Study of scavenging of submicron-sized aerosol particles by thunderstorm rain events

Abstract

Observed scavenging coefficients for 0.013–0.75 μm particles are between 1.08×10⁻⁵ and 7.58×10⁻⁴ s⁻¹. Based on observed results a correlation between scavenging coefficient and rain intensity is obtained to study below thundercloud scavenging of atmospheric aerosols during thunderstorm rain events. When the rain intensity increases from 5.24 to 45.54 mm h⁻¹, the corresponding scavenging coefficient increases from 0.5×10⁻⁵ to 4×10⁻⁵ s⁻¹ for thunderstorm rain episodes. The overall scavenging coefficients for 0.02 – 10 μm particles at different rainfall rates are estimated from contributions of Brownian diffusion, directional interception, inertial impaction, thermophoresis, diffusiophoresis and electrical forces during thunderstorms. The evolutions of PSD are predicted at different time intervals with theoretical scavenging rates. Comparison of observed evolutions of PSD during thunderstorm rain events with predicted evolutions of PSD shows an order of discrepancy between the observed and model results. Possible causes for discrepancy are discussed in terms of uncertainties in observed data and shortcomings in theoretical approach. The present results are useful for recommendations for the type of experimental setup essential for the field study of precipitation scavenging and improvements in theoretical approach close to atmospheric conditions during thunderstorm rain events.

Introduction

A significant fraction of the earth's rainfall in temperate climates comes from thunderstorms. Rainfall associated with electrified and lightning-producing storms plays an important role in the natural washout of atmospheric particles. Raindrops falling through the cloud of aerosol particles collect a fraction of these particles in their path. The fraction of particles in the cylindrical volume swept out by a falling raindrop that makes contact with the droplet is known as collision efficiency. The tendency for particles to be carried around the droplet by the flow, instead of making contact with it, results in collision efficiency less than unity. The collision efficiencies are even less than 10⁻³ for aerosol particles in the aerodynamic diameter (d_ae) range 0.2 – 2 μm (Horn et al., 1988; Garcia et al., 1994; Mircea et al., 2000; Androache, 2003; Ma et al., 2004). Particles in this size range are too large to have sufficient Brownian diffusivity and too small to get effectively collected by raindrops due to mechanisms of inertial impaction and directional interception. The region of the low collision efficiency for particles in the accumulation mode (0.2<d_ae<2.0 μm) is known as “Greenfield gap” following the work of Greenfield (1957). Submicron particles in the Greenfield gap form a most important fraction of the atmospheric aerosols. Particles in this size mode contribute dominantly to visibility degradation in the atmosphere. Also, this size fraction is mainly deposited in the deeper regions of the respiratory tract when inhaled.

Despite its importance, very little is known about the mechanisms by which submicron particles are removed from the atmosphere by precipitation. The few scavenging field measurements available (Davenport and Peter, 1978; Radke et al., 1980; Schumann, 1989; Nicholson et al., 1991; Volken and Schumann, 1993) show significant discrepancies between observation and theory, with measured scavenging efficiencies are higher than predicted efficiencies of raindrops for 0.2 – 2.0 μm particles. Recently, Laakso et al. (2003) reported the results of scavenging coefficients based on 6 years field measurements for the aerosol particles having a diameter of 0.01 – 0.5 μm. Brownian diffusion is the main removal process for 0.01 – 0.5 μm particles. However, the effect of electrical force is one of the major source of uncertainty in the estimation of the scavenging coefficient for particles smaller than 1 μm (McGann and Jennings, 1991; Jaworek et al., 2002). Electrical effects can enhance the efficiency with which aerosol particles are captured by precipitation-sized drops (Grover et al., 1977; Wang et al., 1978; McGann and Jennings, 1991; Byrne and Jennings, 1993; Pranesha and Kamra, 1997; Tinsley et al., 2000, Tinsley et al., 2001; Jaworek et al., 2002).

The effect of electrical forces on collection efficiencies of raindrops collecting submicron particles during thunderstorm rain events was investigated with measurements from field experiments (Chate and Pranesha, 2004a, Chate and Pranesha, 2004b). Theoretical scavenging collection efficiency and scavenging coefficients described in this study included collection mechanisms such as Brownian diffusion, directional interception, inertial impaction and electrical charge effects. When computations were performed with the combination of these collection mechanisms, large discrepancies were found between observed and theoretical results. It seems that phoretic forces due to temperature and concentration gradients likely to play an effective role in the collection of submicron aerosols by raindrops due to the occurrence of condensation and rapid evaporation in atmospheric conditions associated with thunderstorm rain events. The role of thermophoresis mechanism in depleting the number concentrations of aerosol particles due to temperature gradient between the layers of the atmosphere is examined on the basis of field measurements (Chate and Pranesha, 2004b). The phenomena of thermophoresis, diffusiophoresis and condensational growth of hygroscopic particles (Chate et al., 2003) along with electrostatic forces during a thunderstorm may be important as removal processes for the submicron particles (Chate and Pranesha, 2004a, Chate and Pranesha, 2004b).

In order to resolve the discrepancy between theoretical and observed results, the contributions of thermophoresis, diffusiophoresis and electrical effects in collision efficiency computations need to be combined with the Brownian diffusion, directional interception and inertial impaction. It is essential to consider the dependence of the water vapor pressure over a curved surface of droplet and the vapor pressure differences corresponding to the difference in the vapor pressure of water at temperatures associated with the temperature differences in the range of 1 to 5 °C and that of 3 °C between drop surface and ambient air, in the contribution of diffusiophoresis to improve the accuracy of collision efficiency. Computations of collision efficiency and scavenging coefficients need to be performed for a range of temperature differences between temperature of drop surface and ambient temperature to understand the relative influence of phoretic effects in the contributions of various collection mechanisms. Additional aspects which were not considered by Chate and Pranesha, 2004a, Chate and Pranesha, 2004b have been included in the present work to compute overall collision efficiency and thus scavenging coefficients to study their combined effects on evolutions of particle size distributions (PSD) of atmospheric aerosols during thunderstorm rain events. Theoretical scavenging coefficients are averaged over the particles in the diameter range 0.02 – 1 μm at various rainfall intensities (e.g. 5.24, 6.78, 10.17, 15.78, 19.48, 45.54 mm h⁻¹). Raindrop size distributions are adopted from Best (1950) for computations of scavenging coefficients for a range of rainfall intensities instead of lognormal distribution functions (for rainfall intensity of 0.5 and 25 mm h⁻¹) used in the previous study (Chate and Pranesha, 2004a, Chate and Pranesha, 2004b). Direct measurements of evolutions of aerosol PSD for 0.01–1 μm particles during thunderstorm rain events have been lacking in the literature and are very sparse in the Indian region. In this paper for the first time the evolutions of PSD for the most important fractions of the atmospheric aerosols during thunderstorm rain events are presented to understand the mechanisms of precipitation scavenging of submicron aerosols in the atmospheric conditions below the electrified and lightning-producing storms. The present study mainly focuses on the comparison of observed and theoretical scavenging coefficients and evolutions of PSD before and after thunderstorm rain episodes to understand the role of electrical and phoretic forces together in the natural washout of submicron aerosols during thunderstorm rain events. Possible causes for discrepancies between the model and observations are discussed in terms of limitations in theoretical study and uncertainties in available experimental data obtained in field measurements. Recognition of the factors responsible for discrepancies between the model and observations may be useful for recommendations for type of experimental setup needed for further field study. Accordingly, it is possible to develop and verify a more complete model based on parameters close to atmospheric conditions during a thunderstorm rain event.