Air quality expert group - Fine particulate matter (PM2.5) in ... - Defra

PM2.5 in the UK
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4.5.2 Quantifying the secondary inorganic contribution
71. It is relatively straightforward to identify the contribution of secondary inorganic
particles to the PM2.5 mass in the atmosphere. Measurements of the sum
of sulphate and nitrate expressed in chemical equivalents are typically very
similar to the concentration of ammonium expressed in chemical equivalents,
suggesting that within the fine particles these components are chemically
combined as ammonium sulphate and ammonium nitrate, and the combined
sum represents the secondary inorganic contribution to the PM2.5 mass.
Occasionally, there may also be a contribution from ammonium chloride formed
from the reaction of hydrogen chloride, typically emitted from coal burning and
incineration, with ammonia. However, ammonium chloride is semi-volatile (Pio
and Harrison, 1987) and current emissions of hydrogen chloride are frequently
too low to sustain an atmospheric concentration sufficient to lead to the
formation of ammonium chloride particles.
4.5.3 Estimation of the secondary organic aerosol contribution
72. Differentiation of primary and secondary organic matter in particles is
challenging. The most frequently used method is the elemental carbon tracer
method which assumes that primary organic matter exists in combination
with elemental carbon and that if the ratio between primary organic carbon
(OC) and elemental carbon (EC) is known, then any organic carbon in excess
of this ratio is attributable to secondary organic matter. This is a relatively
well accepted concept but estimation of the ratio of OC to EC in primary
emissions is difficult. Typically, this is estimated by plotting OC versus EC and
identifying a minimum ratio in the data as in Figure 4.8. This ratio is assumed
to be representative of time periods when no secondary organic carbon was
present and the extent to which individual data points are above the minimum
line is used to estimate their secondary organic carbon content. This method
tends to fail in rural areas where secondary organic carbon is dominant and the
minimum ratio inevitably includes some secondary organic matter. The presence
of wood smoke, which is a primary material with a high OC:EC ratio, can also
cause difficulties in disaggregating the contributions. Recent work by Pio et al.
(2011) examining data from different locations has indicated that the graphical
method of determining the primary OC:EC ratio frequently overestimates this
ratio and consequently many of the data representing primary and secondary
organic matter in the atmosphere may be in error. A number of assumptions
have to be made to extract estimates of primary and secondary organic matter
from radiocarbon measurements but this method has the important attribute
of being able to distinguish between organic material derived from fossil fuel
sources and that from biogenic sources.
73. A more recent method of estimating secondary organic particles is through the
application of PMF to data from AMS instruments, as described in Section 4.5.1
for cooking particles. Typically, the disaggregation of AMS data using the PMF
programme will identify one or two components enriched in ions indicative of
oxidised carbon species, i.e. secondary organic matter. These are by convention
referred to as OOA1 and OOA2, where OOA refers to oxidised organic aerosol.
Some studies have shown that one of these components correlates relatively
highly with sulphate and is of low volatility, while the other correlates much
more closely with nitrate and is of appreciably higher volatility. The former is
typically more oxidised than the latter.