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Fog is not always a sign of pollution

The situation showed that monitoring the quality of the air is done only in weather stations that collect particulate matter (PM) between 2.5 and 10 microns at the ground level is an insufficient measurement to conclude that the quality of the air should be researched and different variables analyzed with the dynamics proper of the city.


To this end, Universidad Nacional de Colombia (UNal) in Medellín, Faculty of Sciences and Director of the Laser and Optic Spectroscopy Group (GLEO, for its Spanish acronym) Professor Alvaro Bastidas says that measuring PM needs to be accompanied by aerosol particle evolution and particle transportation studies along with the physical, optical, and chemical processes these have in the air at different heights over ground level.


With 2 doctoral, 8 master’s, and 12 undergraduate thesis projects on the analysis of the atmosphere, the GLEO group continues to provide evidence to the research on the atmospheric phenomenon through air columns with scientific support, useful information in this in-depth research of the atmosphere of Medellín since 2009.


Read more:  The Saharan air layer is not dangerous but does require follow-up.
 

“If we understand this phenomenon we could become cognizant where there is pollution and when not and identify where in the city are the sources of the emissions (motorcycles, vehicles, industries or air that comes from other places) that thicken the air and in what percentage, ” says Bastidas.
 

Assessments at several heights


Bastidas adds that the persistence of the aerosol load or high air density in the Valle de Aburrá and a good part of the country, despite the sanitary lockdown, is explained by global atmospheric events that have an impact and govern the regional and local phenomenology.

Besides, the high atmosphere density of the main cities of the country is identified as “pollution” is not produced only in these cities but comes from other places around the world.
 

In this sense, as the main cities are at a height above 1,000m (3300ft.) they turn into air “compounds” or “holders”, ones more topographically isolated than others. Therefore, according to Bastidas, it is not correct to affirm that Medellín for being enclosed by mountains has a roof that suffocates.
 

“That does not exist and it may be demonstrated by observation, measuring and recording the evolution in time of the variables and indicators such as the planetary boundary layer, the optic depth, the Ångström exponent, water vapor, the particle-size distribution functions or retro-trajectories.”
 

Learn to “read” fog


It is common in Medellín to say that when there is fog, mist, and low visibility the pollution levels increase, and for that reason the city should change its production rhythm to diminish the 2.5 PM reading. It is important to remember that these particles are so small they are 100% breathable, passing through the lungs and entering the bloodstream producing cardiovascular diseases and even lung cancer.


Bastidas says that there is always a competition of several phenomena in the atmosphere and that some prevail over others. For instance, the entrance of particles from other parts of the world can prevail or come from forest fires or fog, water vapor, etc. For Medellín and the Valle de Aburrá, we need to highlight that most months have high relative humidity, clouds and thick air, therefore the days with high visibility are rare.
 

Read more: How much has the pandemic impacted urban life? (in Spanish).
 

The predominance of each phenomenon in the air provides a characteristic visual appearance that every person can observe and learn to identify and differentiate at first look like:

High pollution: The atmosphere is yellow colored and may increase turning orange and brown when pollution rates are high. This phenomenon has occurred just a few times in Medellín, being the most recent in March of 2016, when the Saharan sands reached the city, which commonly arrive to Puerto Rico, and at the time were pushed by north polar air currents towards the Equator and a good part of them were deposited in Colombia.


This fact is confirmed by the distribution of particle sizes with values comparable to the number of particles per volume unit, both for fine and thick particles, besides the behavior of the optic depth according to the absorption of the solar radiation. The satellite record of the MEdium Resolution Imaging Spectrometer (Meris) shows a high content of aerosol entering from the Atlantic Ocean.

Mist or fog: this phenomenon is seen as a light grey color that sometimes turns whitish. The distribution functions indicate the highest level of particles per volume unit with respect to thick particles and the water vapor curve is monotonously ascending.

Forest fires, vehicle, and industrial emissions: when particles of this type enter the atmosphere, the air takes on a grey color and the visibility is greatly reduced. In this case, it is even possible to distinguish different aerosol layers that occupy the atmosphere volume limited by the limiting layer.


For Bastidas, a scientific-experimental research of the atmosphere would support or not strategies such as imposing a 24-hour vehicle restriction measure, as last March showed that a drastic measure like this did not improve the air quality, nor showed that the high atmospheric density came only from vehicles and the industry.

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