The seventh annual S. Dexter Squibb Lecture series took place on Thursday, September 9, and Friday September 10.  The speaker was F. Sherwood Rowland, Bren Research Professor of Chemistry and Bren Research Professor of Earth Science at the University of California, Irvine.

Community Lecture

Thursday September 9, 7:30 pm
Lipinsky Auditorium

"Our Changing Atmosphere:  Stratospheric Ozone Depletion and Global Warming"

As the 21^st Century begins, several important alterations in the chemical composition of Earth's atmosphere are well under way, and are likely to cause significant further changes in the forthcoming decades. Many of these can be grouped under three headings: depletion of stratospheric ozone, increased trapping of terrestrial infrared radiation with consequent warming of Earth's surface, and rising levels of surface pollution especially in urban environments. In all three instances, the major changes are occurring because of additional gases released to the atmosphere through the activities of the global population.

Trace gas measurements: Improvements in the sensitivity of detection techniques, especially in the latter third of the 20^th century, have permitted the identification of many pervasive atmospheric components. Corresponding improvements in the precision of measurement have then allowed the detection of atmospheric compositional changes. 

Anthropogenic gaseous emissions:  The presence in the atmosphere of appreciable amounts of carbon as methane, CH4, and as carbon dioxide, CO2, illustrates the extreme disequilibrium of the atmosphere, an indication of the importance of biological processes in controlling the composition of the atmosphere.  During the last two centuries, the burning of coal, gas and oil has greatly increased the concentrations of carbon dioxide, while agricultural activities--such as growing rice and raising cattle--have contributed to more than doubling the concentration of methane.

Synthetic molecules.  A characteristic which distinguishes the 20^th century from all previous centuries is the large number of newly synthesized compounds, not previously present in nature, which have been added to its atmosphere. Most of the organohalogen concentration in the atmosphere is now contained in man-made molecules such as chlorofluorocarbons (CFCs). Many such compounds are chemically inert, allowing transport to the stratosphere where destruction by solar ultraviolet radiation releases highly active atomic chlorine, which attacks the Earth's ozone layer.

The greenhouse effect and global warming:  The greenhouse gases--molecules which contain 3 or more atoms, (carbon dioxide, methane, water, ozone, nitrous oxide), CFCs absorb outgoing terrestrial infrared radiation.  The 21^st century concerns about global warming arise from the stronger infrared absorption associated with increasing atmospheric concentrations of these gases, i.e. an enhanced greenhouse effect of a few degrees C beyond the natural one--about 57 degrees F two centuries ago. 

Stratospheric ozone depletion: Loss of stratospheric ozone allows surface exposure to more damaging ultraviolet radiation. The key to the large-scale depletion of stratospheric ozone is the existence of ClO catalytic chain reactions. The Montreal Protocol which banned the further production of CFCs and halons took effect in 1996 has proven to be very effective with actual observations in the lower atmosphere showing that total organochlorine concentrations at the surface are now decreasing.  

General Chemistry Lecture

Friday September 10,  8:00 am
Lipinsky Auditorium

"Hydrocarbons in Earth's Atmosphere"

The local presence of hydrocarbons in the atmosphere has been known for about two centuries, with identification of specific compounds beginning about a century ago.  The reactive removal of volatile hydrocarbons from the atmosphere is primarily the consequence of attack by hydroxyl radicals, which are formed by ultraviolet attack on tropospheric ozone in (1) with the formation of O(1D) atoms, which react with water vapor, as in (2).  Hydroxyl radicals can attack saturated hydrocarbons by abstracting H in (3), and the residual R radical immediately adds an O₂ molecule to form RO₂ in (4).  Hydroxyl radical formation is favored in the summer because of more hours of more intense

sunlight, and in the tropics by higher humidity which favors (3) in competition with deexcitation by collisions with N₂ or O₂.

The alkanes have estimated atmospheric lifetimes of 8 years for methane, 2 months for ethane, 2 weeks for propane, and a few hours for ethylene.  Because the rate of north/south mixing of the atmosphere is approximately 15 months, methane is the only simple hydrocarbon which survives long enough to provide substantial contributions in both northern and southern hemispheres before being oxidized by HO radicals.   For molecules such as ethane and propane, a strong seasonal variation is observed in the temperate and polar latitudes with minimum concentrations in the summer.   Because most hydrocarbons enter the atmosphere in the north, the concentrations there are much very much higher than in the south.

We began collecting atmospheric samples in remote locations on both sides of the equator in 1978.  Continuation of this series of measurements has shown an increase from a global average of 1.52 ppmv in 1978 to 1.78 ppmv in 2003.  Observations of methane from ice cores by other research groups show a gradual increase toward present levels from 0.75 ppmv at the beginning of the industrial revolution two centuries ago.  The warming of the atmosphere by accumulation of anthropogenic gases was expanded in the 1970s from a "carbon dioxide problem" to a "greenhouse gas problem", with the experimental observation of significant increases over time of methane, nitrous oxide, the chlorofluorocarbons (CFCs), and tropospheric ozone as additional contributors to the trapping of outgoing infrared radiation.

The RO₂ radicals from (4) can react with NO in reaction (5) to form NO₂, and its subsequent photolysis produces O atoms and then ozone.  A very minor product of reaction (5) leads to the formation of alkyl nitrates, RONO₂, which therefore become a marker for the production of ozone from the main channel of (5) + (6).   We have investigated the hydrocarbon composition of the air

in many cities around the world, and have observed not only the importance of vehicular traffic for the release of reactive hydrocarbons and nitrogen oxides, but also the importance of liquefied petroleum gas (typically C₃ and C₄ alkanes) in creation of urban ozone through reactions (4) to (6).  We have also measured very high concentrations of alkane hydrocarbons in the rural southwest United States as the consequence of hydrocarbon leakage from the oil and gas industries.  These alkanes have been accompanied by elevated alkyl nitrates, demonstrating that enough NO is present in these to trigger ozone formation even in these non-urban environments.

We have also participated in numerous aircraft- and ship-based experiments, which have led to other observations of hydrocarbons and their reactions.  These include: 

(1)   their formation by biomass burning, as measured both on the ground and in plumes thousands of miles from the location of the burning;

(2)    removal by chlorine atom reaction in the near-absence of tropospheric ozone at altitudes below 500 feet above frozen Hudson Bay (Canada); and

(3)   increased production of isoprene and alkanes accompanying CO₂ decreases during "iron fertilization" experiments in the Southern Ocean.