Physical Causes of APSD Changes in Aerosols from OIPs and Their Impact on CI Measurements - Good Cascade Impactor Practices, AIM and EDA for Orally Inhaled Products - page 71

Table 3.2 Physical processes associated with aerosol transport and influence on CI-measured
APSD; the upward and downward arrows indicate increases and decreases, respectively; the
sideway arrows indicate variable/unpredictable influences on the magnitudes of MMAD and GSD

Inluence a on
MMAD and GSD
MMAD Process Relevance to OIP aerosols GSD

Agglomeration/coagulation Low, unless extra-fines present ↑ ↑ a

Inertial impaction High ↓ ↓

Gravitational sedimentation High ↓ ↓

Molecular diffusion Low ≈ ≈

Electrostatic charge High and variable—mitigate where

possible

↔ ↔

Evaporation High where volatile species present ↓ ↓

Condensation/hygroscopic
absorption of water

High if hygroscopic species are present ↑ ↑

a Thermal agglomeration/coagulation by itself will increase GSD to about 1.35 and have no further
effect thereafter. This change is therefore unimportant for OIP aerosols

precaution when making measurements of APSD with hygroscopic materials is to
control local environmental conditions, especially keeping the relative humidity
constant and preferably at a value where hygroscopic growth does not take place.

3.3.2.7 Summary

Table 3.2 is a qualitative summary of the effects on a hypothetical OIP APSD,
assumed to be unimodal and lognormal, caused by the various processes that have
been considered.
In this table, an upward facing arrow indicates an increase in the APSD property
concerned, with a downward arrow showing the opposite behavior. The “

≈ ” symbol

indicates a small or insignificant effect, whereas the “ ” symbol represents a

highly variable effect that can affect the indicated variable in either direction.
Agglomeration/coagulation is continuously present but is only likely to be
important in the aerosol sampling and APSD measurement process by CI when
extra-fines are present as a large portion of the total mass sampled. Inertial impac-
tion takes place only when particles are accelerated or decelerated close to a nearby
object in the flow path; this process dominates size classification in the CI, but is
also important during the aerosol transport via the induction port and pre-separator
(if used). Gravitational sedimentation, like agglomeration, is continuously present
but is most important with the largest particles that enter the measurement system.
Molecular (Brownian) diffusion is unimportant, except maybe in cases where
extra-ine particles predominate in the APSD. Electrostatic charge has both a large
and variable effect on aerosol APSD, depending upon the acquired charge distribu-
tion combined with the corresponding charge distribution on adjacent surfaces.
It is therefore highly recommended that electrostatic charge is mitigated or avoided

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