Ternary homogeneous nucleation of H ₂ SO ₄ , NH ₃ , and H ₂ O under conditions relevant to the lower troposphere

Abstract. 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 0.08–20 ppbv, 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.