Monitoring systems of greenhouse gases (GHG) are needed to evaluate the effectiveness of past and current mitigation policies, and for the design of new environmental strategies. These monitoring systems should provide independent scientific information to validate the predicted emissions at scales used in emission inventories covering all urban elements (e.g. buildings, vehicular traffic, vegetation, etc.) of relevance in the GHG control management. To achieve this goal, the monitoring systems have to rely on a combination of measurement techniques at the micro-, local- and meso-scale. At the local scale, the eddy covariance (EC) flux method (i.e. flux towers) has been increasingly used to evaluate the exchange of CO2 and other trace gases over urban surfaces. Short and long term EC flux measurements in Mexico City have demonstrated the usefulness of the method to validate the accuracy of gridded emission inventories, support traffic regulations, investigate the potential carbon sequestration by urban vegetation, and in general, to improve the GHG mitigation and air quality management of the city. In this context, this paper reports the findings from the most recent CO2 flux measurements conducted in a residential/commercial neighbourhood of Mexico City in 2011 and 2012. These findings have contributed to the verification capabilities of the local GHG mitigation management, and are expected to serve as a reference for other subtropical cities with similar urbanization patterns. Finally, the experience achieved in Mexico City and other cities should promote the use of EC flux towers to better understand the urbanization’s influence in the biogeochemical cycles, as well as a functional tool for making policy relevant decisions in the context of a changing climate.

Eddy covariance measurement of carbon dioxide was implemented for one year from November 2012 in the central area of Tokyo. The annual total CO2 flux was estimated to be 4400 gC/m2/yr. The monthly mean CO2 flux was upward to the atmosphere throughout the year. Clear seasonal variation was observed, where the largest flux occurred in winter. Local-scale emission inventory revealed that the seasonal variation of observed flux would be owing to the gas consumption in the residential houses. In winter, clear diurnal variation with the morning and night peaks was observed. Emission inventory with high-temporal resolution indicates that increase of traffic and gas consumption would cause the morning peak, while the night peak would be caused by the gas consumption.

The world is experiencing a historical shift in urbanization which has many and varied consequences. The best-documented phenomenon is the urban heat island (UHI) effect, with both local and global effect on climate change which can greatly affect our sensation of thermal comfort. One way to mitigate the UHI effect is urban greening, as plants can provide evaporative cooling effect and shading benefits. Besides, urban greenery can also sequester CO2 in vegetation and soils. On the other hand, urban greenery systems which are under intense management and maintenance may contribute to the emission of CO2 or other greenhouse gases.

We determined the carbon storage of urban turfs, which was 0.05 to 0.29 kg C m-2 for aboveground grass biomass, and 0.18 to 4.89 kg C m-2 for soils (to 15 cm depth). We also measured CO2 fluxes for urban turfs in the wet season of 2012 and dry season of 2013 using a chamber-based technique. Our data demonstrated that grass species played a dominant role in CO2 fluxes with seasonal changes, with CO2 fluxes of all turfgrass species significantly higher in the wet season than in the dry season. Besides, maintenance practices of turfs in terms of fertilization and irrigation also contributed to CO2 emission, which may affect the carbon balance of urban greenery systems and their environmental benefits.

Key words: urban greenery, CO2 flux, turfgrass, carbon balance, urban heat island (UHI) effect

Exchange of greenhouse gases between the ground and the atmosphere is one of the key processes determining the Earth's climate. This issue is very important, especially in the case of the currently observed climate change. According to the results of measurements conducted for last several years, one of the factors that determined the global warming is the increase of the concentration of atmospheric greenhouse gases. Precise measurements of these gases exchange in environment are crucial for understanding their role in climate system. The appearance about 20 years ago of suitable instruments have enabled the study of turbulent exchange of greenhouse gases between the ground and the atmosphere. These measurements resulted fairly well description of turbulent carbon dioxide flux variability at different time scales (diurnal, seasonal, annual variations), as well as the relation between the intensity of the exchange and land use. Research of turbulent exchange of CO2 between the urban surface and the atmosphere has provided a lot of information about the temporal variability of FCO2 flux in several cities around the world. Therefore, the basic characteristics of the daily and seasonal variation of FCO2 fluxes are known, in some cases, the annual exchange was also estimated. Unfortunately, there is no similar knowledge about methane, which concentration in the air is approximately 200 times smaller, but it’s global warming potential is much higher than carbon dioxide. Measurements of methane flux are carried out for several years but mainly in the areas identified as the main sources of methane to the atmosphere e.g. wetlands and rice fields. Unfortunately, only few measurement campaigns are carried out in urban areas, which may also be an important source of methane to the atmosphere (gas leaks from gas networks, sewer systems, garbage dumps, traffic , domestic heating, etc.). The number of stations equipped with instrumentation enabling measurements of the turbulent flux of methane, especially in comparison with the number of carbon dioxide sites, should be considered as definitely insufficient.

The aim of this work is to present preliminary results of methane turbulent net flux measurements carried out in the center of Lódz (central Poland), in comparison with carbon dioxide fluxes. Continuous measurements of these gases are carried out in the center of Lódz by Department of Meteorology and Climatology, University of Lodz since July 2006 (carbon dioxide) and July 2013 (methane). The values of turbulent fluxes has been calculated with eddy covariance method with use of the standard instrumentation set (sonic anemometers and H2O, CO2 and CH4 infrared open-path gas analyzers). So far, more differences than similarities of temporal variability FCH4 in relation to FCO2 were observed. In both cases annual rhythm exist, but variability of FCH4 is much lower. A significant diurnal variability of FCH4 was observed primarily in the warm half of the year, while, in contrast to FCO2, diurnal rhythm of FCH4 in winter is much less diverse. A characteristic feature of the diurnal variability of FCO2 in the city center in the cold half of the year is the presence of two peaks, associated with morning and afternoon and car traffic. In the case of FCH4 such variability does not occur. Temporal variations of both fluxes is characterized by the weekly rhythm (elevated FCH4 and FCO2 on working days from Monday to Friday). The maximum average monthly exchange of CO2, recalculated to pure carbon, is observed in winter (~ 300 gC•m-2•month-1) and the lowest in summer (~ 140 gC•m-2•month-1). Monthly exchange of methane is much less intense, respectively ~ 0.75 gC•m-2•month-1 and ~ 1.1 gC•m-2•month-1. Funding for this research was provided by National Centre of Science under projects 2011/01/D/ST10/07419.

Measurements of flux (F_c) and concentration (?_c) of carbon dioxide are carried out in different ecosystems since several decades. Nevertheless, multiyear records in the urban environment are rare and, when available, cover at most a few years. In Basel, Switzerland, F_c and ?_c are measured continuously since May 2004 by an eddy covariance system at 38 m above street level at Klingelbergstrasse (BKLI) covering in the meantime a full decade. Analysis of this unique time series contributes to an enhanced understanding of controlling factors of ?_c and F_c as well as their seasonal and inter-annual variation in the urban environment.

The urban ?_c-values compare well with background concentration measurements from Schauinsland (Germany) and Jungfraujoch (Switzerland). Furthermore, a good agreement between local and background ?_c is found with respect to seasonality and long term trend (1.5-2 ppm y-1). Compared to these background measurements, ?_c at BKLI is clearly higher and daily fluctuations as well as seasonal amplitude are larger, as a consequence of distinct local sources. Inter-annual differences of the local ?_c at BKLI, when corrected for long term trend, do not vary more than 10 ppm.

F_c is examined for the diurnal, seasonal and inter-annual variability with the use of sectorial analysis. Furthermore, the controlling factors of F_c, as investigated by a footprint analysis, show clear separation of an eastern “traffic”- and a western “residential”-sector. This is a direct result of the local wind regime with a distinct diurnal pattern. To validate the measurements on a neighbourhood scale (about 400 m) fluxes are scaled up by taking into account the varying relative frequency of occurrence of the different wind sectors. The resulting up-scaled average neighbourhood net ecosystem exchange (NEE) totals up to 4.3 kgC m-2 y-1, and is thus slightly higher when compared to the measured NEE of 4.1 kgC m-2 y-1. Both results are still clearly lower than the 6.33 kgC m-2 y-1 from the inventory based approach by the local authorities.

Current regional climate model projections state the number of hot days with maximum daily temperatures > 30 °C to quadruple until the end of the century in most parts of Germany (scenario A1B; Jacob et al. 2008, Klimaatlas DWD, 2014). Besides global mitigation measures there is need to implement local adaptation strategies in response to urban warming.

Urban green roofs are discussed as one important local urban adaptation strategy. Several studies have already described local meteorological and ecological properties of green roofs, e.g. lowering extreme temperatures by evaporative cooling, rainwater retention and potential air pollution removal (e.g. Ng et al. 2012, Rowe 2011, Yang et al. 2008). However, in situ data on the complete energy balance of extensive green roofs as well as data on the potential of green roofs as carbon sinks is still missing. Such data is important to understand the interaction of green roofs with the urban boundary layer and to validate current green roof models (Sailor 2008). Therefore the eddy covariance measurement technique is applied in order to quantify the turbulent surface-atmosphere exchange of an extensive green roof at the Berlin Brandenburg airport. The green roof covers an area of about 8900 m2 and has a south-west north-east orientation with a length of 168 m. The measurement period started in July 2014 and is planned to be continued until the end of August 2015 in order to get a complete picture of the variability of turbulent fluxes during all seasons of the year.

Seasonal variation of the carbon dioxide exchange and water vapor dynamics of the extensive green roof for the first ten months of the measurement period will be presented and discussed. Furthermore our attention focusses on the evapotranspiration behavior of the green roof during warm periods. First results indicate that during a seven day heat period, maximum daily latent heat fluxes may drop by about 80 % comparing the last versus first day of this period (01.07.2014 - 07.07.2014).

Literature:

Jacob, D., Göttel, H., Kotlarski, S., Lorenz, P. and Sieck, K., 2008: Klimaauswirkungen und Anpassung in Deutschland – Phase 1: Erstellung regionaler Klimaszenarien für Deutschland (Forschungsbericht 204 41 138 UBA-FB 000969). UMWELTFORSCHUNGSPLAN DES BUNDESMINISTERIUMS FÜR UMWELT, NATURSCHUTZ UND REAKTORSICHERHEIT, Dessau-Roßlau, 159 pp.

Klimaatlas DWD, 2014. Deutscher Klimaatlas (Deutscher Wetterdienst, DWD).

Ng, E., Chen, L., Wang, Y., Yuan, C., 2012: A study on the cooling effects of greening in a high density city: An experience from Hong Kong. Building an Environment. 47, 256-271.

Rowe, D.B., 2011. Green roofs as a means of pollution abatement. Environmental Pollution. 159, 2100-2110.

Sailor, D.J., 2008. A green roof model for building energy simulation programs. Energy and Buildings. 40, 1466-1478.

Yang, J., Yu, Q., Gong, P., 2008: Quantifying air pollution removal by green roofs in Chicago. Atmospheric Environment. 42, 7266-7273.