Ppbv Yr Research Articles

Contribution of regional pollution and long‐range transport to the Asia‐Pacific region: Analysis of long‐term ozonesonde data over Japan

Long‐term (1970–2002) ozonesonde data from four sites in Japan (Sapporo, Tsukuba, Kagoshima, and Naha) have been analyzed using a sectoral air mass classification based on back air trajectories. Air masses from “regionally polluted,” “Eurasia,” Pacific, and Japan regions are tagged with daily ozone values at each of the four sites. Ozone in regionally polluted air masses shows a broad maximum in spring and summer and a minimum in winter, whereas ozone in other air masses shows a maximum in spring and minimum in summer at Tsukuba, Kagoshima, and Naha. The wintertime ozone in regionally polluted air masses is highest but summertime ozone is lowest at Naha among all other sites. In the lower troposphere the air masses from southern China also show much lower ozone levels in summer than from the northern part (up to 36 ppbv in August), which is suggested to be due to the additional influence of an Asian monsoon over southern China. Ozone levels at Sapporo are determined mainly by Eurasian air masses and show a broad maximum in spring and summer. The boundary layer ozone levels in the regionally polluted air masses are 10–12 ppbv higher throughout the year than in the Eurasian air masses, and in winter they are about 8 ppbv higher than over Europe. However, ozone levels and seasonal variations are similar over Japan and Europe in the lower troposphere. Ozone levels in the regionally polluted air masses increased by 11–20% (5–11 ppbv) from 1970s to 1990s over Japan, and this is attributed to the large increase in NOx emissions over China in the 1990s. The long‐term trend (between 1970 and 1990) in regionally polluted ozone is observed not to be very different over Japan and Europe, particularly in the lower troposphere. However, during the 1990s the wintertime positive trend (0.2–0.4 ppbv yr−1) in lower tropospheric ozone in the Eurasian air masses at Sapporo is suggested to be due to the intercontinental transport from Europe/North America. However, the inconsistency between the long‐term trend in the regionally polluted ozone and the NOx emissions over China in the 1990s remains unexplained.

Long‐term and seasonal variations in the levels of hydrogen peroxide, methylhydroperoxide, and selected compounds over the Pacific Ocean

The National Aeronautics and Space Administration's Global Tropospheric Experiment program has conducted five sampling programs examining the Pacific troposphere from 1991 to 2001. Flights on two aircraft, a P3‐B and DC‐8, sampled the Pacific troposphere at altitudes from 0.5 km to 13 km and from 130°E to 115°W and from 65°N to 70°S. Long‐term trends in the observed species mixing ratios in different geographic regions are presented. Changes in species mixing ratios are discussed with respect to changes in source, transport, and photochemical production and loss processes over the decade study period. CH4 and CO2 exhibited average annual mixing ratio increases of 6.39 ppbv yr−1 and 1.92 ppmv yr−1, respectively. O3, CO, and NO all exhibit significantly higher mixing ratios in the north Pacific troposphere in the spring than in the fall. However, there is no clear annual trend for these species. H2O2 mixing ratios were 38 ± 16% higher (Δ(fall–spring) 258 ± 34 pptv), and CH3OOH was 61 ± 15% higher (Δ343 ± 76 pptv) throughout the Pacific troposphere in the fall than in the spring. During 2001, SO2 loading in the North Pacific is 2–3 times larger than during earlier years. Anthropogenic tracer gases, C2H6, C3H8, C2H2, and C2Cl4, do not exhibit comparable increases in their average mixing ratios during the decade study period. CH3I, a tracer for oceanic emission, also does not exhibit an increase in 2001. Sulfur loading characterized by SO2 also does not exhibit a clear annual trend. The enhanced sulfur loading during 2001 appears to have resulted in a suppression of H2O2 mixing ratios in the lower troposphere throughout the midlatitude North Pacific.

Trends in methane and sulfur hexafluoride at a tropical coastal site, Thumba (8.6°N, 77.°E), in India

Measurements of methane (CH 4 ) and sulfur hexafluoride (SF 6 ) have been made using gas chromatographic technique at a coastal site, Thumba (8.6°N, 77°E), India from the flask air samples collected during 1996–2000. The growth rates of CH 4 and SF 6 are observed to be 30 and 0.25 pptv yr −1 respectively. The growth rate of CH 4 at Thumba is found to be much higher than its average growth rate of about 7 ppbv yr −1 at other tropical monitoring sites. The observed growth rate of SF 6 at Thumba is similar to that found at other tropical stations. Seasonal amplitudes in CH 4 and SF 6 are higher at Thumba when compared to the other sites. The large growth rate of CH 4 at Thumba could be attributed to the increase in regional sources, and the greater seasonal amplitude is shown to be caused by the change in dynamical structure from Northern Hemisphere summer to winter.

Tropospheric methane retrieved from ground‐based near‐IR solar absorption spectra

High‐resolution near‐infrared solar absorption spectra recorded between 1977 and 1995 at the Kitt Peak National Solar Observatory are analyzed to retrieve column abundances of methane (CH4), hydrogen fluoride (HF), and oxygen (O2). Employing a stratospheric “slope equilibrium” relationship between CH4 and HF, the varying contribution of stratospheric CH4 to the total column is inferred. Variations in the CH4 column due to changes in surface pressure are determined from the O2 column abundances. By this technique, CH4 tropospheric volume mixing ratios are determined with a precision of ∼0.5%. These display behavior similar to Mauna Loa in situ surface measurements, with a seasonal peak‐to‐peak amplitude of approximately 30 ppbv and a nearly linear increase between 1977 and 1983 of 18.0 ± 0.8 ppbv yr−1, slowing significantly after 1990.

Implications of the recent fluctuations in the growth rate of tropospheric methane

Global measurements show that the mixing ratio of tropospheric methane (CH4) increased by 1.1% (19.5 ± 1.7 ppbv) over the five‐year period 1996–2000, with striking fluctuations in its annual growth rate. Whereas the global CH4 growth rate reached 15.9 ± 0.7 ppbv yr−1 in 1998, the growth rate was −2.1 ± 0.8 ppbv yr−1 in 2000. This is the first time in our 23‐year global monitoring program that we have measured a negative annual CH4 growth rate. The CH4 growth rate fluctuates in an unpredictable fashion, and we reemphasize that global CH4 concentrations cannot be extrapolated into the future based on past trends. As a result, we suggest that the slowing of the CH4 growth rate during much of the 1980s and 1990s cannot be used to imply that CH4 will no longer be of concern in greenhouse gas studies during this century.

Trends of atmospheric methane in Beijing

The concentration of atmospheric methane in Beijing is still increasing, although its annual trend has significantly decreased from 2.0% yr−1 in 1985–1989 to 0.5% yr−1 in 1990–1997. The seasonal variability of methane concentration apparently appeared in a double-peak pattern with one peak in winter and the other in summer. It is known that the seasonal inter-annual variations of atmospheric methane in Beijing are different from year to year. From 1986 to 1997, the concentrations of atmospheric methane increased by 184 ppbv, in which about 37% was due to its increase in winter and 21% in summer. After 1993, the trends of methane concentration in summer, which are mainly due to emission from biogenic sources, are negative, while their trends in winter, which are mainly due to emission from non-biogenic sources, are positive with a value of about 25 ppbv yr−1. As a result, the seasonal inter-annual trends from 1993 to 1997 were mainly due to the increase of methane emission from non-biogenic sources in winter. It implies, therefore, that in Beijing the biogenic sources have been decreasing but the non-biogenic ones, such as fossil fuel combustion, have increased.

Effects of the stratospheric quasi‐biennial oscillation on long‐lived greenhouse gases in the troposphere

An analysis is presented of results of an extended integration with a global general circulation model that includes treatment of a long‐lived tropospheric trace constituent as well as a momentum source that forces a realistic stratospheric quasibiennial oscillation (QBO). It is shown that the dynamical QBO may modulate stratosphere‐troposphere exchange in such a manner as to produce a QBO in global‐mean tropospheric tracer mixing ratio. For the growth rate of tropospheric methane this transport‐induced QBO is expected to have a peak‐to‐peak amplitude of 1–2 ppbv yr−1, which, while modest compared with the full range of observed variability in growth rate, is still significant. The observed methane growth rate time series during 1983–1999 is shown to be consistent with the predicted QBO effect, although the record is definitely dominated by other interannual variations.

Carbon monoxide in the U.S. mid‐Atlantic troposphere: Evidence for a decreasing trend

Nearly continuous measurements of carbon monoxide (CO) were made at Shenandoah National Park‐Big Meadows in rural Virginia, a site considered representative of regional air quality, from December 1994 to November 1997. Similar observations were also made at this location from October 1988 to October 1989. These observations combine to indicate a decreasing trend in CO concentration over the U.S. mid‐Atlantic region of about 5.0 ppbv yr−1, with greater than 95% confidence that the slope is significantly different from zero. The decrease suggests U.S. reductions in anthropogenic CO emissions have been effective in reducing pollutant levels. The observed trend is consistent with the U.S. EPA reported trend in emissions and the decrease in Northern Hemisphere tropospheric background CO mixing ratios observed by other researchers.

Trends in stratospheric humidity and the sensitivity of ozone to these trends

Measurements of stratospheric water vapor and methane from the Halogen Occultation Experiment (HALOE) mounted on the Upper Atmosphere Research Satellite (UARS) are used to investigate changes in stratospheric water vapor over the period 1992–1996 inclusive. An increase in water vapor mixing ratio is found at levels between 30 km and 65 km across the globe which fit, to first order, a linear trend varying with altitude from 40 parts per billion by volume per year (ppbv yr−1) to a maximum of 90 ppbv yr−1 at 45 km. These trends appear to be greater than that expected due to the growth in tropospheric methane over the past several decades, and possible mechanisms accounting for this are discussed. The trend of the combined budget of 2 × CH4 + H2O is approximately constant with altitude with a global mean value of 61±4 ppbv yr−1. On the basis of these estimates, sensitivity studies have been performed using a two‐dimensional (2‐D) radiative‐chemical‐dynamical model to assess the impact on concentrations of stratospheric ozone of this degree of change in stratospheric water vapor over timescales consistent with doubling CO2 scenarios. We find that the impact of increased stratospheric water vapor is to enhance the ozone increase in the midstratosphere by ∼1–2% compared to the response due to a doubling of CO2 itself of ∼5–10%. In the upper stratosphere the destruction of ozone is enhanced and the changeover from production to loss is lowered to ∼50 km (from ∼70 km). A chemical mechanism for these processes involving enhanced OH and NO2 is identified.

Sensitivity of the CH4 growth rate to changes in CH4 emissions from natural gas and coal

The observed rate of increase in atmospheric CH4 declined gradually during the late 1980s [Steele et al., 1992] and then fell dramatically by ∼4 to 7 parts per billion by volume (ppbv) yr−1 from 1991 to 1992 [Dlugokencky et al., 1994a]. Post‐1993 the CH4 growth rate has recovered to pre‐1991 levels. In this study, we assess the sensitivity of the CH4 growth rate to changes in CH4 emissions from the coal and gas industries over the last 15 years using a global two‐dimensional (2‐D) model of the atmosphere. Emissions are estimated from available production data, and in the case of natural gas, a spectrum of leakage rates is assumed. Changes in the former Soviet Union (FSU) have been assessed separately from the rest of the world (ROW). Prior to 1989, increased coal and natural gas production led to an increase in the CH4 growth rate (i.e., opposite to that which was observed). During the period 1989 to 1993 the modelled CH4 growth rate decreased by ∼1–2 ppbv yr−2 at northern mid‐latitudes (assuming constant gas leakage rates), showing that declining emissions, primarily in the form of natural gas from the FSU, may have contributed to the general reduction in the CH4 growth rate from 1989 to 1991. However, this does not explain the large reduction which was observed between 1991 and 1992. A model run with FSU gas leakage rates decreasing by 1% yr−1 from 5% in 1990 to 2% in 1993 shows a decrease in the CH4 growth rate of ∼9–12 ppbv yr−1 (or ∼3–5 ppbv yr−2) over this period. This is similar in magnitude to the observed decline over the same period. This sensitivity study illustrates the large potential for significantly lowering the growth rate of atmospheric CH4 by reducing fossil fuel emissions from coal and, in particular, by reducing the leakage rates from natural gas installations.

Legal alert: Opacity monitors

Houston is recognized for its large petrochemical industrial facilities providing abundant radicals for tropospheric ozone formation. Fortunately, maximum daily 8-h average (MDA8) surface ozone concentrations have declined in Houston (− 0.6 ± 0.3 ppbv yr− 1) during the summers (i.e., May to September) of 2000 to 2014, possibly due to the reductions in precursor emissions by effective control policies. However, it is also possible that changes in meteorological variables have affected ozone concentrations. This study focused on the impact of long-term wind patterns which have the highest impact on ozone in Houston. The analysis of long-term wind patterns can benefit surface ozone studies by 1) providing wind patterns that distinctly changed ozone levels, 2) investigating the frequency of patterns and the respective changes and 3) estimating ozone trends in specific wind patterns that local emissions are mostly involved, thus separating emissions impacts from meteorology to some extent. To this end, the 900-hPa flow patterns in summers of 2000 to 2014 were clustered in seven classes (C1–C7) by deploying an unsupervised partitioning method. We confirm the characteristics of the clusters from a backward trajectory analysis, monitoring networks, and a regional chemical transport model simulation. The results indicate that Houston has experienced a statistically significant downward trend (− 0.6 ± 0.4 day yr− 1) of the cluster of weak easterly and northeasterly days (C4), when the highest fraction of ozone exceedances (MDA8 > 70 ppbv) occurred. This suggests that the reduction in ozone precursors was not the sole reason for the decrease in ozone exceedance days (− 1.5 ± 0.6 day yr− 1). Further, to examine the efficiency of control policies intended to reduce the amount of ozone, we estimated the trend of MDA8 ozone in C4 and C5 (weak winds) days when local emissions are primarily responsible for high ambient ozone levels. Both C4 and C5 show a large reduction in the 95th percentile and summertime trends mainly due to effective control strategies. Based on the 5th percentile daytime ozone for C1 (strong southeasterly wind) in coastal sites, this study found that the cleanest air masses that Houston received became more polluted during the summer of 2000–2014 by 1–3 ppbv. Though this study focused on Houston, the analysis method presented could generally be used to estimate ozone trends in other regions where surface ozone is dominantly influenced by both wind patterns and local emissions.

Concentration and 13C records of atmospheric methane in New Zealand and Antarctica: Evidence for changes in methane sources

Measurements of 13C in atmospheric methane made at Baring Head, New Zealand (41°S), over the 4‐year period, 1989–1993, display a persistent but highly variable seasonal cycle. Values for δ13C peak in summer at about −46.9‰ and drop to around −47.5‰ in the late winter. Methane concentration shows a similar cycle, with winter peaks and summer minima. Similar features are observed at the New Zealand Antarctic station, Scott Base, at 78°S. While the phase of the δ13C cycle is consistent with a kinetic isotope effect that preferentially leaves methane enriched in 13C in the atmosphere after oxidation by OH, the amplitude of the cycle is much larger than expected from published laboratory measurements of the effect. We interpret the origin of this cycle and its inter‐annual variability to be due to episodic southward transport of isotopically heavy methane from large‐scale tropical biomass burning, possibly in conjunction with changes in the rate of interhemispheric transport in the troposphere. The Baring Head 13C data show no significant secular trend from 1989 to mid‐1991, followed by a rapid trend toward methane less enriched in 13C. This indicates a major shift in the balance of the sources of atmospheric methane and precludes an increased sink strength. The trend in 13C since mid‐1991 coincided with significant changes to the methane growth rate observed at Baring Head and at Scott Base: an elevated growth rate of about 15 parts per billion by volume (ppbv) during 1991 gave way to less than 3 ppbv yr−1 thereafter. A 2‐box model of atmospheric methane (one box per hemispheric reservoir) suggests that (1) the recent decline in 13C in methane observed at Baring Head and Scott Base cannot have a solely northern hemispheric origin and (2) the most plausible origin is a recent reduction in methane released by biomass burning in the southern hemisphere, combined with a lower release rate of fossil methane in the northern hemisphere.

A dramatic decrease in the growth rate of atmospheric methane in the northern hemisphere during 1992

Global measurements of atmospheric methane have revealed a sharp decrease in the growth rate in the Northern Hemisphere during 1992. The average trend for the Northern Hemisphere during 1983–1991 was (11.6±0.2) ppbv yr−1, but the increase in 1992 was only (1.8±1.6) ppbv. In the Southern Hemisphere, the average increase (1983–1991) was (11.1±0.2) ppbv yr−1, and the 1992 increase was (7.7±1.0) ppbv. Various possibilities for a change in methane sources or sinks are discussed, but the most likely explanation is a change in an anthropogenic source such as fossil fuel exploitation, which can be rapidly and easily affected by man's activities.