ECN publication
Carbonaceous aerosol: where is it coming from?
Published by: Publication date:
ECN Environment & Energy Engineering 8-3-2013
ECN report number: Document type:
ECN-E--13-001 ECN publication
Number of pages: Full text:
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Chemical composition as well as physical properties of organic and elemental carbon fractions (OC and EC, respectively) as well as black carbon (BC) reflect specific emission sources (combustion of fossil fuel by vehicular traffic and shipping, biomass burning in electricity plants and wood stoves, and biogenic contribution). In this report a number of these features are described. For political decision-making, an essential question is the following: when emissions of one source (traffic) go down in the near-future, will the (associated) health risk also be lower? At this time, any answer to this question is highly speculative as current knowledge is incomplete and debatable. We therefore focus on a more basic question, i.e., can we discriminate between contributions of the various sources by looking at the OC and EC concentrations? And, equivalently, does the ratio OC/EC change for different sources? The work presented here is the starting point for a more extended review to be produced at a later stage. Chapter 3 interprets concentration data from in-situ measurements, while Chapter 4 primarily describes emission characteristics based on (global-scale) modelling studies. Some interesting observations from this study are: • Usually organic carbon concentrations exhibit a poor correlation with elementary carbon indicating a different origin for their presence in PM. Also, average concentrations of OC are higher than those of EC. • EC originates from primary sources, i.e. transport, biomass burning, and energy production (using coal). The highest levels are measured near busy roads and inside tunnels. EC accounts for approximately 30% of the total carbon at urban sites. At rural sites, this is 10% (PM2.5) to 20% (PM10). • Similar levels of EC are observed in PM2.5 and PM10 indicating that EC is mainly associated to PM2.5. In contrast, OC levels measured in PM2.5 are clearly lower than in PM10. Its presence in the coarse fraction is some 20-40% of total OC and probably caused by large biogenic particles. • 30% of the organic carbon comes from fossil fuel combustion, even in the city centres. Biomass burning and natural emissions (mainly of gaseous precursors such as terpenes) dominate the emissions of OC. • The OC/EC ratio is higher than 1 for all examined datasets. This indicates that most of the OC in PM2.5 and PM10 is of a secondary origin (SOA), possibly owing to various chemical reactions of volatile gaseous compounds and their subsequent condensation in the atmosphere. EC is entirely related to primary aerosol (POA). • EC and OC levels observed in Asian cities are higher than in US and EU. This is believed to be caused by specific sources like fuel combustion by traffic and wood burning for residential cooking. • The study of the various experimental data sets show that OC/EC ratios vary between around 1 (along busy traffic streets and within tunnels) and 15 (shipping). Ratios for urban background sites and biomass burning are between 4 and 5. The data from ships exhausts were characterized with a (very) low (and rather unexpected) EC content. • The data from modelling studies give a somewhat different perspective. Here, the burning of fossil fuel (traffic, power generation, industry) leads to lower OC/EC ratios (up to 1) compared to biomass burning ( near 6), both in agreement with observational data. For residential activity (mainly cooking) it is 3. Shipping emissions have an OC/EC ratio varying between 2 and 7, much lower than concluded from the measurement. Part of these discrepancies can be understood by considering the effect of dilution and SOA formation on concentration levels: EC-rich exhaust becomes mixed with less EC-rich ambient air, increasing the OC/EC ratio. Modelling studies often include emission inventories, which do not take into account the effect of mixing of air masses. Further examination is needed here.

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