Parameterization of In‐Cloud Aerosol Scavenging Due To Atmospheric Ionization: 2. Effects of Varying Particle Density

Abstract

Simulations and parameterization of collision rate coefficients for aerosol particles with 3 μm radius droplets have been extended to a range of particle densities up to 2,000 kg m−3 for midtropospheric (~5 km) conditions (540 hPa, −17°C). The increasing weight has no effect on collisions for particle radii less than 0.2 μm, but for greater radii the weight effect becomes significant and usually decreases the collision rate coefficient. When increasing size and density of particles make the fall speed of the particle relative to undisturbed air approach to that of the droplet, the effect of the particle falling away in the stagnation region ahead of the droplet becomes important, and the probability of frontside collisions can decrease to zero. Collisions on the rear side of the droplet can be enhanced as particle weight increases, and for this the weight effect tends to increase the rate coefficients. For charges on the droplet and for large particles with density ρ < 1,000 kg m−3 the predominant effect increases in rate coefficient due to the short‐range attractive image electric force. With density ρ above about 1,000 kg m−3, the stagnation region prevents particles moving close to the droplet and reduces the effect of these short‐range forces. Together with previous work, it is now possible to obtain collision rate coefficients for realistic combinations of droplet charge, particle charge, droplet radius, particle radius, particle density, and relative humidity in clouds. The parameterization allows rapid access to these values for use in cloud models.

Parameterization of In‐Cloud Aerosol Scavenging Due to Atmospheric Ionization: Part 3. Effects of Varying Droplet Radius

“…First, the rate coefficients for a particle density less than 1,000 kg/m 3 increase as the particle radius increases, which indicates the intercept effect. Second, the rate coefficients decrease as the particle density increases, which indicates the weight effect (Zhang & Tinsley, , Figure 1); for a particle density of 1 kg/m 3 the particle does not move relative to the air, so there is only the intercept affect and diffusion. Zhang and Tinsley (), their Figure 5, shows that the weight effect varies linearly with particle density for a density less than 1,000 kg/m 3 and for a particle density greater than 1,000 kg/m 3 provided the particle radius is small, while the weight effect for a density greater than 1,000 kg/m 3 varies nonlinearly for the larger particles.…”

“…The inertia of particles plays a small but increasing role in the particle size effect for the larger particle masses and for the larger droplets with increasing fall speeds and larger accelerations of the flows around them. The noninertial particle size effects are more significant for smaller droplets, and they have been discussed in more detail for 3‐μm‐radius droplets (Zhang & Tinsley, , ). In addition, the short‐range effects for large particles include the image electric forces induced by droplet charges (Zhang & Tinsley, , ) as well as the image electric forces induced by particle charges as shown in Figure .…”

“…The noninertial particle size effects are more significant for smaller droplets, and they have been discussed in more detail for 3‐μm‐radius droplets (Zhang & Tinsley, , ). In addition, the short‐range effects for large particles include the image electric forces induced by droplet charges (Zhang & Tinsley, , ) as well as the image electric forces induced by particle charges as shown in Figure . The results of the simulations of collision rate coefficients R A , a ,0, q ,100%, ρ for large particles are given in Appendix A4.…”

“…We determine the collision rate coefficient R A , a , Q , q , RH , ρ between particles and droplets, with droplet radius A , droplet charge Q , particle radius a , particle charge q , relative humidity RH , and particle density ρ , while the droplet density is the constant value of 1,000 kg/m 3 . In our previous work, the rate coefficients have been parameterized for a particle density of 500 kg/m 3 and a relative humidity of 100% with droplet radii up to 15 μm (Tinsley & Leddon, ; Tinsley & Zhou, ) and for varying humidity and particle density for a droplet radius of 3 μm (Zhang, ; Zhang & Tinsley, , ). In this paper we apply our Monte Carlo trajectory model to calculate the collision rate coefficient R A , a , Q , q , RH , ρ for droplet radii A of 3, 6, 9, 12, and 15 μm; droplet charges Q of −100e, −50e, −20e, −10e, 0e, 10e, 20e, 50e, and 100e, where e is the elementary electric charge; particle radius a varying from 0.01 μm to A ; particle charges q of 0e, 10e, 20e, and 50e; particle densities ρ of 1, 500, 1,000, 1,500, and 2,000 kg/m 3 ; relative humidities RH of 95%, 98%, 99%, 100%, and 101%, for 540 hPa and 256.15 K. As the contributing factors to rate coefficients for particles with radius less or greater than 0.2 μm vary differently (Zhang & Tinsley, , ), we parameterize the rate coefficients for the two ranges separately.…”

Parameterization of In‐Cloud Aerosol Scavenging Due to Atmospheric Ionization: Part 4. Effects of Varying Altitude

“…The effect of the image electric force induced by the droplet charge usually is weak and only exists when the size of the droplet and particle are comparable, thus most of the time it can be ignored (Zhang & Tinsley, , ).…”