Particulate matter

Particulate matter, the environment and health

Human exposure to harmful substances in the air, including particulate matter, is one of the most persistent environmental problems. It is clear that the inhalation of fine particles can cause health effects and increased mortality. Policy (EU, national and local) focuses primarily on the mass fraction PM10 (and on PM2.5 since recently). This refers to the number of micrograms of particulate matter per cubic meter with a diameter of less than 10 (respectively 2.5) micrometers. This relates primarily to existing legal threshold values.  

By excluding contributions of a natural origin (such as sea salt and sand and clay dispersed by the wind), these threshold values could be easier to attain. However, this would only involve a technical intervention in the regulations. Potentially hazardous elements and properties remain unchanged, after all. In addition, health effects can also be found at extremely low levels. Attaining threshold values may have priority in accordance with policy, but is insufficient for alleviating the health problem. It is also the question whether natural material does not indeed contribute to health risks.  

The general statistical correlation between PM10 and health effects has been proven. However, there are indications that certain physical or chemical properties of particulate matter are the actual cause of these effects. The first relates to particulate size distribution (the finer they are, the more harmful they are), what the relationship is between their surface area and diameter, or whether or not they adhere to one another, etc. The second relates to the presence of heavy metal bonds (e.g. lead, vanadium, copper, zinc, etc.), polyaromatic hydrocarbons and soot. Exhaust gasses in particular are focused on in this respect.  

An interesting question is whether health effects related to air pollution occur everywhere to the same extent or are connected to a specific situation (e.g. in the vicinity of roads, energy-intensive industry, bio-industry). This may be expected given the differences in the measured impact of particulate matter among locations such as traffic roads, city background and the countryside, but has not been proven yet.

Particulate matter and climate

Particulate matter not only has an effect on health and the environment, but also on the climate. With the exception of soot, it reduces the greenhouse effect. This occurs in two ways: a) the matter itself reflects sunlight; b) it promotes the formation of clouds, which reflect sunlight (even more strongly). Soot, on the other hand, absorbs additional sunlight and also makes snow and ice melt when it falls on them.  

Questions to be answered

Legislation and regulations in the EU and the Netherlands are based on PM10 standards. PM2.5 standards will have to be complied with shortly. These standards are based on the total mass per m3, and therefore not specifically on certain harmful components. As a result, the regulations can be enforced properly, but are not ideally tailored to actual risks. Public safety probably benefits primarily from emission-reducing measures for specific particulate matter elements. The key question therefore is which elements in particulate matter cause health effects. This leads to a following question, namely which properties of particulate matter should be subject to a policy that is properly tailored to the risks concerned. To resolve these questions, measurement methods for the composition of particulate matter with a high time-resolved capability are required. Standard filter equipment requires at least one day to collect an adequate amount of particulate matter. This then has to be taken to a lab for weighing and analysis. However, in order to identify sources and predict health effects, it must be possible to measure a large number of properties simultaneously at short intervals. This must occur at urban hotspots as well as background locations in order to see exposure differences.

Examples of recent results

Particulate matter in the Dutch province of Noord-Brabant

PM10 emissions became the topic of discussion in Noord-Brabant. These related to the consultation exercise of the Environmental Impact Assessment for the reconstruction plans specified by the province of Noord-Brabant in mid-2005. Air quality was not included:

1. When the umbrella plan was drawn up, particulate matter from the livestock industry was not a topical subject. This was not yet known as an important source of PM10 emissions. This component was therefore excluded from the guidelines for the Environmental Impact Assessment.
2. At that moment, the particulate matter problem attracted considerably less political/administrative interest than is the case nowadays. This has since changed due to various rulings by the Council of State, in which construction activities have been suspended, for example, because air quality standards for particulate matter, amongst other things, have been exceeded.  

Measurements carried out by ECN and WUR resulted in the following conclusions:

1. Although traffic and transport are the greatest contributors to PM10 emissions in the Netherlands, this is not the case in the province of Noord-Brabant. The contribution of traffic and transport on the one hand and agriculture on the other is almost equal.
2. Some 22% of the average concentration of PM10 in Noord-Brabant is caused by Dutch sources originating from human activity. Roughly 8% is caused by sources in Noord-Brabant that emit PM10 directly. Approximately 11% is caused by sources in Noord-Brabant that emit substances from which PM10 may originate. With regard to the latter, the agricultural sector in Noord-Brabant is a significant contributor via the emission of ammonia.
3. Concentrations were calculated for each individual company at a distance of fifty metres. In 2005, it was calculated that 270 of the approximately 13,000 companies exceeded the 32 μg/m3 standard. This number will drop to 175 in 2015 due to a significant decrease in background concentration, but is being impeded by the switchover from non-free range to free-range housing by the poultry sector.
4. If the Province’s plans are implemented, exposure to PM10 will improve on average, but may deteriorate locally. This means that in some cases, zoning plans and Article-19 procedures could fail due to air quality. These would therefore have to be subjected to additional tests. The government – central, provincial, and local – could provide subsidies to encourage the inclusion of air scrubbers in zoning plans and environmental permits.  

Particulate matter and NOx with and without green strip

During a pilot study conducted in 2006, these components were measured on both sides of the green strip alongside a motorway – the first time that this has been done in the Netherlands. The number of suitable measurement days was inadequate for final conclusions (also due to the wind direction and speed), but the results demand more.  

The measurements were carried out along two locations: A (with a green strip) and B (without a green strip). It is roughly 7 m wide and 10 m high. At each location, measurements were taken at 3 m from the road (at location A just before the central reservation); at 10 m (just after); at 45 m (pasture) and at 90 m (ditto). The (preliminary) results were:    

↑ = increased
↓ = decreased
0 = no change

For the green strip (location A), the concentration of NOx is about 10% higher compared to no green strip (location B). A possible explanation is that the green strip reduces the mixture of polluted air with cleaner air from higher altitudes. PM10 and PM2.5 concentrations behind the green strip are approximately 20% lower than before it, which is probably because the leaves capture some of it. At a greater distance, the green strip reduces NOx concentrations by 20%. This is possibly because the green strip causes an eddy that actually increases the mixture across a greater distance.