Survival of newly formed particles in haze conditions

Abstract

Intense new particle formation events are regularly observed under highly polluted conditions, despite the high loss rates of nucleated clusters. Higher than expected cluster survival probability implies either ineffective scavenging by pre-existing particles or missing growth mechanisms. Here we present experiments performed in the CLOUD chamber at CERN showing particle formation from a mixture of anthropogenic vapours, under condensation sinks typical of haze conditions, up to 0.1 s^-1. We find that new particle formation rates substantially decrease at higher concentrations of pre-existing particles, demonstrating experimentally for the first time that molecular clusters are efficiently scavenged by larger sized particles. Additionally, we demonstrate that in the presence of supersaturated gas-phase nitric acid (HNO₃) and ammonia (NH₃), freshly nucleated particles can grow extremely rapidly, maintaining a high particle number concentration, even in the presence of a high condensation sink. Such high growth rates may explain the high survival probability of freshly formed particles under haze conditions. We identify under what typical urban conditions HNO₃ and NH₃ can be expected to contribute to particle survival during haze.

Fig. 1. Comparison of measured and modelled growth rates. (a) Particle size distribution from an example CLOUD experiment showing rapid growth from NH₄NO₃ formation once the activation diameter (vapour supersaturation including the Kelvin effect) is reached. (b) Model prediction for the experiment in (a). The black traces in (a) and (b) show the 50% appearance time. The initial experimental conditions are 1891 pptv NH₃, 352 pptv HNO₃, and 3.9 × 10⁷ molecules per cm³ H₂SO₄. The inputs to the model are the production rates of HNO₃, NH₃, and H₂SO₄, and the Kelvin diameter determined from other CLOUD experiments (see ESI – Modelling ammonium nitrate). (c) Measured particle growth rates after activation versus excess [HNO₃] × [NH₃] vapour product (round points, previously shown in Wang et al. (2020)). The excess vapour product is the supersaturation for the formation of ammonium nitrate, and is determined by subtracting the calculated equilibrium vapour product from the measured value. The round points were determined using the 50% appearance time method (see ESI – growth rates). The diamond points show the growth rates obtained by fitting modelled data for each experiment. The growth rates corresponding to panels a and b are indicated by a blue box. The dashed black curve shows a power law fit through the model values of the form y = kx^p, with p = 1.33 and k = 7 × 10⁻⁶. All experiments were performed at 5 °C and 60% relative humidity.

Fig. 2. Nucleation experiments and model simulations with a high condensation sink in the CLOUD chamber. (a–c) A CLOUD experiment with low HNO₃ (∼0.03 ppbv) and high initial condensation sink (CS) (0.06 s⁻¹) from pre-existing particles around 100 nm size. The experimental conditions are ∼2.5 × 10⁶ molecules per cm³ [H₂SO₄], ∼0.03 ppbv [HNO₃], ∼5–20 ppbv [NH₃], 60% RH, and 5 °C. The initial vapour product [HNO₃] × [NH₃] gives an activation diameter of ∼30 nm, i.e. particles less than 29.8 nm are in sub-saturated conditions. The CS gradually falls as the particles are flushed from the chamber (diluted) by fresh makeup gas. At 65 minutes, the CS reaches ∼0.033 s⁻¹ (indicated by an orange line) and new particles begin to appear above 2.5 nm (b and c) and grow steadily at a rate of 6.4 ± 1.0 nm h⁻¹, in the size range 3.2–4.9 nm. The formation rate, J_2.5, continues to increase as the CS falls further. (d) Model simulation using the measured initial HNO₃ and NH₃ concentrations and H₂SO₄ production rates, and an initial lognormal particle size distribution centred around 100 nm. The model closely reproduces the onset of new particle formation near 65 minutes (orange line). (e) Size and time dependent growth rates calculated using the INSIDE method. (f–h) A second CLOUD experiment with higher HNO₃ (0.1–0.2 ppbv) and high initial CS (0.06 s⁻¹) from pre-existing particles around 100 nm size. The experimental conditions are ∼6 × 10⁶ cm⁻³ [H₂SO₄], ∼1.7 ppbv [NH₃], 60% RH, and 5 °C. The initial vapour product [HNO₃] × [NH₃] gives an activation diameter of ∼7.5 nm, i.e. particles larger than 7.5 nm are in supersaturated conditions. Under these conditions, we measure steady new particle formation (J_2.5 = 5–10 cm⁻³ s⁻¹; panel f) and rapid growth of both the newly formed and the pre-existing particles, which maintains a high CS despite dilution of the chamber contents (panel g). (i) Model simulation of the second CLOUD experiment. The initial vapour product for the simulation has an activation diameter of 2.5 nm, i.e. particles larger than 2.5 nm are in supersaturated conditions. The model predicts continuous new particle formation as well as rapid growth of both the new particles and the pre-existing particles. The reason for the different appearance of the measured (h) and simulated (i) size distributions is due to varying experimental conditions (see text). (j) Size and time dependent growth rates calculated using the INSIDE method.

Fig. 3. Survival parameter of newly formed particles versus condensation sink: the survival parameter is defined as the particle formation rate at 6 nm divided by the formation rate at 2.5 nm, i.e. J₆/J_2.5. CLOUD measurements are indicated by diamond symbols and model simulations by square symbols without outlines. The points are coloured according to the particle growth rate at 3 nm, calculated from the measured HNO₃ and NH₃ concentrations (Fig. 1), the fuchsia colour indicates conditions of either no growth (GR = 0) or evaporation of NH₄NO₃ (GR < 0). The CLOUD experiments are those shown in Fig. 2, the experimental conditions are listed in its caption. All the model simulations assume kinetic nucleation (zero evaporation), and ∼10 nm h⁻¹ early growth (from H₂SO₄) for non-activated particles, in the absence of any particle condensation sink. The model assumes a constant J_2.5 of 10 cm⁻³ s⁻¹. The model conditions are 5 °C, HNO₃ and NH₃ between 400 pptv and 4 ppbv, and a condensation sink between 0.01 and 0.13 s⁻¹. Experiments where the activation diameter is sufficiently low that the non-activated growth surpasses it result in activation of particles. Activated particles grow rapidly enough to survive loss in the presence of high condensation sinks whereas non-activated particles have very low survival probabilities. The experimental measurements show that the rapid particle growth rates from ammonium nitrate formation are sufficient to overcome losses of newly formed particles in high condensation sink environments. The good agreement of the model with the experimental data confirms that particle scavenging involves unit sticking probability, as expected from previous measurements in low condensation sink environments.

Fig. 4. Illustration of the binary behaviour of modelled survival of newly formed particles due to ammonium nitrate formation: (a–c) modelled particle survival parameter as a function of condensation sink and concentrations of HNO₃ and NH₃ at 5 °C. The model assumes constant HNO₃ and NH₃ concentrations and H₂SO₄ production rate, and simulates a variable CS at 300 nm. The red square in panel c is a model where the CS and J rates did not stabilise within the time of the model. (d) Particle survival parameter versus the calculated growth rate at 3 nm for different condensation sinks. When the growth is not positive, i.e. no condensation, the particle survival parameter, J₆/J_2.5, is extremely low and around 5 × 10⁻⁴ for CS = 0.005 s⁻¹. However, above the activation diameter, the total particle growth rates increase by up to a factor of 100 or more and the survival parameter approaches unity, even for condensation sinks as high as 0.1 s⁻¹. The dashed line in between positive and negative growth rates represents a range with no data points. (e) An inset of (d) with only growth rates above 0.