Scavenging Coefficient Research Articles

Modelling wet deposition in simulations of volcanic ash dispersion from hypothetical eruptions of Merapi, Indonesia

The statistical impact of including the process of wet deposition in dispersion model predictions of the movement of volcanic ash is assessed. Based on hypothetical eruptions of Merapi, Indonesia, sets of dispersion model simulations were generated, each containing four simulations per day over a period of three years, to provide results based on a wide range of atmospheric conditions. While on average dry sedimentation removes approximately 10% of the volcanic ash from the atmosphere during the first 24 h, wet deposition removes an additional 30% during seasons with highest rainfall (December and January) but only an additional 1% during August and September. The majority of the wet removal is due to in-cloud rather than below-cloud collection of volcanic ash particles. The largest uncertainties in the amount of volcanic ash removed by the process of wet deposition result from the choice of user-defined parameters used to compute the scavenging coefficient, and from the definition of the cloud top height. Errors in the precipitation field provided by the numerical weather prediction model utilised here have relatively less impact.

Wet precipitation scavenging of soluble atmospheric trace gases due to chemical absorption in inhomogeneous atmosphere

We analyze the effects of irreversible chemical reactions of the first and higher orders and aqueous-phase dissociation reactions on the rate of trace gas scavenging by rain in the atmosphere with non-uniform concentration and temperature. We employ an one-dimensional model of precipitation scavenging of chemically active soluble gaseous pollutants that is valid for small gradients of temperature and concentration in the atmosphere. It is demonstrated that transient altitudinal distribution of concentration under the influence of rain is determined by the partial hyperbolic differential equation of the first order. Scavenging coefficients are calculated for wet removal of chlorine, nitrogen dioxide and sulfur dioxide for the exponential and linear initial altitudinal distributions of trace gases concentration in the atmosphere and linear and uniform altitudinal temperature distributions. Theoretical predictions of the dependence of the magnitude of the scavenging coefficient on rain intensity for sulfur dioxide are in a good agreement with the available atmospheric measurements.

Precipitation scavenging of aerosol particles at a rural site in the Czech Republic

The influence of in-cloud and below-cloud scavenging, described by the obscurities (mist, fog and shallow fog) and precipitation, on submicron atmospheric aerosol (AA) particle number size distributions (PNSDs) was studied using 5 years of measurements at the rural background station Košetice. The typical PNSDs during individual meteorological phenomena were compared, and the change in the concentrations before and after the beginning of the phenomenon, the scavenging coefficient λs, and the rate of change of the AA concentrations with time were computed. It was found that both obscurities and precipitation have a strong influence on the AA concentrations, both on the total number concentrations and on the PNSDs. The presence of phenomena even changes the number of modes on the PNSDs. The PNSD main mode is shifted to the larger particles, and the concentrations of particles smaller than 50 nm in diameter are considerably lower. In nucleation mode, however, wet scavenging does not seem to be the main process influencing the AA concentrations, although its considerable effect on the concentration was proved. During obscurities, there is a typical PNSD to which the PNSDs converge at any mist/fog/shallow fog event. The concentrations of AA particles smaller than 80 nm are lower than they are during periods without any phenomenon recorded, and the concentrations of the strongly prevailing accumulation mode are higher. During liquid precipitation, PNSDs are lower when compared to non-event periods. With larger droplets of the phenomenon, the position of the main mode of the bimodal PNSDs is shifted to the smaller particles. The process of gas-to-particle conversion takes place in the breaks from precipitation during a rain showers period. Precipitation containing frozen hydrometeors behaves differently from liquid precipitation. Concentrations of AA particles larger than 200 nm during precipitation containing solid particles do not differ from non-event cases, suggesting insignificant scavenging.

Scavenging of black carbon in Chilean coastal fogs

In November/December 2013 a pilot experiment on aerosol/fog interaction was conducted on a coastal hill in the suburbs of Valparaíso, Chile. Passages of garúa fog were monitored with continuous recordings of a soot photometer and an optical aerosol spectrometer. An optical fog sensor and an automatic weather station provided meteorological data with which the aerosol could be classified. High-resolution back trajectories added meteorological information. From filter samples, optical and chemical aerosol information was derived. Scavenging coefficients of black carbon (BC) and measured particulate mass below 1μm diameter (PM1) were estimated with three approaches. Averaging over all fog periods of the campaign yielded a scavenging coefficient of only 6% for BC and 40% for PM1. Dividing the data into four 90°-wind sectors gave scavenging factors for BC ranging from 13% over the Valparaíso, Viña del Mar conurbation to 50% in the marine sector (180°–270°). The third, and independent approach was achieved with two pairs of chemical aerosol samples taken inside and outside fogs, which yielded a scavenging coefficient of 25% for BC and 70% for nonseasalt sulfate. Whereas fogs occurred rather infrequently in the beginning of the campaign highly regular daily fog cycles appeared towards the end of the experiment, which allowed the calculation of typical diurnal cycles of the aerosol in relation to a fog passage.

Concentration Changes Of PM10 Under Liquid Precipitation Conditions

Abstract This study reports the results of field research into variability of the scavenging coefficient (Λ) of suspended dust comprising particles with aerodynamic diameters less than 10 mm. Registration of PM10 over 7 years in conditions of the occurrence of rainfall (convective light showers, large-scale precipitation and storms) was undertaken in an undeveloped rural area. The analysis involved 806 observations taken at constant time intervals of 0.5 hour. The measurements of the concentration of PM10 were performed by means of a reference method accompanied by concurrent registration of basic meteorological parameters. It was found that, for PM10, the scavenging efficiency is considerably influenced by rainfall intensity R and the type of precipitation. In the case of convective precipitation, data on Λ are only partially related to “classical approach” of rain scavenging. Within the range of comparable values of rainfall intensity, the type of wet deposition (except for storms) does not influence the effectiveness of scavenging PM10 from the ground-level zone. The large number of observations conducted in real-time conditions yielded a proposal of simple regression model, which can be deemed suitable for the description of variability Λ (DPM10), but only to a limited extent for large-scale precipitation. The collected results can be applied in air pollution dispersion models and deposition and were found to be generally representative for areas with similar climatic characteristics.

Modelling of the atmospheric dispersion of mercury emitted from the power sector in Poland

Poland belongs to the group of EU countries with the highest levels of mercury emissions, with a large portion of these emissions being related to coal combustion. This paper presents a modelling analysis of the impact that the Polish power sector has on the atmospheric concentrations of mercury. A detailed mercury emission inventory is used to analyse the concentration and deposition of mercury. For this study, a chemical scheme devoted to mercury transformations in the atmosphere was implemented into the Polyphemus air quality system. The system was then used to perform simulations for 2008 in two domains i.e. over Europe and over Poland. The impact of various parameters on concentration and wet scavenging of mercury has been analysed. The results of the mercury ambient concentrations and depositions, are presented. Additionally, the contribution of natural and anthropogenic sources to mercury deposition in Poland is shown. The performed works showed that the national sources have low impact to overall deposition, however local contribution in wet deposition of big emitters may reach 50%. Sensitive analysis showed a significant impact of reaction with bromine compound and scavenging coefficient on modelled results of mercury concentration and deposition.

Scavenging of radioactive soluble gases from inhomogeneous atmosphere by evaporating rain droplets

We analyze effects of inhomogeneous concentration and temperature distributions in the atmosphere, rain droplet evaporation and radioactive decay of soluble gases on the rate of trace gas scavenging by rain. We employ a one-dimensional model of precipitation scavenging of radioactive soluble gaseous pollutants that is valid for small gradients and non-uniform initial altitudinal distributions of temperature and concentration in the atmosphere. We assume that conditions of equilibrium evaporation of rain droplets are fulfilled. It is demonstrated that transient altitudinal distribution of concentration under the influence of rain is determined by the linear wave equation that describes propagation of a scavenging wave front. The obtained equation is solved by the method of characteristics. Scavenging coefficients are calculated for wet removal of gaseous iodine-131 and tritiated water vapor (HTO) for the exponential initial distribution of trace gases concentration in the atmosphere and linear temperature distribution. Theoretical predictions of the dependence of the magnitude of the scavenging coefficient on rain intensity for tritiated water vapor are in good agreement with the available atmospheric measurements.

Source identification of total mercury (TM) wet deposition using a Lagrangian particle dispersion model (LPDM)

Event-based precipitation samples for total mercury (TM) were collected with a modified MIC-B sampler concurrent with atmospheric gaseous oxidized mercury (GOM) and particulate bound mercury (PBM) concentrations in Seoul, Korea from January 2006 to December 2009 to: 1) determine the seasonal variations in TM wet deposition, 2) investigate contributions of GOM and PBM scavenging to mercury wet deposition, and 3) identify source areas contributing to the high TM wet deposition using a Lagrangian particle dispersion model (LPDM). The volume-weighted mean (VWM) TM concentration was highest in winter, while the wet deposition flux was highest in summer. Multiple linear regression showed that the scavenging coefficient (SC) for GOM was much higher than the SC for PBM indicating that GOM was more effectively scavenged by wet deposition than PBM. Joint-probability LPDM (JP-LPDM) indicated that the main sources of TM wet deposition were the industrial and coastal areas in China and Japan.

工业蒸汽管道吹扫的探讨

在工业蒸汽管道吹扫过程中，准备工作充分、吹扫程序合理是安全吹扫的重要保障，满足吹扫流速、吹管系数要求是保证吹扫效果的重要指标。本文根据理论计算并结合工作中的实际吹扫经验，探讨工业蒸汽管道吹扫的实施过程。 It is an important guarantee of purging safety with sufficient preparation work and reasonable purging procedure in the process of industrial steam pipe purging, and there is an important index of purging effect meeting purging velocity and pipe scavenging coefficient. This paper discusses the actual work based on the theory of simulation calculation and the previous purging experience for the implementation of the industrial steam pipe purging process.

Bulk or modal parameterizations for below‐cloud scavenging of fine, coarse, and giant particles by both rain and snow

AbstractBulk or modal parameterizations for below‐cloud mass and number scavenging coefficients Λm (s−1) and Λn (s−1) of three aerosol modes—fine (PM2.5), coarse (PM2.5–10), and giant (PM10+)—for both rain and snow scavenging are developed for use in modal‐approach aerosol transport models. The new bulk parameterizations are based on the size‐resolved Λ(d) parameterization of Wang et al. (2014), using assumed lognormal mass and number size distributions for PM2.5, PM2.5–10, and PM10+. The resulting modal‐mean formulas for Λm and Λn follow power law relationships with precipitation intensity R, consistent with most existing studies. The empirical parameters in the power law relationships obtained in this study are also within the range of parameter values obtained in previous field and theoretical studies. Uncertainties in Λm due to the size distribution or size range assumed for each aerosol mode are generally smaller than 30% for PM2.5–10 and PM10+ but could be on the order of factor of 2 for PM2.5. These uncertainties, however, are much smaller than other known uncertainties in existing Λ formulations, which are typically larger than 1 order of magnitude. Moreover, the new bulk parameterizations are believed to be more representative than most existing schemes because the size‐resolved parameterization of Wang et al. (2014), which they are based on, was developed with consideration of all available theoretical formulations and field‐derived estimates for size‐resolved Λ and their associated uncertainties.

Precipitation scavenging of gaseous pollutants having arbitrary solubility in inhomogeneous atmosphere

We investigate scavenging of gaseous pollutants in the atmosphere under the combined influence of rain and varying temperature distribution that affects the rate of soluble gas scavenging. We employ a one-dimensional model of precipitation scavenging of gaseous pollutants having arbitrary solubility that is valid for small gradients and for non-uniform initial vertical distributions of temperature and soluble trace gases concentration in the atmosphere. It is showed that transient altitudinal distributions of temperature and concentration under the influence of rain are determined by linear wave equations that describe propagation of temperature and scavenging wave fronts. Scavenging coefficient and the rates of precipitation scavenging are calculated for wet removal of methanol (\({\text{CH}}_{3} {\text{OH}}\)) using measured initial distribution of methanol and temperature in the atmosphere. Theoretical predictions of the dependence of the magnitude of the scavenging coefficient on rain intensity for tritium oxide and sulfur dioxide are in good agreement with the available atmospheric measurements.

Below-cloud scavenging of aerosol particles by precipitation in a typical valley city, northwestern China

To fill the blank information for aerosol precipitation-scavenging research in north-west of China, the aerosol particle and raindrop size distributions were measured simultaneously during 1 September 2012 to 31 August 2013 in urban Lanzhou. The scavenging coefficients of thunderstorm and non-thunderstorm rain and snow events were studied and presented on the basis of nine selected precipitation cases including 3 snow and 6 rain events. The variation of scavenging coefficients of snowfall across the size distribution clearly exhibited a trough of lower values for particles of 1000 nm–2000 nm in diameter, while the particles smaller than 500 nm were scavenged efficiently by non-thunderstorm rain, and thunderstorm rain more effectively scavenged the particles in 500–1000 nm. The snow scavenging coefficients varied between 3.11 × 10−7 s−1 and 1.18 × 10−3s−1 in the 10–10,000 nm size range. The scavenging coefficients of thunderstorm (non-thunderstorm) rain were between 8.25 × 10−7s−1 (7.48 × 10−6s−1) and 1.23 × 10−3s−1 (7.46 × 10−4s−1). Additionally, the number of particles in 10–50 nm was more sensitive to duration of snow, while snowfall intensity was more responsible for particle number concentrations in 50–100 nm and 100–1000 nm. The longer period of precipitation with lower raindrop velocity can more effectively scavenge the particles in the size range of 10–50 nm.

Minimum Collection Efficiency Diameter during Snow Scavenging Process

The scavenging of atmospheric aerosols by below-cloud scavenging processes during rain or snow precipitation plays an important role in the removal of aerosols in the atmosphere. Because of a variety of frozen precipitation types and their physical properties, the snow scavenging is a more complicated process compared with the rain scavenging. This study determined the minimum collection efficiency and minimum collection efficiency diameter for polydispersed hydrometeor size distributions during the snow precipitation. Three types of collectors (graupel, dendrite, and column) were considered and the obtained minimum collection efficiency and the corresponding minimum collection efficiency diameter were intercompared. Results showed that the minimum collection efficiency diameters for snow were smaller than those for rain. The minimum collection efficiency diameters of graupel decreased as the collector diameter increased, whereas other collectors (i.e., dendrite and column) did not show this decreasing tendency. The minimum collection efficiencies of graupels were larger than those of dendrite and column collectors. It was also shown that column collectors had the largest scavenging coefficients.

Validation of the TRMM Multi Satellite Rainfall Product 3B42 and estimation of scavenging coefficients for 131I and 137Cs using TRMM 3B42 rainfall data

The TRMM rainfall product 3B42 is compared with rain gauge observations for Kaiga, India on monthly and seasonal time scales. This comparison is carried out for the years 2004–2007 spanning four monsoon seasons. A good correlation is obtained between the two data sets however; magnitude wise, the cumulative precipitation of the satellite product on monthly and seasonal time scales is deficient by almost 33–40% as compared to the rain gauge data. The satellite product is also compared with APHRODITE's Monsoon Asia data set on the same time scales. This comparison indicates a much better agreement since both these data sets represent an average precipitation over the same area. The scavenging coefficients for 131I and 137Cs are estimated using TRMM 3B42, rain gauge and APHRODITE data. The values obtained using TRMM 3B42 rainfall data compare very well with those obtained using rain gauge and APHRODITE data.

Non-isothermal scavenging of highly soluble gaseous pollutants by rain in the atmosphere with non-uniform vertical concentration and temperature distributions

We suggest a non-isothermal one-dimensional model of precipitation scavenging of highly soluble gaseous pollutants in inhomogeneous atmosphere. When gradients of soluble trace gases’ concentrations and temperature in the atmosphere are small, scavenging of gaseous pollutants is governed by two linear wave equations that describe propagation of a scavenging and temperature waves in one direction. If wash-down front velocity is much larger than the velocity of the temperature front, scavenging is determined by propagating scavenging front in the atmosphere with inhomogeneous temperature distribution. We solved the derived equation by the method of characteristics and determined scavenging coefficient and the rates of precipitation scavenging for wet removal of sulfur dioxide using measured initial distributions of trace gases and temperature in the atmosphere. It is shown that in the case of exponential initial distribution of soluble trace gases and linear temperature distribution in the atmosphere, scavenging coefficient in the region between the ground and the position of a scavenging front is proportional to rainfall rate, solubility parameter in the under-cloud region, adjacent to a bottom of a cloud and to the growth constant in the formula for the initial profile of a soluble trace gas in the atmosphere. The derived formula yields the same value of scavenging coefficient for sulfur dioxide scavenging by rain as field estimates presented by McMahon and Denison (Atmos Environ 13:571–585, 1979). It is demonstrated that in the case when the altitude variation of temperature in the atmosphere is determined by the environmental lapse rate, scavenging coefficient increases with height in the region between the scavenging front and the ground. In the case when altitude temperature variation in the atmosphere is determined by temperature inversion, scavenging coefficient decreases with height in a region between the scavenging front and the ground. Theoretical predictions of the value of the scavenging coefficient for sulfur dioxide washout by rain and of the dependence of the magnitude of the scavenging coefficient on rain intensity are in good agreements with the atmospheric measurements of Martin (Atmos Environ 18:1955–1961, 1984).

Development of a new semi-empirical parameterization for below-cloud scavenging of size-resolved aerosol particles by both rain and snow

Abstract. A parameter called the scavenging coefficient Λ is widely used in aerosol chemical transport models (CTMs) to describe below-cloud scavenging of aerosol particles by rain and snow. However, uncertainties associated with available size-resolved theoretical formulations for Λ span one to two orders of magnitude for rain scavenging and nearly three orders of magnitude for snow scavenging. Two recent reviews of below-cloud scavenging of size-resolved particles recommended that the upper range of the available theoretical formulations for Λ should be used in CTMs based on uncertainty analyses and comparison with limited field experiments. Following this recommended approach, a new semi-empirical parameterization for size-resolved Λ has been developed for below-cloud scavenging of atmospheric aerosol particles by both rain (Λrain) and snow (Λsnow). The new parameterization is based on the 90th percentile of Λ values from an ensemble data set calculated using all possible "realizations" of available theoretical Λ formulas and covering a large range of aerosol particle sizes and precipitation intensities (R). For any aerosol particle size of diameter d, a strong linear relationship between the 90th-percentile log10 (Λ) and log10 (R), which is equivalent to a power-law relationship between Λ and R, is identified. The log-linear relationship, which is characterized by two parameters (slope and y intercept), is then further parameterized by fitting these two parameters as polynomial functions of aerosol size d. A comparison of the new parameterization with limited measurements in the literature in terms of the magnitude of Λ and the relative magnitudes of Λrain and Λsnow suggests that it is a reasonable approximation. Advantages of this new semi-empirical parameterization compared to traditional theoretical formulations for Λ include its applicability to below-cloud scavenging by both rain and snow over a wide range of particle sizes and precipitation intensities, ease of implementation in any CTM with a representation of size-distributed particulate matter, and a known representativeness, based on the consideration in its development, of all available theoretical formulations and field-derived estimates for Λ (d) and their associated uncertainties.

Sensitivity of the modelled deposition of Caesium-137 from the Fukushima Dai-ichi nuclear power plant to the wet deposition parameterisation in NAME

This paper describes an investigation into the impact of different meteorological data sets and different wet scavenging coefficients on the model predictions of radionuclide deposits following the accident at the Fukushima Dai-ichi nuclear power plant in March 2011. Three separate operational meteorological data sets, the UK Met Office global meteorology, the ECMWF global meteorology and the Japan Meteorological Agency (JMA) mesoscale meteorology as well as radar rainfall analyses from JMA were all used as inputs to the UK Met Office's dispersion model NAME (the Numerical Atmospheric-dispersion Modelling Environment). The model predictions of Caesium-137 deposits based on these meteorological models all showed good agreement with observations of deposits made in eastern Japan with correlation coefficients ranging from 0.44 to 0.80. Unexpectedly the NAME run using radar rainfall data had a lower correlation coefficient (R = 0.66), when compared to observations, than the run using the JMA mesoscale model rainfall (R = 0.76) or the run using ECMWF met data (R = 0.80). Additionally the impact of modifying the wet scavenging coefficients used in the parameterisation of wet deposition was investigated. The results showed that modifying the scavenging parameters had a similar impact to modifying the driving meteorology on the rank calculated from comparing the modelled and observed deposition.

Lidar profiling of aerosol scavenging parameters at a tropical station, Pune, India

In this study, we deal with observations of aerosol column content (height integration of vertical distribution of aerosol number density) that have been carried out using an Ar+ lidar for three different measurement cycles (each cycle consisting of three experimental days associated with non-rain, rain, and non-rain, respectively) of weekly spaced observations for pre-monsoon (March/April 1994), monsoon (September 1991), and post-monsoon (October 1998). Based on these observed profiles of aerosol number concentration on rainy days with respect to those on non-rainy days, vertical distributions of scavenging collection efficiencies (SCEs) are computed and discussed in this article. The SCE is found to decrease from 0.3 to 0.01 between the heights, 100 and 800 m for thunderstorm rain in April 1994, and during monsoon, it increases from 0.1 to 0.7. In the October 1998 episode, SCE was found to increase initially from 0.35 to 0.75 for heights between 40 and 200 m and thereafter decrease to 0.35 in the height interval of 200–800 m. For the rainfall intensity increase from 1 to 10 mm hour−1, the corresponding scavenging coefficient (SC) for atmospheric layer 50–100 m varies from 4 × 10−6 to 4 × 10−5 s−1 for thunderstorm in April 1994 and between 5 × 10−6 and 5 × 10−5 s−1 in October 1998, respectively. During monsoon, these values vary from 3 × 10−5 to 5 × 10−4 s−1. They lie in the range of those observed in the earlier field studies. The results are found useful to establish links between aerosols and cloud properties, and the influence of such interactions on weather and climate.

Water spray geoengineering to clean air pollution for mitigating haze in China’s cities

In the past 30 years, China has suffered from air pollution and heavy haze created by fast industrial growth and economic expansion. This article reviews the techniques for remediation of air pollution. Then, I propose a geoengineering method for mitigating air pollution and haze in China’s cities by using water to scavenge air pollution. Here, water should be sprayed into the atmosphere like watering garden. The scientific rationale and mechanism for the geoengineering scheme are explained. It is found that precipitation scavenging coefficients are very sensitive to the size distributions of both aerosol and raindrops, and rain intensity. I found that the water spray geoengineering method can reduce the PM2.5 pollution in the atmosphere very efficiently to 35 μg m−3 level in a very short time period from few minutes to hours or days, depending on the precipitation characteristics. In addition, the water spray geoengineering method has excellent advantages such as rapidity, already available technology, low cost, and a nature-like process. This proposed geoengineering scheme can be one of the answers for fighting air pollution in the cities globally.

The influence of electric charge on minimum particle scavenging efficiency particle size during below-cloud scavenging processes

Below-cloud wet deposition scavenging processes are important for removing coarse and very fine particles from the atmosphere. This study investigated the influence of electric charge on particle deposition and estimated the scavenging gap particle size (the most penetrating particle size) along with the corresponding minimum collection efficiency and minimum scavenging coefficient particle size by using equations for below-cloud scavenging processes. A harmonic mean type approximation was used for calculations and the Cunningham correction factor was applied to include the slip effect of nuclei mode particles. Results based on the analytically approximated solution compared well with the numerically calculated results and previous studies that did not consider electric charging effects. The results showed that charge effects play an important role in determining the most penetrating particle size. This study also showed that use of the Cunningham correction factor may increase the most penetrating particle size under certain conditions. Subsequently, these results indicate that consideration of the electric charge effect with a Cunningham correction factor is important for estimating the most penetrating particle size.