Methane (CH4) is considered to be a serious atmospheric
pollutant, both for its role as a greenhouse gas and its immediate effect on
the area where it is released, where it can contribute to raised ozone levels
at ground level, which is harmful to human health. It is currently calculated
that methane is present in the atmosphere at levels 2.5 times as high as in the
pre-industrial period, and that the level of atmospheric methane, which rose
steadily till the mid 1980s, then slowed and levelled off by the early 2000s,
has begun to rise sharply again since 2007. Methane emissions can come from a
number of sources, including landfill sites and sewage treatment works, but the
majority of the released methane is thought to come from leaks in the natural
gas infrastructure (i.e. gas extracted from fossil hydrocarbon sources and
intended for use as a domestic or industrial fuel). However the exact
contribution of different sources to atmospheric pollution is unclear (and
sometimes controversial), and current levels of atmospheric methane are much
higher in many areas than predicted to arise from leaks in the gas distribution
network.
In a paper published in the Proceedings of the National Academy ofSciences of the United States of America on 23 January 2015, a team of scientists led by KathrynMcKain of the School of Engineering and Applied Sciences and Department ofEarth and Planetary Sciences at Harvard University discuss the results of an
experiment in which they monitored methane levels in and around the city of
Boston, Massachusetts, for one year (twelve calendar months) in 2012-13, in
order to determine levels of the gas in the city, and attempt to determine its
origin.
McKain et al. monitored gas
levels at four sites, Boston University and Copley Square in the city, and Harvard
Forest and Nahant outside the urban area. Both the urban sites showed raised
methane levels compared to the countryside year round, with the Boston
University site with the Boston University site being 37.3 parts per billion
above background in summer and 58.5 parts per billion above background in
winter, while the Copley Square site produced levels 23.6 parts per billion
above background in summer and 39.3 parts per billion above background in
winter.
Location of two city [Boston University (BU), 29-m
height; CopleySquare (COP), 215-m height] and two peripheral [Harvard Forest
(HF);Nahant (NHT)] measurement stations (black points) in Boston, and the
surrounding area, overlaid on a map of the number of housing units with NG per
square kilometer (14). The 90-km radius circle delineates the ∼18,000km2
land area for which CH4 emissions and the NG loss rate were calculated.The
magenta and purple contours enclose 50% of the average footprint(sensitivity area)
of the BU and COP afternoon measurements, respectively.The two city sites are
difficult to distinguish at this scale because the horizontaldistance between
them is ∼2 km. The influence area is ∼80% largerfor COP than BU because the former station is higher. McKain et al. (2015).
In order to determine the origin of the gas, McKain et al. also monitored ethane (C2H6)
levels at each site. Ethane forms a significant proportion of supplied natural
gas, but is not produced by microbial sources such as wetlands, landfill or
sewage farms. Since Boston lacks other potential sources of atmospheric ethane,
such as natural geological gas seeps or hydrocarbons refineries, this is taken
as a good proxy for gas emitted by the natural gas infrastructure. Based upon
this McKain et al. calculate that 98%
of atmospheric methane in winter and 67% of atmospheric methane in winter
originates from the natural gas infrastructure (a higher percentage in winter
would be expected, as humans use less fuel and microbial production is higher).
This suggests that 2.7% of all natural gas supplied to the Boston
area during the study period was leaked into the atmosphere. It is currently
estimated that 0.7% of all supplied natural gas in the US is lost to the
atmosphere, though this figure is not broken down by regions. The State ofMassachusetts produces its own figures, and estimates that 1.1% of supplied
natural gas is lost (68% of the study area lies within Massachusetts, while 88%
of the supplied gas in the state is consumed within the study area).
Reconstructed geographical distribution of NG
consumption, in unitsof CH4 mass flux, during September 2012 through
August 2013 for the fourstates included in the study region. McKain et al. (2015).
McKainet al.’s figures
suggest that both the US national figures and the independent figures produced
in Massachusetts are severe underestimates. They suggest that this is because
these figures are based upon detected leeks from the system (i.e. leeks large
enough to be noticed and responded to), and that numerous smaller leeks could
go undetected, particularly if occurring in inaccessible places. They also note
that leeks on private property are not included in the official figures, but
will still contribute to atmospheric pollution.
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