After the severe air pollution episodes in various parts of the US in the ‘50s and ‘60s, the US Congress passed the Clean Air Act and subsequent Amendments. As per this Act, the Environmental Protection Agency was formed to regulate environmental pollution. The six major airborne pollutants are called ‘criteria pollutants’, and include ozone, lead, nitrous oxides (NOx), sulfurous oxides (SOx), carbon monoxide (CO) and particulate matter (PM, or Total Suspended Matter, TSP). The smaller size fractions of PM are divided into PM10 and PM2.5 (the subscript refers to the upper size limit on the particle’s aerodynamic diameter).
While the other pollutants can be easily measured, PM, especially the carbonaceous fraction, is much harder to characterize:
(a)
Many organic
compounds (that are collectively called organic carbon or OC) are
present in both the gas
and solid
phases. Hence, when a measurement is made, both phases can be
captured, leading to an overestimate of the particulate OC
concentration
in the
atmosphere. At the same time, some of the captured particles can
transform to the gas-phase and escape from the filter, causing a
negative bias in the measured OC. We
developed a better measurement technique for OC and compared it to a
conventional sampler, to estimate the
errors (artifacts) associated with the conventional method.
• R Subramanian, Andrey Khlystov, Juan Cabada and
Allen Robinson (2004). “Positive and negative artifacts in
particulate organic carbon measurements with denuded and undenuded
sampler configurations.” Aerosol Science and Technology,
38(S1):27–48.
(b)
The carbonaceous fraction is made of two physically distinct, but
analytically somewhat inseparable components - OC (compounds with a
substantial
non-carbon component, i.e. hydrogen, oxygen, even nitrogen and sulfur)
and "elemental" carbon (EC, which is mostly carbon). EC is used
as a marker for diesel engine emissions, and a good example is the
black cloud of smoke you might see behind a diesel bus or truck as it
accelerates (this cloud is made of "soot", a combination of OC and
EC). The common thermal/optical technique to measure OC and EC
leads to an operational definition of the OC/EC split - i.e. the
method used (temperature steps, optical measurement) determines the
answer. With many different instruments and techniques in use,
there are substantial differences when analytical results from
different laboratories are compared. This has implications not
only for source-apportionment of diesel vehicle emissions, but also for
climate models (EC, similar to BC or black carbon, is an absorbing
aerosol that warms the atmosphere around it, in contrast to sulfates
that cause cooling). My research explored the reasons for the
operational discrepancies, and we developed a model that helps estimate
the range of uncertainty associated with the OC/EC split.
• R Subramanian, Andrey Khlystov and Allen
Robinson. “Effect of peak inert-mode temperature on elemental
carbon measured using thermal-optical analysis.” Aerosol Science
and Technology, in press (as of March 2006).
The
third major component of my doctoral research
was quantification of the contribution of primary organic aerosol
sources (motor vehicles, wood smoke, cooking, etc.) to
the ambient particulate OC concentrations in Pittsburgh,
PA. This knowledge
is
essential to successful regulation and control. We
performed source-receptor modeling using the EPA's CMB8 model
with organic molecular markers. Much of this work is currently
under review at different journals. [ask
me more: subra REMOVE @randomsubu REMOVE
.com]
R Subramanian,
Allen Robinson,
Anna Bernardo-Bricker and Wolfgang Rogge (2005)
Organic carbon mass balance and source apportionment of
primary organic carbon in the Pittsburgh region using molecular markers.
AAAR Particulate Matter Supersites Program & Related
Studies 2005, Atlanta, GA
R Subramanian,
Allen Robinson and Andrey Khlystov (2005)
OC/EC analysis with Thermal-Optical methods: Effects of
temperature protocol and non-carbonaceous compounds.
AAAR Particulate Matter Supersites Program & Related Studies
2005, Atlanta, GA
R Subramanian,
Andrey Khlystov, Juan Cabada and Allen
Robinson (2002)
Sampling artifacts during measurement of ambient
carbonaceous aerosol.
AAAR Annual Conference 2002, Charlotte, NC
(Abstracts of
past American Association for Aerosol Research conferences are here.)