INTERÉS GEOESTRATÉGICO

The urban heat island phenomenon intensifies the effects of meteorological and chemical parameters in the urban atmosphere. UHI increases the photochemical reaction rates and pollutant emissions from biogenic and anthropogenic sources. UHI causes an increase in cooling energy demands, thus producing more pollutants from fossil fuel combustion. In addition, high temperature leads to smog formation and increased ozone concentrations in urban areas. Ozone has a close interaction with meteorological parameters (temperature, cloud, radiation, wind speed) as well as chemical parameters (NOx, CO, VOCs). Ozone is a photochemical pollutant. O3 reactions

take place in the presence of sunlight and involve volatile organic compounds (VOC) and oxides of nitrogen (NOx). It is formed during daytime and destroyed during the night within complex chemical reaction chains. The ozone concentration increases during periods with hot, sunny and calm conditions and thus negatively affects the air quality in urban areas (Seinfeld & Pandis, 2012). The heat island also intensifies the processes of ozone formation in the urban environment. Thus, the effects of UHI mitigation strategies on temperature and ozone concentrations need to be investigated.

The UHI impacts on urban climate and air quality are typically studied through a one-way approach at local, regional and global scales (Arnfield, 2003; Ban-Weiss et al., 2015; Taha, 2008 and 2009; Salamanca et al., 2012; Li and Bou-Zeid, 2014; Bhati and Mohan, 2016). In these studies, the interaction between regional atmosphere and local climate is neglected. The one-way approach cannot simulate the complete interactions between urban climate and air quality. The meteorological processes and photochemical reactions in the urban atmosphere magnify the UHI effects. These interactions in the urban environment cause changes in regional climate. The changes in regional atmosphere affect local pollution. A two-way nested approach provides an integrated simulation setup to capture the full impacts of meteorological processes and photochemical interactions in the atmosphere. This approach decreases the uncertainties associated

with scale separation and grid resolution. In addition, this method reveals more details of the effects of surface modifications on urban climate and regional air quality.

Another important factor that affects air quality in urban areas is aerosols. Aerosols affect the radiative balance of the Earth-Atmosphere system by scattering and absorbing the incoming solar radiation directly and by influencing cloud formation and precipitation indirectly (IPCC 2013; Zhang et al., 2014 and 2008). The aerosols impact cloud properties by convective potential energy such as radiation, relative humidity and wind shear (Fan et al., 2013). The evaporative cooling of water bodies during daytime is recognized to modulate the influence of aerosols on the processes of convective systems (Tao et al., 2011). Aerosols also act as cloud condensation nuclei (CCN) and may impact the life-time, albedo, and precipitation of cloud systems, through a complex interaction between cloud micro-physics and dynamics (Chen et al., 2011; Archer-Nicholls et al., 2015). There are two opposite effects of aerosol on cloud formation and precipitation because of aerosol radiative properties and CCN potentials: aerosols reduce the downward solar radiation to the ground, decreasing sensible heat fluxes to evaporate water and thus lessening precipitation; or absorbing solar radiation and gain heat and enhancing the convective clouds formation, thus increasing precipitation (Kluser et al., 2008; Levin and Brenguier, 2009; Koren et al., 2005; Fan et al., 2013). But current understanding of aerosol effects on the radiative budget and hydrological cycle of the climate system is still inadequate at the fundamental level. Some uncertainties also exist in aerosol estimation because of their heterogeneous distribution and complex interactions with radiation and clouds in the atmosphere (IPCC AR5, 2013).

Increasing surface albedo results in reflecting more short wave radiation and decreasing air temperature and photochemical reaction rates (Akbari et al., 2001 and 2009; Arnfield, 2003; Ban- Weiss et al., 2015; Taha, 2008 and 2009; Taha et al., 2000; Salamanca et al., 2012; Li and Bou- Zeid., 2014; Bhati and Mohan., 2016). By increasing surface reflectivity (ISR), Taha (2008) found 2 o_{C decrease in maximum air temperature in urban areas in California. Similar results were found}

in Greece (Synnefa et al., 2008) and New York City (Lynn et al., 2009). Taha (2015) found 3 o_C

and 5–10 ppb decreases in air temperature and ozone concentrations respectively in Sacramento. Salamanca and Martilli (2012) have shown that a higher albedo decreases urban temperature by 1.5–2 o_{C during hot summer days in Madrid. Fallmann et al. (2013 and 2014) showed that}

increasing surface albedo led to a decrease in 2-m air temperature and ozone concentrations, by 0.5

al. (2015) found that by increasing surface albedo, the air temperature was reduced by 2–3°C in Sacramento and the ozone concentrations decreased by up to 5–11 ppb during the daytime. The results of increasing albedo in Houston showed a reduction in temperature by up to 3.5 °C (Taha, 2008).

Few studies have also addressed the effects of albedo enhancement on a global scale. Akbari et al. (2012) found that by increasing roofs’ (0.25) and pavements’ (0.15) albedos, the total radiative forcing will decrease by 0.044 Wm-2_{. Menon et al. (2010) found an increase of 0.5 Wm}-2 _{in total}

outgoing radiation over global land area with albedo enhancement in urban areas (0.1). Oleson et al. (2010) found a decrease of 0.8 to 1.2 o_{C of urban heat island by increasing roof albedo (0.9).}

Most previous studies have used a one-way simulation (climate simulations first, followed by air quality simulations). This approach does not provide a feedback of the atmospheric pollutants on the climate. One objective of this dissertation is to develop a two-way nested approach to

simulate the full impacts of meteorological processes and photochemical reactions on urban climate and air quality. This approach provides an integrated simulation setup to investigate the

effects of UHI and its mitigation strategy over a larger geographical area through urban areas. Increasing surface albedo may induce impacts on the hydrological cycle and radiative budget in the atmosphere. Yet, the effect of surface modification on aerosol-radiation-cloud interactions has not been investigated. Thus, it is necessary to illustrate the effects of heat island mitigation strategy on aerosols through case studies at different scales with a proper simulation tool. The model is required to combine the nonlinear effects of aerosols and simulate the interaction of aerosols, meteorology, chemistry and radiation in a fully interactive manner. Another objective of this

dissertation is to develop an approach to investigate the effects of UHI and albedo enhancement on aerosols’ direct, semi-direct and indirect effects in the atmosphere and at the surface.

2.3. Meteorological and Photochemical Models to Investigate the Effects of UHI and