol (SOA) forms when volatile organic compounds oxidize into products with sufficiently low volatility to partition into particles, or through multiphase chemistry. Gas–particle partitioning depends on product volatility, aerosol mass, temperature and composition; aqueous processing can create additional low-volatility material.

ResearchResearch frontier: particles, forcing and policy

In the United States, Title IV of the 1990 Clean Air Act Amendments established the Acid Rain Program, including a cap-and-trade system for power-plant SO₂. Europe’s LRTAP Convention and protocols coordinate transboundary air-pollution controls. Substantial sulfur reductions demonstrate policy effectiveness, though ecological recovery can lag because soils and watersheds recover slowly.

Control policies can reduce precursors without eliminating all particles: natural sea salt, dust, wildfire smoke and biogenic emissions remain. Monitoring should combine aerosol composition and size with deposition chemistry, ecosystem buffering and meteorology.

References

Atmospheric Chemistry and Physics: From Air Pollution to Climate Change, 3rd ed. · J. H. Seinfeld, S. N. Pandis, 2016
Atmospheric aerosol properties and climate impacts · U. Lohmann, B. Feichter, 2005
Towards a theory of cloud formation by condensation nuclei · H. Köhler, 1936
Aerosol indirect effect on climate · S. Twomey, 1977
Atmospheric new particle formation from sulfuric acid and ammonia: nucleation and growth · J. Kirkby et al., 2011
Direct kinetic measurements of the reaction of the Criegee intermediate CH2OO with SO2 · J. M. Welz et al., 2012
Secondary organic aerosol formation in cloud droplets and aqueous particles (aqSOA): a review of laboratory, field and model studies · B. Ervens, B. J. Turpin, R. J. Weber, 2011