*2.1.1. Optical counters*

Condensation nuclei particle counter (TSI-3760) is an instrument that increases the size of the particles with a forced growth in an environment artificially saturated with butanol. This increases the efficiency of detection and counting of very small droplets (Twomey, 1991).


**Table 1.** Aerosol and cloud particle instrumentation on the C130 aircraft

The CCN is a subset of the total concentration of particles, which can form droplets in an environments of over-saturation, as in a cloud (101 - 110% SS). The CCN counter model 100, determines the concentrations of these atmospheric particles. The operating principle is based on measuring the variation within a thermal gradient diffusion chamber, to create an environment of over-saturation. An electrical system controls the temperature of two plates that create the conditions of over saturation. A beam of laser light passes through the chamber and the instrument measures the amount of light scattered and estimates the concentration of CCN per volume. Delene et al (1998) and Delene and Deshler (2000) have a more detailed description and analysis of this instrument operation.

The PCASP, FSSP300, and FSSP100 provide information on particle concentrations in size defined ranges. The instruments pass a beam of light with specific wavelength and intensity, through the air sample. The particles in the sample scatter the light beam with an intensity that depends on their size, shape and composition. The instrument measures the amount of light scattered with sensors. By knowing the intensity of scattered light and particles composition (water for FSSP100 and FSSP300) or sodium chloride (for PCASP), it is possible to infer the size range of particles to which they belong. A more detailed description of the operation and measurement uncertainty associated with the PCASP can be obtained on Strapp et al (1992).

## *2.1.2. Instruments location*

224 Atmospheric Aerosols – Regional Characteristics – Chemistry and Physics

rectify strongly onto climate time scales in this region.

Horizon reversed its track along 95º W and then returned to port.

and might infer possible causes about their changes in concentrations.

instruments used in the EPIC 2001 flights for the study.

process study and long-term monitoring.

**2. Methodology** 

**2.1. Instrumentation** 

*2.1.1. Optical counters* 

particles.

ITCZ are highly variable on daily to intra-seasonal time scales. The effects of such variability

EPIC 2001 was conceived as an intensive process study along and near 95ºW during September and October 2001. This longitude was chosen to coincide with the Tropical Atmosphere Ocean project (TAO) mooring array in order to provide an overlap between the

In addition to the TAO moorings, two aircrafts, the National Center for Atmospheric Research's (NCAR) C-130 and NOAA's P-3, plus two ships, NOAA's R/V Ron H. Brown and the National Science Foundation's (NSF's) R/V New Horizon, and Galapagos-based soundings, were used to make measurements of the atmosphere and ocean in this region. The aircraft were based from 1 September to 10 October 2001 in Huatulco, Mexico. The ships spent approximately 3 weeks in the vicinity of 10º N, 95º W, and then traversed the 95º W line to the equator. After a short stop in the Galapagos Islands, the Ron H. Brown then proceeded south along 95º W and then to the Woods Hole Oceanographic Institute Improved Meteorological Recorder (IMET) mooring at 20º S, 85º W. Meanwhile the New

On this study we use a P-3 aircraft belonging to the National Oceanic and Atmospheric Administration (NOAA) and a C-130 Hercules property of the National Science Foundation (NSF) operated by the National Center of Atmospheric Research (NCAR). El Centro de Ciencias de la Atmósfera, at the Universidad Nacional Autónoma de México, (UNAM) installed and operated instrumental at the C-130 to measure some properties of atmospheric

We used instruments to measure the physical properties of atmospheric particles. The particles chemical composition is inferred from their optical properties as it is explained later in this chapter. Since the atmospheric particles are micro and sub-micron range sizes, the number of particles per volume may provide information on their origin or formation

We use optical counters to estimate the concentration of particles. Their operating principle is based on the extinction of a beam of known wavelength, having gone through an air sample with a certain amount of particles. Table 1 contains the technical details of the

Condensation nuclei particle counter (TSI-3760) is an instrument that increases the size of the particles with a forced growth in an environment artificially saturated with butanol. This increases the efficiency of detection and counting of very small droplets (Twomey, 1991).

The external instruments are installed in a pod on the wing and internal within the fuselage in the cabin. The location characterizes the way that each machine takes the air sample to analyze. The external instruments are in direct contact with the air all the time. Thus, the air sampling is instantaneous. But the internal instruments take the air sample by an air inlet and a section of hose that transports it to the device. In this case, the instrument uses a

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 PCASP and the FSSP.

Interaction Between Aerosol Particles and Maritime Convective Clouds:

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

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

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

composition.

**2.3. Sampling** 

and precipitation clouds.

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

Measurements in ITCZ During the EPIC 2001 Project 227
