Introduction
The temporal and spatial variability of greenhouse gas fluxes in the
atmosphere is currently one of the most widely discussed climatological
problems in the scientific community. Methane, despite its trace presence in
the air (ca. 1.8 ppm; Hartman et al., 2013), plays an important role in the
environment. It participates in the global carbon cycle and is one of the
greenhouse gases whose concentration in the atmosphere affects the radiation
balance of the earth's surface. An increase in the concentration of methane
contributes to an enhancement of the greenhouse effect (Ciais et al., 2013).
Therefore, emissions of this gas into the atmosphere should be carefully
monitored.
Methane is produced during the process of methanogenesis under anaerobic
conditions, from the decay of organic plant debris in water. The most
important source of methane in the world is wetlands (Shurpali and
Verma, 1998; Rinne et al., 2007; Baldocchi et al.,
2012; Hatalaa et al., 2012), but paddy fields (Miyata et al., 2000), cattle
farms (Laubach and Kelliher, 2005; Dengel et al., 2011; Hartmann et al.,
2013; Nicolini et al., 2013) and emissions from the soil are all important
sources (Smeets et al., 2009; Denmead et al., 2010; Wang et al., 2013).
Moreover, emissions of methane accompany forest fires and grass vegetation.
The effect of the combustion of natural gas (which contains at least 80 %
methane) is mainly water vapour and carbon dioxide. The combustion of fossil
fuels is, however, predominantly incomplete and is therefore an important
factor causing anthropogenic methane emissions. This happens in the case of
combustion of both natural gas and hydrocarbons contained in petrol and other
fuels (Nam et al., 2004; Nakagawa et al.,
2005; Wennberg et al., 2012). Another important source of methane in
urbanized areas is leakage from urban gas pipelines (Lowry, et al., 2001;
Gioli et al., 2012; Wennberg et al., 2012; Phillips et al., 2013). Methane
may also be emitted during the anaerobic respiration of bacteria in urban
soils (Bogner and Matthews, 2003) and in the decomposition of solid waste and
wastewater in sewage systems and at landfill sites (Bogner and Matthews,
2003; Laurila et al., 2005; Lohila et al., 2007; Wennberg et al., 2012; Jha
et al., 2008). In contrast, methane is removed
from the air by consumption by soil bacteria (Goldman et al., 1995; Kaye et
al., 2004; Groffman et al., 2006; Groffman and Pouyat, 2009). Methane is
involved in some of the reactions leading to photochemical smog formation
(Seinfeld and Pandis, 2006). The disintegration of methane also results from
its reacting with the hydroxyl group in the atmosphere (Whalen, 2005). Annual
global emissions of methane into the atmosphere have been estimated as
∼ 5000 Tg. Emissions from landfills and waste (87–94 Tg) or fossil
fuels (85–105 Tg) are 2–3 times lower than estimated emission from
wetlands (177–284 Tg) (Ciais et al., 2013).
Research into the methane content in the air is now a priority because the
literature indicates that cities could be a significant source of this gas
(Elliot et al., 2000; Gioli et al., 2012; O'Shea et al., 2014; Nicolini et
al., 2013; Phillips et al., 2013; Christen, 2014; Kumar and Sharma, 2014;
Morin et al., 2014). The measurements of changes in methane concentration
have been carried out for decades (Ciais et al., 2013; Hartmann et al.,
2013). However, the analysis of its flux in urban areas is extremely rare. In
recent years, there have been approximately 500 stations measuring the fluxes
of carbon dioxide (CO2) around the world of which approximately 20 are
located in cities and only a few are able to measure methane flux (Nordbo et
al., 2012; Oliphant, 2012; Christen, 2014; Helfter et al., 2016). It can be
concluded that the measurement of methane flux in cities is in its infancy
and challenges like the need for long-term measurements (beyond a month) and
the relationship between methane fluxes and land use are yet to be overcome.
The development of the theory and measurement techniques of turbulent
exchange of mass, energy and momentum fluxes has been progressing for decades
(Stull, 1988; Lee et al., 2005; Foken, 2008; Aubinet et al.,
2012). Historical measurements of methane flux
have been severely limited due to the lack of suitable sensors, which have
only recently become available (Pattey et al., 2006; Hendriks et al., 2008;
Eugster and Pluss, 2010; Dengel et al., 2011; Detto et al., 2011; Sakabe et
al., 2012). At present, one of the most widely used instruments is the LI7700
Open Path CH4 Analyser (Burba and Anderson, 2010; McDermitt et al.,
2011), which uses eddy covariance as a measurement technique (Aubinet et al.,
2012). Worldwide, there are only a few long-term, continuous measurement
stations measuring turbulent fluxes of water vapour and carbon dioxide in
urban areas (Christen, 2014; Helfter et al., 2016). For methane flux in urban
areas, such data are probably at the implementation phase because previous
studies focused on areas which are the largest source of methane, i.e.
natural wetlands (Shurpali and Verma, 1998; Rinne
et al., 2007; Baldocchi et al., 2012; Hatalaa et al., 2012; Aubinet et al.,
2012), agricultural land (paddy fields, Miyata
et al., 2000) or forests (Smeets et al., 2009; Wang et al., 2013). The
chamber method, widely used in rural areas, has only a limited relevance in
the city. This method makes it possible to take measurements of methane
emissions from specific areas like urban lawns (Baciu et al., 2008); however,
it cannot be used in larger urban areas. A variety of techniques have
recently been applied to provide independent estimates of urban methane
emissions such as airborne observations (O'Shea et al., 2014; Mays et al., 2009), Fourier transform spectrometry (Wunch et
al., 2009) or isotopic source apportionment studies (Lowry et al., 2001).
Moriizumi et al. (1996) suggested the
occurrence of covariability of radon 222Rn concentration and methane flux, which they estimated to be 20 mg m-2 day-1. In
Poland, the issue of exchange of greenhouse gases in an urban area has also
been studied in Kraków, where, based on the measurements of methane
concentrations and the height of the atmospheric boundary layer, the average
monthly nocturnal flux of methane has been estimated to be 0.8 to
3 mg m-2 h-1 (Kuc et al., 2003; Zimnoch et al., 2010).
The aim of this study is to analyse the temporal variability of the
turbulent flux of methane (FCH4) based on a long-term series of
measurements recorded for over 2 years in the centre of Łódź
(July 2013 to August 2015). The diurnal variability of FCH4 was
analysed and monthly values of the flux were determined. An assessment of
the cumulative annual exchange of methane between urban Łódź and
the troposphere was completed to determine whether it was an equally
efficient source of methane to the troposphere as carbon dioxide. The
measurement results were compared to the variability of selected
meteorological elements. As the methane emissions in the city are determined
mainly by anthropogenic factors, the value of fluxes on weekdays and at
weekends were compared. No comparison was made to the fluxes estimated using
specific inventory methods because of a lack of data.
The western part of the centre of Łódź (top). Solid white
lines indicate the source area with P=25, 50, 75 and 90 %, calculated
for the turbulent fluxes measured at 10:00 to 14:00 during unstable
stratification (all available data from the period July 2013–August 2015).
The dashed red lines represent 250, 500, 750 and 1000 m distances from
the measurement point. The white circle indicates liquid petroleum gas station and the blue rectangle
indicates area of road tunnel construction. Bottom figures show spatial
distribution of gas network (bottom left) and sewage system (bottom right) in
the neighbourhood of the measurement site (white dots). Schemes are based on
data from Geodesy Center of Łódź (www.mapa.lodz.pl). Photo
source: www.google.com.
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Measurement site and instrumentation
Study area and site location
Łódź is one of the largest cities in Poland. The area of the city
is about 295 square kilometres (km2), and its population is estimated at
706 000 residents. The city is located in central Poland on relatively flat
terrain which slopes south-westwards. Its altitude varies from 280 to 160 m
above mean sea level. The most densely built-up city-centre area covers
80 km2 and the altitude difference in this part of the city does not
exceed 60 m. In the immediate vicinity of Łódź, there are no
large bodies of water, rivers or orographic obstacles impacting the climate
of the city that are worthy of investigation. Another factor making it easier
to take measurements of turbulent fluxes of mass and energy in
Łódź is that the city does not have a standard central sector of
tall buildings towering over an urban canopy layer, unlike other large cities in Poland.
A FCH4 measurement site in Łódź (left) and
instrumentation (middle). The right figure shows the frequency of measured
1 h blocks of raw data in relation to the RSSI (Received Signal Strength
Indicator) of Li7700 methane open-path analyser. Data recorded only in the
case RSSI > 20 % were taken into account.
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The measurements of turbulent fluxes of methane are conducted in the western
part of the city centre (51∘47′ N, 19∘28′ E) as shown
in Fig. 1. This part of the city has the highest population density, reaching 17.2 thousand people per km2. The station for measurements of
fluxes of mass, energy and momentum has been operating in the western part of
Łódź since 2000 (Offerle et al., 2006a, b; Pawlak et al., 2011;
Fortuniak et al., 2013; Fortuniak and Pawlak, 2015), but methane fluxes have been studied since July 2013. The
measurement set is mounted on top of a mast at a height of z=37 m
(Fig. 2, left) which, given the average height of buildings of 11 m, enables
the assumption that the measurements are taken above the blending height in
the inertial sub-layer (Fig. 2). The source area of turbulent fluxes was
estimated (Fig. 1) for data collected during unstable stratification
((z-zd)/L<-0.05) around midday
(10.00–14.00 UTC + 1), following the method of
Schmid (1994), and ranged from 250 to 750 m away from the measurement station
(Fig. 1).
The percentage of artificial surface coverage from buildings, pavements,
streets and squares in this part of the city is 62 %. The remaining part
of the area is covered in vegetation of which only 10 % is trees (Kłysik,
1998). The vegetation is distributed unevenly in the form of lawns and trees
growing along street canyons. In the immediate vicinity of the measurement
location, 3- to 5-storey buildings dominate, ranging from 15 to 20 m in height. Most of the buildings have flat roofs covered with black tar paper
or sheet metal. The trees growing in the area are mostly deciduous and their
height usually does not exceed the height of the buildings. This results in a
well-formed roof surface with an average height of 11 m. The density of
built-up areas north and east of the measurement point, compared to the
southern and western sectors, is 10–20 % greater (Fig. 1). The
displacement height zd is estimated at 7.7 m. According to the
classification by Stewart and Oke (2012), the local climate zone can be
described as “compact low rise”. The roughness coefficient z0m estimated
for the neutral stratification surrounding the measurement point was 2.5 m
on average. More information on the city's structure and the local climate
conditions can be found in Kłysik (1996), Kłysik and Fortuniak (1999),
Fortuniak et al. (2006, 2013), Offerle (2006a, b), Pawlak et al. (2011)
and Zieliński et al. (2013). The gas distribution network and sewerage
system around the flux tower are shown in Fig. 1.
Instrumentation and data processing
Measurements of turbulent fluxes of methane were carried out using a standard
measurement set consisting of the ultrasonic anemometer R.M. Young
model 81000 (R.M. Young, Traverse City, Michigan, USA) and a fast-response
methane concentration sensor with an open-measurement-path LI7700 (LI-COR,
Lincoln, Nebraska, USA). The measurements were carried out with a precision
of 0.001 m s-1 and 1 ppb respectively. As the final calculation of
methane flux also requires values of sensible heat and water vapour fluxes in
the place of observation (LI-7700 Open Path CH4 Analyzer, 2011), the
measurement set also included a sensor measuring the concentrations of water
vapour and carbon dioxide. This was a LI7500 infrared CO2/H2O
open-path analyser (LI-COR, Lincoln, Nebraska, USA).
The whole measurement system was attached approximately 1 m below the top of
the mast (Fig. 2, middle). The LI7500 head was placed on the horizontal arm
on the south-eastern side of the mast at a distance of about 60 cm from the
mast. The ultrasonic anemometer was then installed at a distance of 20 cm.
The LI7700 methane sensor was installed on an additional arm, 30 cm lower,
so that the centre of its measurement path, which is about 4 times longer
than the paths of LI7500 and ultrasonic anemometer, was at a similar level.
Previous studies have shown that the influence of a mast of diameter 0.15 m
is negligible and does not generate flow distortion (Fortuniak et al.,
2013).
All the aforementioned sensors sampled with a frequency of 10 Hz.
Immediately before starting the measurements in July 2013, the sensor for
measuring H2O and CO2 mole fractions was calibrated (the zero and
span values were set). The methane concentration analyser was installed
directly after purchase, so the zero and span had been set by the
manufacturer. The two sensors and the ultrasonic anemometer were cleaned
approximately once a month. This was pertinent to the methane sensor because
its mirrors proved to be highly susceptible to grime (air impurities, bird
droppings, atmospheric deposits, drying raindrops or melting snowflakes).
The manufacturer equipped the instrument with a mirror heating and
condensation anti-freezing system and a cleaning system. Using a pump, this
applied cleaning liquid to the lower mirror. However, in practice and
particularly in autumn and winter this was insufficient, particularly on days
with humidity of up to 100 % when the signal strength dropped by several
tens of percent in just a few hours. According to the manufacturer of the
instrument, if the signal strength Relative Signal Strength Indicator (RSSI)
is less than 10 %, this means that the measurement path is blocked by
external factors. However, it was decided to tighten this criterion. In
order to calculate the fluxes, the methane mole fraction values observed at
RSSI > 20 % were chosen. Of these, the RSSI exceeded 70 % in
only 8 % of cases (Fig. 2, right), while observations at 20 < RSSI < 70 % had a much greater share. Most often and in 20 % of
cases the signal strength was between 30 and 40 % (Fig. 2, right).
The 10 Hz fluctuation data for the vertical wind velocity and the
concentrations of water vapour and methane were recorded by a CR21X
data logger (Campbell Scientific, Logan, Utah, USA) so that all parameters
could be recorded at the same time. The measurement station was also
equipped with sensors recording the general weather conditions (air
temperature and humidity, atmospheric pressure, wind direction and velocity,
radiation balance components, precipitation). These data were recorded every
10 min by a CR10 data logger (Campbell Scientific, Logan, Utah, USA) and
were archived together with the 10 Hz data on a PC.
The FCH4 was determined directly from the definition as the covariance
of the vertical wind velocity fluctuations and the methane concentration
fluctuations in the air (Lee et al., 2005; Foken, 2008; Burba and Anderson,
2010; Aubinet et al., 2012):
FCH4=w′ρCH4′‾=1N∑Ni=1w-w‾ρCH4-ρCH4‾.
The w′ and ρCH4′ parameters are, respectively, the
fluctuations of vertical wind velocity and the concentration of methane in
the air, while w‾ and ρCH4‾ are their
averaged values. A positive flux means the turbulent transport of methane
into the troposphere; a negative flux is its uptake by the urban surface.
Block averaging of 1 h was used as an averaging period. Since the
measurements were carried out at a considerable height, a shorter averaging
period could lead to underestimating the fluxes (Pawlak et al., 2011). During
the calculations, all necessary procedures and corrections were applied. Any
data with non-real values were rejected, the spike detection procedure was
performed (Vickers and Mahrt, 1997), the double rotation of the wind
coordinate system was applied (Kaimal and Finnigan, 1994) and the impact of
separation of the sensors was eliminated by maximizing the covariance in the
interval ±2 s. Furthermore, sonic temperature was corrected for humidity
in the air (Schotanus et al., 1983) and the WPL correction was added (Webb et
al., 1980). According to LI7700 manufacturer's recommendations, the
correction terms related to air density fluctuations affecting both the
spectroscopic response and the mass density retrieval were applied (LI-7700
Open Path CH4 Analyzer, 2011).
A detailed control of the quality of the calculated fluxes was also carried
out, which focused primarily on the assessment of data stationarity. The most
commonly used Foken's test (Foken and Wichura, 1996) is not always fit for
this
purpose. Therefore two other tests were used as proposed by Mahrt (1998) and
Dutaur et al. (1999) and modified by Affre et al. (2000). During the data
quality assessment, a very strict criterion was adopted to classify the data
as suitable for further analysis when three tests confirmed that
stationarity was met. A milder criterion, indicating good data quality when at
least one test suggested stationarity, did not meet the expectations. This
criterion accepted data with unrealistically high positive values and a
substantial number of fluxes with high negative values whose existence
cannot be explained. However, the restrictive evaluation of the data reduced
the amount of data suitable for further analysis by 23.8 %. Uncertainty
regarding their quality was kept to a minimum. Approximately 10 % of the
data were not registered due to problems with power supply in autumn 2013,
and 29.8 % of the recorded data were rejected because the measurements had
been taken in weather conditions which made it impossible for the LI7700
sensor to measure the concentration of methane properly. This was a result
of such factors as precipitation and atmospheric deposits, saturation of air
with water vapour and impurities. This problem occurred particularly in
autumn and winter (Table 1) when frequent cleaning of the sensor placed on
the mast was impossible. As a result, the percentage of acceptable data was
36.4 % as shown in Table 1.
Data capture of 1 h values recorded for FCH4 in the centre
of Łódź in the period July 2013–August 2015.
Spring MAM
39.1 %
Summer JJA
47.4 %
Autumn SON
26.5 %
Winter DJF
31.1 %
July 2013–August 2015
36.4 %
Results
Climate background
The climate of central Poland is a typical transitional climate of moderate
latitudes. It is characterized by marine air masses flowing from the west
and by continental air from the east. The mean monthly air temperature in
the study period varied from 0.1 ∘C in winter (January 2014) to
22.8 ∘C in summer (August 2015). The study period was considered
to be hot with heat waves occurring and the winters were relatively warm
with mean temperatures in 2014 and 2015 of 2.7 and
2.4 ∘C respectively. The average temperature in 2014 was
10.9 ∘C. The total precipitation in the same year was 584 mm, with
a greater amount of precipitation of 360 mm (61.6 % of the annual total)
recorded in the warm half of the year. The maximum solar radiation was
observed in July (688 MJ month-1 in 2014 and 697 MJ month-1 in 2015) while the minimum occurred in the
winter months when they fell below 80 MJ month-1. The
monthly radiation balance totals were almost 400 MJ month-1
in July, while in the winter months they became negative and reached even
-56 MJ month-1 (December 2013). The average wind speed in
the period was 3.1 m s-1, with slightly higher values in
winter (3.4 m s-1) and lower values in the summer (2.8 m s-1 on average). The study area of the city is dominated
by air flow from the west (Fortuniak et al., 2013).
During the measurement period, atmospheric instability or neutral conditions
prevailed in the city centre. Stable air stratification was observed in the
centre of Łódź in only 7.6 % of cases (from 2.9 % in winter to
10.4 % in summer). The frequency of neutral and unstable stratification
was similar and was 46.0 and 46.4 % respectively. Unstable conditions
prevailed in summer (51.6 % of cases), while neutral conditions were
observed in 61.7 % of cases in winter. In the diurnal cycle, stable
stratification was also a rarity. In the daytime (10:00 to 14:00),
this type of stratification was observed in only 0.3 % of cases on average
throughout the year, while at night the condition ξ > 0 was
met by 15.0 % of the data. Other types of atmospheric stability appeared
in the daytime in 19.7 % (neutral) and in 80 % (unstable) of cases. At
night, neutral conditions prevailed (67.0 % of cases), while unstable
conditions were observed in 18.0 % of cases on average throughout the
year.
Clear annual and diurnal cycles characterized the fluxes of energy and mass.
Both the sensible heat flux QH and the latent heat flux QE were
largest in summer (about 190 and 120–150 MJ month-1 respectively). The Bowen ratio
B=QH/QE was typically urban (i.e. greater than 1 and up to 2.25
in May 2015). The annual variability of carbon dioxide flux (FCO2) was
also marked by an annual cycle. The maximum values occurred in winter when
anthropogenic CO2 emissions, a result of burning fossil fuels for
vehicle traffic and domestic heating, were the largest. The typical values
exceeded 20 µmol m-2 s-1 and had a
maximum of ∼ 55 µmol m-2 s-1. In summer, the consumption of CO2 by urban vegetation
and the
lack of domestic house heating contribute to a decrease in the intensity of
net exchange. The minimum values of FCO2 were observed as -10 to 10 µmol m-2 s-1.
One-hour means of air temperature, 1 h net turbulent FCH4
flux (approved by three stationarity tests), and mean and median of FCH4
measured in the period July 2013–August 2015.
![]()
Annual variability of FCH<inline-formula><mml:math display="inline"><mml:msub><mml:mi/><mml:mn mathvariant="normal">4</mml:mn></mml:msub></mml:math></inline-formula>
The 2-year measurements of FCH4 revealed a number of characteristics
of the exchange of methane in the city–troposphere system. Irrespective of
the season, mainly positive values of FCH4 were observed (Fig. 3). On
average, the percentage of positive values over the study period was
93.7 %, which was slightly greater in the cold season (94.6 %) than in
the warm season (93.2 %). This means that regardless of the season the
centre of Łódź is a source of methane to the atmosphere. In
addition, the time variability of FCH4 shows a clear annual cycle
with a maximum in the cold season and a minimum in the warm season (Fig. 3).
The highest recorded values exceeded 100 nmol m-2 s-1 and were observed in November, December, January and February. The
least intense exchange of methane was observed from May to September, when
FCH4 was rarely greater than 50 nmol m-2 s-1. The exception was the summer of 2013 when the recorded values of
FCH4 were close to winter values in July and August. However, only
average values were elevated, while the median values are similar to those
of July and August 2014 and 2015. It can be assumed that in the summer of
2013 additional sources of methane were present which could be the result of
damages to the gas network. It is likely that this occurred south-east of
the station, where the deep excavations associated with the construction of a
tunnel for one of the main streets of the city centre were completed (Fig. 2).
Monthly values of mean, median and standard deviation values of
FCH4 in the centre of Łódź in the period
July 2013–August 2015 (all fluxes in nmol m-2 s-1).
J
F
M
A
M
J
J
A
S
O
N
D
2013
Mean
–
–
–
–
–
–
22.1
29.5
39.6
38.1
35.3
45.3
2013
Median
–
–
–
–
–
–
18.2
22.0
26.3
27.7
26.6
38.6
2013
SD
–
–
–
–
–
–
27.1
35.7
56.2
43.7
39.3
35.9
2014
Mean
62.9
66.6
37.4
33.1
21.8
22.9
20.3
19.2
20.8
27.0
43.4
47.8
2014
Median
60.2
64.4
30.6
31.7
20.8
22.1
19.3
18.2
20.2
23.6
34.2
38.4
2014
SD
46.7
42.6
32.9
21.8
20.8
16.8
14.7
15.2
11.3
20.9
31.9
39.3
2015
Mean
52.5
54.2
48.6
25.2
22.8
18.4
17.6
21.5
–
–
–
–
2015
Median
47.6
51.8
46.5
22.4
22.1
17.7
17.3
20.3
–
–
–
–
2015
SD
34.5
39.4
32.4
23.7
16.2
14.9
12.9
14.6
–
–
–
–
It seems that the annual cycle of turbulent methane exchange should be
attributed to the anthropogenic origin of this gas in the centre of the
city. In the cold season, there is an increase in methane emissions
associated with the combustion of fossil fuels, which results from the
increased discharge of motor vehicle exhaust gas (Heeb et al., 2003;
Nakagawa et al., 2005). Another important factor is the increased natural
gas consumption in winter, its leakage from distribution networks and its
use in domestic gas burners. Methane is also produced by heating ovens
(Ciais et al., 2013). The absence of inventory data makes it difficult to
verify these dependencies for Łódź. However, the increased values
of the flux of methane are clearly visible where there are rapid drops in
air temperature i.e. in late October or late November and December 2014
(Fig. 3). A pronounced annual cycle can also be seen in the temporal
variability of the mean monthly values of FCH4 (Fig. 3, Table 2). The
highest monthly averages of FCH4 were recorded in January and February
2014 when the average exchange exceeded 60 nmol m-2 s-1. In the same months of 2015, the FCH4
values were lower and slightly exceeded 50 nmol m-2 s-1, which was a consequence of winter 2014/2015
being warmer than 2013/2014. The mean monthly values of FCH4 in summer
rarely exceeded 20 nmol m-2 s-1. The median
values in the warm half of the year were very similar to the average values.
In the cold season, the median was lower due to the sporadically occurring
elevated levels of FCH4. Regardless of the measurements, some
differences in the time variability of methane flux in transitional seasons
can also be observed. In late winter and early spring, a rapid drop in
FCH4 by approximately 30 nmol m-2 s-1
can be observed, while FCH4 starts to increase at the end of summer and
slowly continues until winter. The cold half of the year is also
characterized by a greater variability of the fluxes of methane (Table 2).
In the summer months, the standard deviation of FCH4 did not exceed 20 nmol m-2 s-1, whereas during the winter
months it was more than 2 times greater. An exception is the
aforementioned summer of 2013.
Mean diurnal variability of FCH4 flux in the period
July 2013–August 2015 (top left figure) and for months. Thin and thick black
lines indicate, respectively, variability of FCH4 and three-element running
average of FCH4. Thin vertical lines indicate standard deviation of
FCH4.
![]()
Diurnal variability of FCH<inline-formula><mml:math display="inline"><mml:msub><mml:mi/><mml:mn mathvariant="normal">4</mml:mn></mml:msub></mml:math></inline-formula>
Figure 4 shows the average daily flux of methane in the centre of Łódź calculated for the entire study period (top graph) and for the
successive months of the year (middle and bottom graphs). The average daily
variability in the successive months confirms the above described annual
variability; i.e. higher values of FCH4 occurred in the cold season.
Furthermore, the average daily variability, regardless of month and time of
day, is always positive. This means that the emissions of methane dominate
over its uptake by the urban surface. The daily pattern, averaged for the
entire measurement period, shows a clear diurnal cycle with two maxima and
two minima. The maximum values occurred in the morning (07:00–08:00 UTC + 1) and in the evening (19:00–20.00 UTC + 1). During the maxima, the
values of FCH4 reached almost 40 nmol m-2 s-1, whereas during the noon hours and at night they
dropped to 26–28 nmol m-2 s-1. Such a daily pattern suggests
that the average flux of methane can be divided into two components. One has
an approximately constant value of up to 26–28 nmol m-2 s-1 and its source may be the sewerage system and
the natural gas distribution system. In the morning and in the afternoon,
additional sources of methane (vehicle traffic, combustion of natural gas,
leaks from gas network associated with the increasing gas consumption) are
activated, increasing the flux by 10–12 nmol m-2 s-1. However, it should be noted that due to the lack of inventory
data the above considerations are only hypothetical.
In the warm half of the year (May–October), the average daily variability
was low and from April to September it ranged between 10 and 40 nmol m-2 s-1. In May, June and July it was
difficult to see clear maxima during a 24 h period. In August, September
and October there was a maximum in the morning. In the cold half of the year
(November–April), the average daily variability of FCH4 was
characterized by distinctly higher values from 20 to 90 nmol m-2 s-1. In this period, the double daily maximum was
easier to identify and in November, December, January and February the
afternoon peak seemed to be greater than in the mornings. In March and
April, the maximum values were comparable. The presence of two maxima in
the variability of FCH4 in the cold season could be explained by the
increased consumption of natural gas, the combustion of fossil fuels in the
morning and afternoon hours from cooking and domestic heating, and the
diurnal variability of motor vehicle traffic, which can cause road congestion
in winter. In the warmer seasons and particularly during the holiday
periods, motor vehicle traffic become less intense and the city's
inhabitants stop heating their homes. In the cold season, FCH4 is
also characterized by greater variability throughout the day. The standard
deviation of FCH4 in this season can reach 50 nmol m-2 s-1
while in the warm season it rarely exceeds 20 nmol m-2 s-1.
Monthly and annual exchange of FCH<inline-formula><mml:math display="inline"><mml:msub><mml:mi/><mml:mn mathvariant="normal">4</mml:mn></mml:msub></mml:math></inline-formula>
Based on the average daily patterns of FCH4 calculated for each month
(the sum of the average hourly FCH4 multiplied by the number of days in
the month), the exchange of methane in the successive months of the study
period was determined (Fig. 5). The highest values occurred in winter; in January and February 2014 they exceeded 2.5 g m-2 month-1. The summer values were more than 2 times
lower and dropped to 0.7–0.8 g m-2 month-1.
Autumn of 2013 was characterized by elevated values of FCH4. A
comparison between the monthly exchange of methane and the mean monthly air
temperature reveals a clear link between these parameters (coefficient of
determination = 0.731; Fig. 5, bottom right). Thus, the anthropogenic
sources of methane gain intensity at low air temperatures, which can be seen
by comparing the results of FCH4 measurements in winter 2013/14 and
2014/15 (Fig. 5). In the first case, an increase of the monthly values of
the flux was recorded starting from November, with a maximum in January and
then a decrease until April–May. Between November 2013 and January 2014, the
exchange almost doubled (from 1.36 to 2.67 g m-2 month-1). The next winter, the monthly exchange of methane between
November and March differed little, and FCH4 increased from November 2014
to January 2015 only by ca. 0.27 g m-2 month-1. The differences were associated with thermal contrasts during
the two winters. In winter 2014/2015, the monthly average temperature
remained at 2.2–2.5 ∘C, while in the previous winter season the
mean January temperature dropped to 0.1 ∘C. The greater activity
of the anthropogenic sources of methane in the centre of the city is also
confirmed by the measurements of methane concentrations (Fig. 5, bottom
left). The high winter values of the flux of methane are accompanied by
higher concentrations of the gas in the air and seasonal changes in OH
concentration.
Monthly totals of FCH4 (top) in relation to mean monthly
CH4 concentration (bottom left) and mean monthly air temperature (bottom
right) in the period July 2013–August 2015.
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Based on the data on the exchange of methane in Łódź obtained
between January 2014 and August 2015, an attempt was made to assess the
cumulative annual exchange of this gas in the centre of Łódź
between the city centre and the atmosphere (Fig. 6). To date, there have
been no standard methods for filling gaps in the long-term data series of
turbulent fluxes of methane in urbanized areas. Difficulties with their
development arise primarily from the fact that the continuous measurements
of FCH4 in cities are still rare. Furthermore, as in the case of carbon
dioxide fluxes, data on anthropogenic sources of the gas and the parameters
of natural processes (e.g. air temperature) may be useful for the data gap-filling procedures (Aubinet et al., 2012). The annual exchange of methane in
the city centre was therefore estimated using two simple methods. Firstly,
on the basis of the average daily patterns of FCH4, the monthly
exchange in the successive months was determined and then the accumulation
was made (Fig. 6, solid step plots). Secondly, the gaps were filled in a
series of 1 h values of FCH4 in two ways. When a data gap was not
longer than 3 h, interpolation was used, while for longer gaps data were
inserted from the average daily pattern in the respective month for the
respective hour. Both methods yielded very similar results (the difference
was approximately 1 %), although it is obvious that the cumulative fluxes
obtained in this manner should be regarded as an approximation. Therefore it
can be stated that the annual exchange of methane in the centre of Łódź in 2014 was equal to about 17.6 g m-2 (Fig. 6).
The graph shows the impact of the annual variability of methane flux: the
cumulative flux grows fastest in the cold half of the year. Due to the
differences in the exchange of methane described in Sect. 3.4, with
reference to changes in air temperature in the study period, the cumulative
exchange in the period January–August 2015 was calculated in a similar
manner. The relatively warmer beginning of 2015 caused the exchange to be
less intense and the cumulative flux of FCH4 in August 2015 was by
9.2 % lower than in 2015.
Cumulative fluxes of FCH4 in the period January–December 2014
(light blue lines) and January–August 2015 (dark blue lines). Dotted and
solid lines indicate cumulative annual FCH4 calculated, respectively, on
the basis of all 1 h data and on the basis of integrated mean daily
courses of consecutive months.
![]()
Weekly differences of FCH<inline-formula><mml:math display="inline"><mml:msub><mml:mi/><mml:mn mathvariant="normal">4</mml:mn></mml:msub></mml:math></inline-formula>
Since fluxes of methane in the city are associated with anthropogenic
sources, a weekly cycle of FCH4 should be expected that is similar to
that seen for CO2 exchange (Pawlak et al., 2011). Based the on 1 h
data for FCH4 recorded in the period July 2013 to August 2015, an
average daily flux of 44.3 mg m-2 day-1 was
determined (Fig. 7). Having taken only working days for the calculation
(Monday to Friday), it was found that the exchange was higher, i.e. 45.2 mg m-2 day-1. In contrast, the average daily
exchange of methane during weekends (Saturday and Sunday) amounted to 42.3 mg m-2 day-1 and was therefore lower by
4.5 %. These results suggest that in the study period
anthropogenic sources of methane are likely to be, on average, less at weekends compared
to working days. It should be emphasized that on Saturday, the average flux
was lower by 6.9 % in comparison with working days, while on Sunday it was
lower by 12 %. The difference is a result of significantly lower peak on
Sunday morning, which can be attributed to less intensive human activity on
Sunday morning and lower traffic load in comparison to the same time of day
on Saturday. Similar results were observed in summer and winter, when the
average daily exchange on working days was higher by 1.6 % (summer) and
1.9 % (winter) compared with the average for the whole week. The average
daily exchange at weekends was lower by 4.0 and 4.7 % respectively. An
exception is the transitional seasons when the average daily exchange of
methane on working days was comparable (spring, -0.3 %) or slightly lower
(autumn, -1.6 %) than the average for the entire week (Fig. 7). However, the average daily exchange during the weekend turned out to be
higher and amounted to +1.6 % (spring) and +3.8 % (autumn). Without
the inventory data, it is difficult to explain why the fluxes vary,
particularly in the case of CO2 fluxes where higher values are observed
on working days as compared to weekends throughout the year (Pawlak et al.,
2011).
Mean daily values of FCH4 in the period July 2013–June 2015
calculated for study period and seasons. Red, black and blue bars indicate mean daily exchange during, respectively, working days (Monday to Friday),
weeks (Monday to Sunday) and weekends (Saturday and Sunday).
![]()
Mean FCH4 calculated for all data (left), cold (middle) and
warm period (right) in relation to wind direction in the period
July 2013–August 2015.
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Methane fluxes and wind direction
As mentioned in Sect. 2.1, the centre of Łódź is the most
densely built-up area of the city. The measurement point is located in an
area of uniform building density while, as mentioned in Sect. 2.1, this
density is slightly greater to the east and north of the station. An
analysis of the average value of FCH4 depending on the wind direction
confirms, at least in part, the impact of building density on the value of
methane turbulent exchange (Fig. 8). The fluxes of methane recorded during
airflow from the north, and especially from the south-east, were by far the
largest in the study period and reached 35–45 nmol m-2 s-1 (Fig. 8, left). However, it is difficult to be
confident in the direct relationship between urban design and FCH4
because of the increased values of FCH4 from the south-western sector
(approximately 40 nmol m-2 s-1). Such a
relationship cannot be ruled out, but the local point sources of methane
may play an important role even though they are difficult to identify. In
the case of the south-western sector, the liquid petroleum gas station
located approximately 800 m from the measurement station may be such a
source. It lies approximately 200 m to the west of the large intersection
where traffic load is usually larger than the surrounding streets.
Significantly lower values of FCH4 (less than 20 nmol m-2 s-1) observed with airflow from the south and west
may be due to the presence of large urban parks (Fig. 1). Heavy traffic does
not run through these streets and the density of the gas network and sewage
system is also significantly smaller in comparison with other sectors (Fig. 1).
The distribution of average FCH4 depending on the wind direction,
calculated for the cold half of the year (Fig. 8, middle), suggests that in
this season local anthropogenic methane sources were more intense. The
relationship between FCH4 and the wind direction was much the same
throughout the study period, while the average values of fluxes were higher
and amounted to 55–70 nmol m-2 s-1.
Therefore, the sources could be clusters of houses with leaks from gas
installations or vehicles at nearby intersections which are heavily jammed
in the cold half of the year. In summer, the average fluxes of CH4 were
significantly lower (less than 30 nmol m-2 s-1; Fig. 8, right) regardless of the wind direction. The contrast
between the sectors was not clear. An exception is the clearly visible
elevated value of FCH4, associated with the airflow from the
south-western sector.
Methane fluxes in relation to carbon dioxide fluxes
During more than 2 years of measurements in Łódź, both FCO2
and FCH4 were measured and therefore the question about temporal
covariability of both fluxes can be addressed, and, consequently, whether
the exchange of methane can be estimated based on the knowledge of the flux
of CO2. The average daily variability (Fig. 9, left) and the average
monthly variability (Fig. 9, middle) of the value of methane flux were
compared to the fluxes of CO2. As the figure indicates, such
covariability exists and bigger fluxes of CH4 are accompanied by larger
fluxes of CO2. Unfortunately, the low coefficients of determination
(0.57 and 0.56) mean FCO2 cannot be used as a proxy for FCH4 in the
centre of Łódź. An even weaker relationship was observed between
the average daily patterns of FCH4 and FCO2 (Fig. 9, right).
Although the two fluxes have a characteristic pattern with two maxima in a
24 h period, the coefficient of determination is only 0.25. We therefore
conclude that FCO2 data cannot be used to facilitate gap filling of
FCH4 data in the centre of Łódź.
Mean daily (left), monthly (middle) and hourly FCH4 fluxes
against FCO2 in the period July 2013–June 2015.
![]()
Monthly FCH4 to FCO2 ratio (up) and mean diurnal courses
of FCH4 to FCO2 ratio in the period July 2013–August 2015 and for
seasons.
![]()
The comparison of FCH4 and FCO2 fluxes allow analysis of the
relative contribution of each of the fluxes to total emissions to atmosphere.
The average value of the FCH4/ FCO2 ratio in 2013–2015 was 3.71×10-3 (Fig. 10, top). Rather stable values of the ratio in months
(minimum 2.41×10-3, maximum 5.3×10-3) and the lack of
a clear annual course suggest rather comparable magnitude of both fluxes.
However, a clear diurnal course in the ratio has been observed (Fig. 10,
bottom) with reduced values in the day and elevated values at night. On
average, over the study period and in the transitional seasons, the daily
variation of FCH4/ FCO2 was similar. Between the hours of 09:00
and 17:00 FCH4/ FCO2 was approximately constant of the order 2.5
to 3.5×10-3. At night, these values grow to about 5–7×10-3, which can be explained by relatively constant methane emissions
related to leaks from pipelines and reduced emission of CO2, which is
the result of minimum of traffic load. In winter, the average daily
variability of the FCH4/ FCO2 ratio can be characterized by
slightly higher values during the day (about 4.4×10-3) and
significantly higher at night, reaching 12×10-3 between the hours
of 02:00 and 06:00 (Fig. 10, bottom). The causes again are clear: a minimum
traffic load giving reduced fluxes of FCO2 but also increased methane
leaks from pipelines associated with higher gas consumption for heating of
the surrounding buildings. The exception was the daily course
FCH4/ FCO2 in the summer, which is reversed. The minimum (of the
order of 3–5×10-3) was observed at night and the maximum (more
than 8×10-3) around noon (Fig. 10, bottom). Elevated values of
the ratio during the day are the result of photosynthesis reducing FCO2 flux.
Summary and conclusions
The measurements of FCH4 carried out in the centre of Łódź
for more than 2 years provided information on the time variability of methane
exchange between the urban surface and the atmosphere. The measurement
results showed that, as in the case of other greenhouse gases, i.e. water
vapour (Offerle et al., 2006a, b) and CO2 (Pawlak et al., 2011), the
centre of Łódź is a source of methane to the atmosphere. Another
feature indicating the similarity in the time variability of greenhouse gases
is the annual cycle of the exchange of methane in the system: city
centre/atmosphere, which seems to result from an annual cycle of
anthropogenic methane emissions. Other characteristics such as diurnal
variability, and notably weekly variability, are not as pronounced as for
FCO2 (Pawlak et al., 2011). The annual exchange of methane in terms of
pure carbon in the centre of Łódź was estimated at
13.2 g C m-2 year-1, which, compared to the exchange of
CO2 estimated in Łódź at 2.93 kg C m-2 year-1,
does not seem too large. At the same time, it must be noted that the centre
of Łódź is a source of methane comparable in intensity to the most
productive natural areas, i.e. wetlands. The annual exchange of methane in
Łódź was estimated to be 17.6 g m-2 year-1, while at
the same time (2014) the exchange in the wetlands of the Biebrza National
Park (north-eastern Poland) was approximately 18 g m-2 year-1
(Fortuniak et al., 2016). Comparable values of the annual exchange of methane
were also observed at other stations located in wetlands: approximately
16.5 g m-2 year-1 (Finland; Rinne et al., 2007) or
14.0–18.5 g m-2 year-1 (Sweden; Nilsson et al., 2008).
Unfortunately, the possibility of comparing the results obtained with those from
other cities is limited at this time. The only longer-term measurements of
methane flux were performed in Florence (March–May 2011; Gioli et al., 2012)
and in London (3-year campaign; Helfter et al., 2016). The mean values of the
methane fluxes obtained in these cities were higher than in Łódź.
In Florence, the average methane exchange in the spring of 2011 was estimated
to be 135 nmol m-2 s-1. The average FCH4 in Łódź
in the same season was 4 times lower and equal to
31 nmol m-2 s-1. However, a comparison of the obtained
results of FCH4 measurements with inventory research does not
necessarily yield a positive outcome (Gioli et al., 2012). In London, the
average exchange was also several times higher than that observed in
Łódź (142 and 32 nmol m-2 s-1 respectively). The
results of this type, however, allow only a very general comparison and the
limited sampling period prevents analysis. For example, the mean variability
of daily FCH4 in Florence in spring was characterized by one maximum
during the day, while two maxima were observed in Łódź for the
same period: morning and evening. The measurements in Florence showed no
correlation between FCH4 and air temperature (R2=-0.04; Gioli et
al., 2012), while in Łódź a strong relationship occurs
(R2=0.71). It is also impossible to compare the annual exchange and, in
the absence of measurements in other cities, it is difficult to determine the
relationship between the intensity of annual methane exchange and a parameter
characterizing the study area of the city in general. In the case of CO2
flux, a clear relationship between the annual FCO2 and the percentage of
artificial surfaces in the vicinity of the measurement point (Nordbo et al.,
2012; Oliphant, 2012) was observed. Based on the existing measurements, it is
difficult to attempt to seek a similar dependence for the flux of methane
since only in London a relationship between FCH4 and population has been
found (Helfter et al., 2016). There are also several published results of
urban methane emissions obtained using eddy covariance techniques such as
using alkanes (Los Angeles, Peischl et al.,
2013), aircraft measurements (Indianapolis, Mays et al., 2009) or a
ground-based Fourier transform spectrometer (Los Angeles, Wunch et al.,
2009). All of them report the existence of FCH4 fluxes higher than those measured
in Łódź.