Ozone depletion is important to us for mainly one reason - effects of solar ultraviolet (UV) radiation to life on earth. This includes land vegetation, animals, sea creatures and vegetation, and of course humans. Among the most noticeable effects of UV radiation is skin erythema, more commonly known as sunburn. Such is typically harmless - research indicates that our sun is a significant source of vitamin D. An old saying is applicable - anything with moderation is fine - but overdoses of UV radiation can cause skin diseases (1), (2), such as (squamous) cell carcinomas, and much worse, melanomas. The former is often well-treatable, especially if detected early. The latter is very dangerous, and can often be fatal. Both are caused by sustained overexposure to UV radiation, though the milder forms from brief intense exposure. Some researchers estimate that 12-30 % of our population may sometime be affected by such skin diseases.
A very small portion of all solar radiation is UV, and even less reaches ground. As mentioned last week, our atmosphere contains a sort of natural protection from UV radiation. Sunlight is a contributor to both formation and destruction of ozone, as an idealized diagram indicates :
Even vegetation is a natural protection, abundantly emitting (tropospheric) ozone near ground during very warm and hot days, which effectively absorbs much of the UV radiation which penetrates stratospheric ozone. UV radiation is commonly classified according to
3 types, depending on wavelength :
Shortest wavelengths are most harmful to us (higher frequency, thus greater energy). UVC is most (very) harmful, UVB harmful, and UVA not so harmful. As indicated above, UVC is essentially all absorbed in our upper atmosphere. Stratospheric ozone absorbs most of UVB, but that which penetrates to ground is of concern. Most UVA penetrates to ground, but is healthy for us. Thus, discussion of tropospheric UV radiation effects and actions to protect from it mainly involves UVB.
UVB is measured many ways - from ground, balloons, and space. Ground-based measurements are most relevant, being where the problem is. The Yankee Broadband UVB-1 Pyranometer and the Scanning UV Spectroradiometer are examples of instruments which measure UVB. The former measures total energy amount in the UVB portion of the solar spectrum, the latter measures amount at specific regions of the UV spectrum, among other spectral regions. Such measurements can be used to monitor UVB amounts, as demonstrated with current amounts in Philadelphia, PA measured by Solar Light Co. You may notice from the graph that measurements are often expressed as Minimum Erythema Dosage (MED). 1 MED (1 J/cm2) is thought to be the amount necessary to cause a fair-skinned person to begin to become sunburned. For reference, it is about 1/350 the equivalent energy of an hour of strong summer noontime sunshine at midlatitudes. Not much, but enough to cause lots of trouble. As hinted at, this depends greatly on who is exposed to UVB. Negroid people tend to be very resilient, one skin disease case occurring for approximately every 60 fair-skinned people. Hispanics tend to be about 10 times more resilient than fair-skinned people. More cases are observed for men than women, but that may be mainly because they are outdoors more.
To quantify effects mentioned regarding public awareness, the National Weather Service uses a UV Index, routinely included with other official current weather forecast products (image) (text). UV Index is also forecast by the Australian Bureau of Meteorology, among others. It indicates expected danger of UVB rays. Rays are most dangerous near solar noon, as indicated in the idealized diagram and tabulated, so near-noontime conditions are a main consideration for calculating the UV Index.
Daily ozone measurements (N Hem) (S Hem), combined with stratospheric wind forecasts aid forecasting of the UV index. Fortunately, UV solar radiation is less difficult to forecast than total solar radiation because clouds attenuate UVB significantly less than other solar energy wavelengths (much of which is near infrared). Thus, a good cloud forecast is required, but not so much as it might otherwise be - which leads to next week's topic.
Text and embedded image are copyright of Joseph Bartlo, though may be used with proper crediting.