domingo, 8 de julho de 2018


TRACKING A COUNTRY’S CONSUMPTION OF OZONE-DEPLETING SUBSTANCES. Dr Daniel Say, postdoctoral research assistant working within the Atmospheric Chemistry Research Group (ACRG) at the University of Bristol and Tim Harrison is the Bristol ChemLabS Director of Outreach, School of Chemistry, University of Bristol, UK.

The Montreal Protocol was introduced to regulate and reduce emissions of chlorofluorocarbons (CFCs) which, along with other man-made (anthropogenic) chemicals, are known to destroy stratospheric ozone (O3). Ozone prevents harmful ultraviolet radiation reaching the surface of the Earth and there, among other interactions, could damage living cells. The stratosphere is a layer of the atmosphere ~12- 50 km above the Earth’s surface. Under the Montreal Protocol, all 195 countries agreed to freeze, reduce and ultimately cease the production and consumption of CFCs through a series of step changes. Crucially, it was recognised that developed countries such as the UK had been using these chemicals for longer and had the financial means to reduce their emissions at a greater rate, than developing countries. Hence, developing countries were given longer to reduce their CFC emissions.

Since CFCs were used across a wide range of applications, from coolants in refrigerators, to propellants in aerosol and foam blowing agents, they required immediate replacement. Hydrochlorofluorocarbons (HCFCs) were introduced as interim replacements for CFCs. Since HCFCs contain a hydrogen atom, they are more susceptible to reaction with the hydroxyl free radical (‘OH) in the troposphere. Free radicals have unpaired electrons and are highly reactive. Hence, their ability to destroy ozone in the stratosphere is reduced relative to CFCs. In 1990, hydrofluorocarbons (HFCs) were introduced to replace both CFCs and HCFCs. Since they do not contain chlorine, HFCs have ozone-depletion potentials of zero. However, they are potent greenhouse gases, able to warm the atmosphere many thousands of times more effectively than carbon dioxide!

Figure 1: Common examples of CFCs, HCFCs and HFCs

Estimates of CFC, HCFC and HFC emissions from developing countries are important for improving our understanding of global trends. Owing to rapidly expanding economy and large population, developing countries such as India, have emissions of these gases that are of importance. While a ban on CFCs came into effect in 2010, some countries emissions of these gases are likely to persist for several decades in the form of ‘banks’, such as within old refrigerators and rigid foams, and will continue to emit CFC for many years after installation. In contrast, India is not required to eliminate the use of HCFCs until 2030, with its timeline for a phase-down of HFCs longer still. In the absence of detailed emissions information from some governments, we can use atmospheric measurements to help track a developing countries progress under the Montreal Protocol.

Aircraft measurements

Given some country’s size, aircraft offer potentially the best means by which to make atmospheric measurements of anthropogenic halocarbons. The FAAM (Facility for Airborne Atmospheric Measurements) research aircraft provides a platform for the collection of ‘whole air samples’ – 3 litre stainless steel flasks that are filled with air whilst flying over e.g. India. These flasks are then returned to suitable laboratories, such as at the  University of Bristol, for analysis via the Medusa GCMS analytical system. This instrument is capable of measuring gases present at very low concentrations – down to parts-per-trillion (ppt) levels. By analysing variability in the concentration of these halocarbons with respect to the background level (i.e. the concentration of each gas already present in the atmosphere), we can say something about a country’s emissions.

In the figure below, we compare variability in the mole fraction (ppt) of CFC-11, HCFC-22 and HFC-134a, all of which were/are used extensively as coolants for refrigeration and air-conditioning applications. Based on the lack of mole fraction variability, our measurements suggest that, in this case, India’s CFC-11 emissions are small, which is consistent with its commitment to cease CFC production and consumption by 2010. Conversely, we see very large enhancements (relative to the baseline) for both HCFC-22 and HFC-134a, gases used predominantly as coolants in stationary air-conditioning units. Since we see considerable variability in the mole fraction of both gases, we can reasonably conclude that in 2016 (when the flasks were collected) India was at a mid-point in terms of replacing HCFC consuming units with non-ozone depleting (HFC) alternatives.

Figure 2 Sample profiles of air over India and their HCFC, CFC and HFC concentrations

Like many developing country, India’s demand for (and emissions of) refrigerants such as HCFCs and HFCs is expected to grow exponentially over the next decade. Long-term atmospheric measurements are therefore required, to independently track the expected increase in emissions and provide a basis for future emissions reduction legislation.


To find the chemical formula of a CFC, given the first add 90 to the ‘number’, to obtain a 3-digit def number, where:
d = the number of carbons
e = the number of hydrogens
f = the number of fluorines
and 2d + 2 - e - f = the number of chlorines.

For CFC-11:
90 + 11 = 101, the number of carbons is 1, the number of hydrogens is 0, the number of fluorines is 1, and the number of chlorines is (2 + 2 - 0 - 1 = 3).
So, the chemical formula for CFC-11 is CFCl3. (see figure 1)
Note CFC-11 could also be written CFC 011.

For, HCFC-22:
90 +22 = 112; C=1; H=1; F=2; Cl 4-3 = 1 Cl; the chemical formula is: CHClF2 (see figure 1)

Note the addition of subscript letters is to differentiate possible isomers

Daniel Say is a postdoctoral research assistant working within the Atmospheric Chemistry Research Group (ACRG) at the University of Bristol. Dan’s work focusses predominantly on the measurement of hydrofluorocarbons and explores the use of inverse modelling techniques for the estimation of regional emissions.

Tim Harrison is the Bristol ChemLabS Director of Outreach, School of Chemistry, University of Bristol, UK.

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