Ternary homogeneous nucleation of H2SO4, NH3, and H2O under conditions relevant to the lower troposphere.

Ternary homogeneous nucleation (THN) of H₂SO₄, NH₃ and H₂O has been used to explain new particle formation in various atmospheric regions, yet laboratory measurements of THN have failed to reproduce atmospheric observations. Here, we report first laboratory observations of THN made under conditions relevant to the lower troposphere ([H₂SO₄] of 10⁶-10⁷ cm^-3 [NH₃] of 0.08-20ppbv, and a temperature of 288 K). Our observations show that NH₃ can enhance atmospheric H₂SO₄ aerosol nucleation and the enhancement factor (EF) in nucleation rate (J) due to NH₃ (the ratio of J measured with vs. without NH₃) increases linearly with increasing [NH₃] and increases with decreasing [H₂SO₄] and RH. Two chemical ionization mass spectrometers (CIMS) are used to measure [H₂SO₄] and [NH₃], as well as possible impurities of amines in the nucleation system. Aerosol number concentrations are measured with a water condensation counter (CPC, TSI 3786). The slopes of Log J vs. Log [H₂SO₄], Log J vs. Log RH, and Log J vs. Log [NH₃] are 3-5, 1-4, and 1, respectively. These slopes and the threshold of [H₂SO₄] required for the unity nucleation vary only fractionally in the presence and absence of NH₃. These observations can be used to improve aerosol nucleation models to assess how man-made SO₂ and NH₃ affect aerosol formation and CCN production at the global scale. [ABSTRACT FROM AUTHOR]