**2.2. Composition of atmospheric particles derived by their physical properties**

To calculate the particles size we used Mie's theory. It says that the intensity of light scattered in all directions depends on the size and composition of the particle and the wavelength of incident light. Mie's theory considers spherical particles, so the optical counters used this assumption to obtain a value about the size of atmospheric particles.

A particle exposed to a beam of light will eliminate some of the energy that hits on it. This phenomenon of extinction is given by the combination of absorption and scattering of light. Water droplets do not absorb radiation and only scatters light. With this justification, we assume that the dispersion coefficient values are a good approximation to the extinction coefficient of particles.

The amount of light scattered by a particle depends on three combined effects: 1) reflection, 2) refraction and 3) diffraction. Refraction depends on the composition of the particle, while the diffraction depends on the wavelength of incident light, the size, and the shape of the particle. Optical counters use these concepts to calculate the size of particles in an air sample. With this information it is possible to characterize the particles counted in certain size ranges, obtaining the size distribution spectrum of particles.

The instruments are calibrated using refraction indexes for water (1.33), ammonium sulphate (1.48) or sodium chloride particles (1.54). Figure 1 shows the dispersion efficiency depending on the particles diameter with the three refraction indexes from Mie's theory.

Figure 1 shows the sensitivity of dispersion efficiency by varying the composition of particles. If we consider a particle with 0.4 μm diameter, the changes on its composition from water (n = 1.33) to sodium chloride (n = 1.58) is approximately 30 times. This indicates that the difference in the amount of light scattered by a cloud droplet and a NaCl particle increases by several orders of magnitude.

The coefficient of dispersion of a population of particles with size r0 to rn is defined by

$$
\sigma\_{\rm s} = \int\_{\rm r\_0}^{\rm r\_0} Q\_{\rm s}(\eta, \rm r, \lambda) \pi r^2 n(r) dr \tag{1}
$$

where Qs is the scattering efficiency, which is a function of refractive index (η) radius of the particle (r) and the wavelength of scattered light (λ). And n(r) represents the concentration of a population of particles according to radius r.

$$\sigma\_{\rm s} = \sum\_{l=1}^{n} \mathbf{Q\_{s}(\eta, \mathbf{r\_{l}}, \lambda)} \pi r\_{l}^{2} \mathbf{n(r\_{l})} \text{dr} \tag{2}$$

With this equation and the data obtained by particle counters we calculated the particles dispersion coefficients for the vicinity of the clouds. We used 28 different refractive indexes, ranging from 1.33 to 1.60, resulting in a matrix of 28 dispersion coefficients for each data. The values were compared against the dispersion coefficients obtained directly from a nephelometer, inferring the approximate refractive index and a possible particles composition.

**Figure 1.** Dispersion efficiency and particle diameter for three different refractive indexes

### **2.3. Sampling**

226 Atmospheric Aerosols – Regional Characteristics – Chemistry and Physics

PCASP and the FSSP.

coefficient of particles.

suction pump to enter the air. The path that the air sample covers must be considered when comparing the data from internal and external instruments. For instance, the CN counter readings show a 1 second delay compared with those obtained instantaneously from the

**2.2. Composition of atmospheric particles derived by their physical properties** 

To calculate the particles size we used Mie's theory. It says that the intensity of light scattered in all directions depends on the size and composition of the particle and the wavelength of incident light. Mie's theory considers spherical particles, so the optical counters used this assumption to obtain a value about the size of atmospheric particles.

A particle exposed to a beam of light will eliminate some of the energy that hits on it. This phenomenon of extinction is given by the combination of absorption and scattering of light. Water droplets do not absorb radiation and only scatters light. With this justification, we assume that the dispersion coefficient values are a good approximation to the extinction

The amount of light scattered by a particle depends on three combined effects: 1) reflection, 2) refraction and 3) diffraction. Refraction depends on the composition of the particle, while the diffraction depends on the wavelength of incident light, the size, and the shape of the particle. Optical counters use these concepts to calculate the size of particles in an air sample. With this information it is possible to characterize the particles counted in certain

The instruments are calibrated using refraction indexes for water (1.33), ammonium sulphate (1.48) or sodium chloride particles (1.54). Figure 1 shows the dispersion efficiency depending on the particles diameter with the three refraction indexes from Mie's theory.

Figure 1 shows the sensitivity of dispersion efficiency by varying the composition of particles. If we consider a particle with 0.4 μm diameter, the changes on its composition from water (n = 1.33) to sodium chloride (n = 1.58) is approximately 30 times. This indicates that the difference in the amount of light scattered by a cloud droplet and a NaCl particle

The coefficient of dispersion of a population of particles with size r0 to rn is defined by

σ� � ∑ Q��η, r�, λ�πr�

��

σ� � � Q��η, r, λ�πr�n�r�dr ��

where Qs is the scattering efficiency, which is a function of refractive index (η) radius of the particle (r) and the wavelength of scattered light (λ). And n(r) represents the concentration

With this equation and the data obtained by particle counters we calculated the particles dispersion coefficients for the vicinity of the clouds. We used 28 different refractive indexes,

�n�r��dr �

(1)

��� (2)

size ranges, obtaining the size distribution spectrum of particles.

increases by several orders of magnitude.

of a population of particles according to radius r.

In EPIC 2001 project we did 19 flights to investigate the ocean-atmosphere interaction, and clouds and aerosol particles properties in the Eastern Pacific. Nine flights were conducted within the ITCZ. The flights were in the area between 8° - 12° North latitude, and 93° - 97° West longitude (figure 2). During the flights were searched and selected young convective and precipitation clouds.

**Figure 2.** EPIC 2001 research area.
