The physical dimension (size, shape), relative position, spatial arrangements between different canopy elements determine the amount and spatial distribution of fraction of photosynthetic radiation (fPAR) within and below the canopy, which control the absorption, reflectance, transmission, and scattering of solar radiation. A single live leaf reflects green light and near-infrared light due to its internal structure. When scalling to the individual tree or forest stand level, non-random distribution and multi-layer structure of canopy elements result in multiple scattering of radiation between the different layers of foliage elements and other parts of canopy. This results in the obvious difference in reflectance for the individual leaf, tree canopy and a stand at landscape level.

The denser a canopy, the more absorption and reflectance
Formaldehyde (CH2O) is a carcinogenic pollutant emitted as an intermediate Drug_discovery product in the oxidation of most biogenic and anthropogenic hydrocarbons. It is also known as a primary emission product of incomplete hydrocarbon combustion [1]. This makes formaldehyde an ubiquitous component of both remote and polluted urban atmospheric environments. The CH2O concentrations in polluted urban environments are in the order of 10 �C 20 ppbv (parts per billion in volume), whereas in non-urban locations concentrations from 0.01 to 10 ppbv have been observed [2�C5]. Moreover, formaldehyde is a chemical widely used in the manufacture of building materials and many household products like foams, consumer paints and polymer products.

Outgassing of formaldehyde from these materials may lead to elevated CH2O concentration levels in indoor air. Even at low concentrations, formaldehyde can lead to health risks and may be associated with various diseases, such as bronchial asthma, atopic dermatitis and ��sick building�� syndrome. At concentrations of 100 �C 500 ppbv irritation of eyes, nose, and throat has been reported. At higher concentrations CH2O leads to headaches and dizziness, and at 100 ppmv (parts per million in volume) exposure can be fatal [6]. Thus, strict regulation and controls on CH2O emissions are required. As an example, an upper limit of 0.75 ppmv for long-term exposure (8 h time-weighted average) and 2 ppmv for short-term exposure (15 min) has been imposed by the US Occupational Safety and Health Administration to protect workers from exposure to formaldehyde [7].A number of different methods for monitoring the formaldehyde level in both the environment and industrial exhausts have been developed and validated [8�C10], e.g.