University of Houston researchers: Climate change helped to reduce ozone levels –
Houston findings should hold for coastal regions around the world
Researchers at the University of Houston have determined that climate change – in the form of a stronger sea breeze, the result of warmer soil temperatures – contributed to the drop in high-ozone days in the Houston area.
Robert Talbot, professor of atmospheric chemistry, said that also should be true for coastal regions globally.
The researchers describe their findings in a paper published this week in the journal Atmosphere. In addition to Talbot, they include first author Lei Liu, a doctoral student, and post-doctoral fellow Xin Lan.
The study relied upon ground-level ozone data collected over the past 23 years by the Texas Commission on Environmental Quality. The meteorological data was collected by the National Oceanic and Atmospheric Administration.
The researchers said they did not set out to find a connection between climate change and lower ozone levels – the number of days in which ozone levels exceeded federal standards varied from year to year but overall, dropped dramatically between 1990 and 2013. For example, in Aldine, one of four sites studied, the number of days during which ozone levels exceeded federal standards over an eight-hour period dropped to an average of 11 days per year during 2001-2013, down from 35 days per year during 1990-2000.
Talbot said the steep decline made him suspect something was happening beyond a city-led effort to reduce nitrogen oxide emissions, one of the components of ozone.
Liu said they first ruled out other meteorological factors, including temperature, humidity and solar radiation. After they discovered the lower ozone readings coincided with days the southerly flow was strongest, they realized that climate change – in the form of warmer soil temperatures – had increased the southerly flow, she said.
“The frequency of southerly (air) flow has increased by a factor of ~2.5 over the period 1990-2013, likely suppressing O3 (ozone) photochemistry and leading to a ‘cleaner’ Houston environment,” they wrote. “The sea breeze was enhanced greatly from 1990 to 2013 due to increasing land surface temperatures, increased pressure gradients, and slightly stronger on-shore winds. These patterns driven by climate change produce a strengthening of the sea breeze, which should be a general result at locations worldwide.”
Industrial plants and vehicle exhaust mix with heat and sunlight to produce ground-level ozone, which can worsen asthma and other conditions. The city’s rapid population growth – more people means more cars – and the refineries and petrochemical plants along the Houston Ship Channel are key factors in Houston’s ground-level ozone.
The U.S. Environmental Protection Agency in 1997 classified Houston as a “severe” nonattainment area due to ozone levels measured over an eight-hour period. By 2008, the city was classified as a “moderate” nonattainment area.
For the study, researchers focused on data from four locations: Galveston, Clinton Drive near the Houston Ship Channel, Aldine and a site in Northwest Harris County. They also used data on background ozone levels collected from the roof of Moody Tower, a high-rise residence hall on the UH campus.
Background ozone levels have remained constant over the past seven years, they report, dropping just one part per billion. The average background level over that period was 30 parts per billion.
But the number of days in which ground-level ozone exceeded federal standards in one-hour and eight-hour measures dropped sharply at all four sites between 1990 and 2013. (Data for the Galveston site is available only back to 1997.)
In contrast, “the length of time per year Houston is under the influence of southerly flow has more than doubled from 1990 to 2013,” the researchers wrote. ” … We propose that the increased flow of ‘cleaner’ air is diluting the dirty Houston air, lowering the mixing ratios of NOX, O3 and precursor hydrocarbons. It also would advect the polluted air away from Houston,” leading to lower potential to produce ozone.
They compared land and sea temperatures over the 23-year period to determine how temperature differences impact southerly flow. Land temperatures increase faster than water temperatures on a daily time scale, Liu said. As the heated air over land rises, cooler air from the sea rushes in, dispersing both ozone and the chemical elements that contribute to ozone.
“We weren’t looking at it from a climate change perspective at first,” Talbot said. “Then once we saw it was the sea breeze, we knew it had to be climate.”
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The drop in temperature over the southern polar circle causes the appearance of the ozone hole at a height of between 20 and 30 km.
The primary results of this study are as follows:
The ion temperature in the upper atmosphere during the winter season of 2007–2009 is significantly lower than during earlier periods of low solar activity. This is consistent with multiple reports of a colder thermosphere during the recent solar minimum. The largest differences are observed during the daytime and reach ~100 K.
A strong temperature variation with ~10–13 h period and ~50–90 K amplitude is found in the altitude range of 200–250 km and interpreted as an evidence of an enhanced semidiurnal tide that propagated to the upper thermosphere. This enhancement is observed during the first part of the campaign (18–24 January) and is likely to be associated with the sudden stratospheric warming.
A weaker 20–50 K variation with a period of ~6.5–8 h is retrieved from the data below ~250 km altitude. This variation could result from the enhancement of the terdiurnal tide during sudden stratospheric warming.
Ion temperature variations of different strength are found at multiple nontidal periods, with the largest variations reported for periods of 16–17 h, 30–40 h, 10–13 d, and 3–4 d. This variability appears to be driven by planetary waves with periods of 4 d, 5 d, and 10–13 d.
The oscillations with a 3–4 d period and a 10–13 d period can result from upward propagation of planetary waves with these periods. The oscillation with 16–17 h period could result from the nonlinear interaction of the quasi 2 d wave with a semidiurnal tide.
Analysis of data for one experimental campaign limits our ability to distinguish whether temperature oscillations with nontidal periods (16 h, 30–40 h, 3–4 d, and 10–13 d) are solely associated with sudden stratospheric warming or are part of the regular ionospheric variability.
Multiple studies of ionospheric disturbances with planetary wave periods are focused on variations in electron density and interpret their findings using the E region dynamo mechanism, as planetary waves are not expected to propagate to F region altitudes. However, our observations of ion temperature variations with planetary wave periods (and periods resulting from interaction of planetary waves with tides) do not necessarily require a distinct E region dynamo driver. These results indicate that such waves can propagate up to ~250 km, at least in the case of deep solar minimum, and dissipate at altitudes above ~250 km. To the best of our knowledge, this is the first experimental evidence of direct (or indirect) propagation of planetary waves to the upper thermosphere.
http://onlinelibrary.wiley.com/doi/10.1029/2012JA018251/full
At first blush, this looks more like an Urban Heat Island effect than global climate change.
“Atmospheric chemists have already studied the effects of cosmic ray ionization on CFCs, according to Sanche, but no one has looked at the effects of cosmic rays inside polar clouds. To simulate a dense, antarctic cloud, Sanche and Lu cooled a metal rod down to temperatures between 20 and 100 K and condensed water vapor and CFCs onto its surface. They then bombarded the condensate with low-energy electrons like those created by cosmic rays ionizing atoms in the atmosphere. The electrons reacted with the CFCs and made active chlorine, and the team determined the likelihood of this reaction by measuring the charge buildup on the end of the rod.
The results suggest that electrons from cosmic rays are about a million times more likely to interact with CFCs inside polar clouds than anyone previously believed, says Sanche. Robert Compton of the University of Tennessee says that Sanche and Lu’s revised estimations could help atmospheric scientists and meteorologists to improve their models of ozone loss. Sanche says these observations may also change the way we understand the ozone hole. A rise in global temperatures could cause an increase in polar cloud cover that would lead to more cosmic-ray-induced CFC reactions, he says. “So here you would predict some link between global warming and [the ozone hole].”
http://physics.aps.org/story/v8/st8
“There are two ways in which the charged secondary particles are produced by cosmic rays. High-energy gamma rays can decay into pairs of electrons and positrons (anti-electrons); and cosmic rays can collide with atmospheric atoms and kick out electrons. Each of these charged products will create a radio-frequency pulse as it moves in the geomagnetic field. In general, the radio waves are polarized in a direction that depends on the relative orientation of the particles’ motion and the magnetic field.”
http://physics.aps.org/articles/v8/37
If all the ozone in this column were to be compressed to standard temperature and pressure (STP) (0 deg C and 1 atmosphere pressure) and spread out evenly over the area, it would form a slab approximately 3mm thick.
1 Dobson Unit (DU) is defined to be 0.01 mm thickness at STP.
The total ozone maps for the northern hemisphere are based on near-real time NASA Earth Probe Total Ozone Mapping Spectrometer (TOMS) gridded satellite data available from the NASA TOMS home page, NOAA SMOBA (Stratosphere Monitoring Ozone Blended Analysis) data (if TOMS data are not available) and on ground-based measurements. Ground-based data are provided by Environment Canada, by the Russian Central Aerological Observatory, and by other agencies. Over the polar night area Dobson and Brewer moon observations and/or NOAA’s TIROS Operational Vertical Sounder (TOVS) satellite data are used. TOVS data are also used when the more reliable TOMS data are not available. To see ozone maps from the individual data sources (TOMS, SMOBA, TOVS and ground-based) click here. The mapping algorithm is similar to those used by the WMO Ozone Mapping Centre . Total ozone values are given in Dobson Units.
http://exp-studies.tor.ec.gc.ca/ozone/images/graphs/gl/current.gif
http://exp-studies.tor.ec.gc.ca/e/ozone/Curr_allmap_g.htm