# Chemicals from Brominated Flame Retardants: Analytical Methods, Occurrence, Transport and Risks
## 4 Transport Mechanisms of BFRs
### Atmospheric Dispersion and Deposition Dynamics  
BFRs can be emitted into the atmosphere during their production, use, and disposal phases. Once in the atmosphere, BFRs can undergo long-range transport through processes such as advection (horizontal movement of air masses, which can transport BFRs over long distances) and diffusion (random movement of BFR molecules in the air, leading to their dispersion). These mechanisms significantly influence the deposition of BFRs from the atmosphere. Studies have shown that the deposition of BFRs can occur through diffusive gas exchange or by precipitation scavenging from the gas phase, which include wet deposition processes, and is influenced by seasonal atmospheric variability [91]. Diffusive gas exchange refers to the transfer of BFRs in their gaseous form from the atmosphere directly onto surfaces through molecular diffusion and air-surface exchange processes [92]. This process covers the dry deposition of airborne pollutants. The deposition of BFRs in soils, surface waters, and vegetation can result in their accumulation over time, leading to persistence and bioaccumulation [93]. Precipitation scavenging plays a crucial role in the wet deposition of hydrophobic organic chemicals, including BFRs, and is typically more significant than dry deposition [94]. It is considered the most important process for removing many semivolatile and non-volatile organic contaminants from the atmosphere [94]. Wet deposition involves the removal of BFRs from the atmosphere through precipitation, such as rain or snow [91]. The wet process can be characterized by a wet scavenging coefficient (defined as the ratio of the pollutant mass removed by precipitation to the total pollutant mass in the atmosphere) [95]. The wet scavenging of atmospheric pollutants includes in-cloud scavenging processes and below-cloud scavenging processes [95; 96]. Below-cloud atmospheric particles are removed by raindrops via Brownian diffusion, interception, and impaction [97]. The below-cloud scavenging coefficient can be estimated using Equation (1) [66]:  
\[\lambda\big{(}d_{p}\big{)}=\int_{0}^{\bowtie}\frac{\pi}{4}D^{2}V(D)E\big{(}D,d_ {p}\big{)}N(D)dD \tag{1}\]  
The variables in this case are the aerosol diameter (d\({}_{\text{p}}\)), raindrop size distribution function (N), raindrop fall velocity (V), and capture efficiency (E). There are two components to \(\lambda\): (1) the surface-to-volume ratio's impact is incorporated into the complex function of the raindrop size distribution to determine the fractional area swept by the falling raindrops; (2) E represents the proportion of aerosol particles in the raindrop sweep volume that are genuinely caught [66]. Therefore, factors such as collection efficiency, the terminal velocity of raindrops, raindrop size distribution, and particle size distribution play important roles in below-cloud scavenging [9; 98]. During the later stages of rainstorms, high concentrations of aerosols can significantly enhance precipitation efficiency, leading to more intense and localized precipitation clusters [98]. These processes were demonstrated in a study by [PERSON] et al. [91], who quantified 1,2-dibromo-4-(1,2-dibromoethyl)cyclohexane (TBECH) and brominated diphenyl ethers 47 (BDE-47) in almost all precipitation samples from Saturna Island and Tadoussac in Canada. The movement of BFRs in the atmosphere is mainly influenced by their physical and chemical properties, along with weather conditions such as temperature etc. as it directly affects vapor pressure and octanol-air partition coefficients (K\({}_{\text{OA}}\)) [55]. [PERSON] et al. [91] showed how seasonal changes affect the distribution of BFRs in the air in Canada. They discovered that the levels of TBECH and BDE-47 measured by active air samplers closely followed the average air temperature. These levels were higher in the warmer months (April to August) and lower in the colder months (September to March).