Review of “Remote Sensing of Particulate Pollution 1 from Space: Have we
reached the promised land?”
R. M. Hoff and S. A. Christopher
Review by R. B. Husar
The authors are to be commended for the thorough review and assessment of
the current literature on satellite the remote sensing of PM and I concur with their
major conclusions. My comments address some qualifications but not the
essence of the review.
I support their assessment that ‘ground-based measurements, models, and
satellite measurements should be viewed system, each component of which is
necessary to better understand air quality’. However, an equally important aspect
of full air quality characterization is the integration, consistency checking and
reconciliation among the components. Also, the pollutant mass and composition
closure will only be possible if the emissions are included as the fourth
component of an integrated air quality characterization system.
The authors state that up until now, regulatory compliance is not aided by
satellite aerosol measurements. For instance, they state that for the new
Exceptional Event (EE) Rule, satellite data are presently of limited utility because
of the poor precision (±20-30%) of the current AOD measurements and they can
not be used for the “but for” test. Actually, the EE Rule is the first regulation that
explicitly includes the use of satellites in enforcing a regulation. Three of the four
clauses of the rule allows and encourages the use of satellites: (1) The source is
not reasonably controllable or preventable, (2) There is clear causal relationship
between the exceedance and the event, (3) The event is in excess of the
"normal" values. The fourth “but for“ clause by law can only be established using
the actual surface measurement of the mass concentration with the Federal
Reference Method.
Discussion of the fundamental problems of aerosol sensing properly includes
most of the sampling and detection issues. However, just like the majority of
contemporary literature, no consideration is given to the inherent ‘fundamental’
problem of satellite remote sensing: It requires the inversion of a radiative signal
to extract the intristic properties of particle size, composition, etc. This
notoriously difficult inversion problem arises from the mismatch in dimensionality.
The aerosol size-composition system has much more dimensions than the
available observations. The spectral columnar back-scattering measurement is
restricted to a narrow spectral transmission window of the atmosphere (0.4-1
microns). Consequently, the inversions are inherently undetermined and require
major a priory assumptions about aerosol properties. Lastly, given the extensive
coverage of the satellite detection gaseos species in the review, it would have
been appropriate to include gaseous in the title.