During the past ten years, it has become increasingly clear that the inhalation of airborne combustion-related ultra fine particles (UFPs) presents a significant health-hazard to humans, owing to the fact that these particles tend to deposit on and eventually encapsulate in the lung tissue. Such UFPs comprise both solid particles and liquid-like particles. A significant part of the combustion-related solid particles is composed of soot particles that comprise or largely consist of unburned elemental carbon. A smaller part of the combustion-related solid particles is composed of inorganic ashes. Ultrafine combustion-related liquid-like particles are typically composed of more-or-less volatile hydrocarbon/H2SO4/H2O material together with small amounts of inorganic species. Combustion-related UFPs measure between approximately 5 nm and 500 nm in diameter (most particles measuring less than 200-300 nm in diameter), and normally comprise or are at least partially covered with carcinogenic polycyclic aromatic hydrocarbons (PAHs) and other volatile organic compounds (VOCs). These UFPs are emitted into air from the exhaust of combustion sources such as automobile traffic and other local combustion sources and are formed as the result of an incomplete combustion process. In particular diesel motors are notorious for emitting large amounts of soot particles and other UFPs into air.
Apart from the neighborhood of industrial combustion sources and other stationary combustion sources, the concentration of combustion-related UFPs, hereafter simply referred to as UFPs, in the western world is generally highest on or near locations where motorized traffic is present. Very high local concentrations may be encountered particularly in tunnels, traffic intersections and/or in traffic queues under conditions of limited ventilation and/or windspeed. There is increasing evidence that the impact of these UFPs on human health is more significant than that of the common gaseous exhaust pollutants (CO, NOx, SO2, VOCs) emitted by combustion motors. The local air pollution is thus to an important extent correlated with the local UFP concentration. In addition, the local UFP concentration is to a large extent correlated with the local concentrations of the common gaseous exhaust pollutants because they all originate from the same pollution sources.
In the prior art, the seriousness of the airborne particle pollution level is primarily established in terms of the airborne particle mass concentration associated with in particular those airborne particles that are respirable, i.e. that can reach and deposit into the alveolar region of the lungs. Since all airborne particles with dp≦10 μm are respirable, the total respirable particle mass Mtotal is considered to be a relevant parameter, Mtotal being defined as
      M    total    =            ∫              dp        =        0                    dp        =                  10          ⁢                                          ⁢          µm                      ⁢                  ρ        p            ⁢                        π          ⁢                                          ⁢                      ⅆ            p            3                          6            ⁢                        ⅆ                      N            ⁡                          (                              ⅆ                p                            )                                                            ⅆ            ln                    ⁢                                          ⁢                      ⅆ            p                              ⁢                          ⁢              ⅆ        ln            ⁢                          ⁢              ⅆ        p            wherein ρp is the particle density, and wherein dN(dp)/d ln dp denotes the particle size distribution, dN(dp) denoting the number concentration of particles that have a diameter dp. The integral is taken over the entire range of respirable particle diameters dp, in practice up to dp=10 μm (being the upper aerodynamic size limit for respirable particles). The respirable particle mass concentration Mtotal can e.g. be measured by light scattering and/or by particle sampling/weighing.
It has been found that Mtotal is in general mainly derived from particles with dp>300-500 nm, even if these particles only have a very small number concentration, i.e. the number of particles per unit volume. The smaller particles do not substantially contribute to Mtotal in spite of the circumstance that their number concentration is generally very high. A human health impact parameter Hfp, whose numerical value is an indication of the relative seriousness of impact on human health, taking account of respirable fine particles (FP) with dp>300-500 nm can be shown to be approximately proportional with Mtotal under normal environmental conditions.
In view of increasing evidence that especially UFPs with a particle diameter dp smaller than 300-500 nm are hazardous to human health, a mere measurement of only the respirable particle mass concentration Mtotal would not provide reliable and/or sufficient data on the total human health impact parameter Htotal being the sum of the separate human health impact parameters for FP's and UFP's, respectively. Further, the total UFP number concentration and the number-averaged UFP diameter cannot be obtained with low-cost means.