Abstract. Heat stress is one of the most severe climate threats to human society in a future warmer world. The situation is further exacerbated in urban areas by urban heat islands (UHIs). Because the majority of world's population is projected to live in cities, there is a pressing need to find effective solutions for the heat stress problem. We use a climate model to investigate the effectiveness of various urban heat mitigation strategies: cool roofs, street vegetation, green roofs, and reflective pavement. Our results show that by adopting highly reflective roofs, almost all the cities in the United States and southern Canada are transformed into white oases – cold islands caused by cool roofs at midday, with an average oasis effect of −3.4 K in the summer for the period 2071–2100, which offsets approximately 80 % of the greenhouse gas (GHG) warming projected for the same period under the RCP4.5 scenario. A UHI mitigation wedge consisting of cool roofs, street vegetation, and reflective pavement has the potential to eliminate the daytime UHI plus the GHG warming.

A wedge strategy for mitigation of urban warming in future climate scenarios

Animals interpret their environment by combining information from multiple senses. The relative usefulness of different senses may vary between species, habitats and sexes; yet, how multimodal stimuli are integrated and prioritized is unknown for most taxa. We experimentally assessed foraging preferences of great tits (Parus major) to test whether urban and forest individuals prioritize visual and olfactory cues differently during foraging. We trained 13 wild-caught birds to associate multimodal (colour + odour) cues with a food reward and assessed their foraging preferences in a cue-separation test. In this, the birds could choose between the multimodal training cue and its olfactory or visual components. Our results suggest that the birds did not perceive multimodal cues in an integrated way, as their response was not stronger than for unimodal cue components. Urban birds preferred olfactory cues, while forest birds preferred visual cues. Nevertheless, female birds preferred the multimodal cue, while males foraged more randomly with respect to which cue was present. These findings contribute to our understanding of the relative roles of vision and olfaction in bird foraging behaviour. Future work should focus on how habitat- and sex-specific sensory prioritization modifies bird foraging behaviour and foraging success in the context of urban adaptations across populations.

Birds differentially prioritize visual and olfactory foraging cues depending on habitat of origin and sex

Impacts of land use and land cover changes on local meteorology and PM2.5 concentrations in Changchun, Northeast China

Abstract. Urbanization or rural–urban transformation (RUT) represents one of the most important anthropogenic modifications of land use. To account for the impact of such process on air quality, multiple aspects of how this transformation impacts the air have to be accounted for. Here we present a regional-scale numerical model (regional climate models RegCM and WRF coupled to chemistry transport model CAMx) study for present-day conditions (2015–2016) focusing on a range of central European cities and quantify the individual and combined impact of four potential contributors. Apart from the two most studied impacts, i.e., urban emissions and the urban canopy meteorological forcing (UCMF, i.e., the impact of modified meteorological conditions), we also focus on two less studied contributors to the RUT impact on air quality: the impact of modified dry deposition due to transformed land use and the impact of modified biogenic emissions due to urbanization-induced vegetation modifications and changes in meteorological conditions affecting these emissions. To quantify each of these RUT contributors, we performed a cascade of simulations with CAMx driven with both RegCM and WRF wherein each effect was added one by one while we focused on gas-phase key pollutants: nitrogen, sulfur dioxide (NO2 and SO2), and ozone (O3). The validation of the results using surface observations showed an acceptable match between the modeled and observed annual cycles of monthly pollutant concentrations for NO2 and O3, while some discrepancies in the shape of the annual cycle were identified for some of the cities for SO2, pointing to incorrect representation of the annual emission cycle in the emissions model used. The diurnal cycle of ozone was reasonably captured by the model. We showed with an ensemble of 19 central European cities that the strongest contributors to the impact of RUT on urban air quality are the urban emissions themselves, resulting in increased concentrations for nitrogen (by 5–7 ppbv on average) and sulfur dioxide (by about 0.5–1 ppbv) as well as decreases for ozone (by about 2 ppbv). The other strongest contributor is the urban canopy meteorological forcing, resulting in decreases in primary pollutants (by about 2 ppbv for NO2 and 0.2 ppbv for SO2) and increases in ozone (by about 2 ppbv). Our results showed that they have to be accounted for simultaneously as the impact of urban emissions without considering UCMF can lead to overestimation of the emission impact. Additionally, we quantified two weaker contributors: the effect of modified land use on dry deposition and the effect of modified biogenic emissions. Due to modified dry deposition, summer (winter) NO2 increases (decreases) by 0.05 (0.02) ppbv, while there is almost no average effect for SO2 in summer and a 0.04 ppbv decrease in winter is modeled. The impact on ozone is much stronger and reaches a 1.5 ppbv increase on average. Due to modified biogenic emissions, a negligible effect on SO2 and winter NO2 is modeled, while for summer NO2, an increase by about 0.01 ppbv is calculated. For ozone, we found a much larger decreases of 0.5–1 ppbv. In summary, when analyzing the overall impact of urbanization on air pollution for ozone, the four contributors have the same order of magnitude and none of them should be neglected. For NO2 and SO2, the contributions of land-use-induced modifications of dry deposition and modified biogenic emissions have a smaller effect by at least 1 order of magnitude, and the error will thus be small if they are neglected.


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Impact of urbanization on gas-phase pollutant concentrations: a regional-scale, model-based analysis of the contributing factors

The deadline for submission of abstracts is 01 June 2022. Remember that this low-fee conference is focused on our graduate and early career researchers and will feature daily keynotes, a multiple time-zone friendly format, and will provide significant points of difference from other virtual conferences. Remember also that this is a stepping-stone to our flagship ICUC-11 conference to be held face-to-face in 2023. Please enjoy this current Urban Climate News – David Pearlmutter and his dedicated team have once again provided a newsletter packed with useful and informative material on various urban projects, features of special interest and newsworthy reports. With my very best wishes to you in these turbulent times. I wish to begin this column by briefly reflecting on the tragic situation that continues to unfold in Ukraine as a result of the totally unjust war inflicted on that country. Our IAUC membership includes a number of colleagues from Ukraine and there have been several memorable IAUC activities in that country. I wish to express my concern for them and their relatives in these very difficult times; please feel free to reach out if there is anything that we can do to help you in these difficult circumstances. In a recent column I mentioned the increased focus on urban areas in the current deliberations of the IPCC. This is confirmed with the recent release (February 28 and April 4) of the reports from IPCC Working Groups (WGs) II and III. This is especially the case for the WG II report “Impacts, Adaptation and Vulnerability” where a substantial number of the headline statements in the Summary for Policymakers (SPM) explicitly mention the key vulnerabilities of cities and their important role in addressing issues of climate change. This is impressive justification for our work in the area. For example, some key statements drawn from the WGII SPM include that... “4.3 billion urban people, property and critical infrastructure are increasingly adversely affected by climate change (high confidence). Key impacts include heat stress and flooding, and cascading impacts through supply chains and resource flows, which damage the lives, health and livelihoods of urban residents and connected rural places (high confidence).” And in relation to adaptation...“Urban systems are critical adaptation spaces for advancing the health and well-being of the majority of the world’s population (high confidence). Adaptation of natural, physical and social infrastructure play a critical role in building climate resilient urban systems.” The statement ...“Increasing evidence shows that nature-based solutions in urban areas (e.g., shade trees, natural ecosystems, green roofs) can provide important livelihood options and health benefits and reduce poverty while also supporting mitigation and adaptation (high confidence)”... and the underlying material in the background report draws heavily on the work of our community. I am sure that our membership will find many ways of using material from the current assessment for promoting their work. The August 2022 IAUC Virtual Poster Conference Organising Committee has met several times since the beginning of the year and has developed an very exciting program. I am delighted to announce the opening of the website (iaucposter2022.com) and the call for abstracts https://iaucposter2022.com/submission-and-registration. From the IAUC President

New Research Shows How Health Risks to Children Mount as Temperatures Rise

. Urban areas are hot spots of intense emissions, and they influence air quality not only locally but on a regional or even global scale. The impact of urban emissions over different scales depends on the dilution and chemical transformation of the urban plumes which are governed by the local- and regional-scale meteorological conditions. These are influenced by the presence of urbanized land surface via the so-called urban canopy meteorological forcing (UCMF). In this study, we investigate for selected central European cities (Berlin, Budapest, Munich, Prague, Vienna and Warsaw) how the urban emission impact (UEI) is modulated by the UCMF for present-day climate conditions (2015–2016) using two regional climate models, the regional climate models RegCM and Weather Research and Forecasting model coupled with Chemistry (WRF-Chem; its meteorological part), and two chemistry transport models, Comprehensive Air Quality Model with Extensions (CAMx) coupled to either RegCM and WRF and the “chemical” component of WRF-Chem. The UCMF was calculated by replacing the urbanized surface by a rural one, while the UEI was estimated by removing all anthropogenic emissions from the selected cities. We analyzed the urban-emission-induced changes in near-surface concentrations of NO2 , O3 and PM 2.5 . We found increases in NO2 and PM 2.5 concentrations over cities by 4–6 ppbv and 4–6  µg m−3 , respectively, meaning that about 40 %–60 % and 20 %–40 % of urban concentrations of NO2 and PM 2.5 are caused by local emissions, and the rest is the result of emissions from the surrounding rural areas. We showed that if UCMF is included, the UEI of these pollutants is about 40 %–60 % smaller, or in other words, the urban emission impact is overestimated if urban canopy effects are not taken into account. In case of ozone, models due to UEI usually predict decreases of around −2 to −4  ppbv (about 10 %–20 %), which is again smaller if UCMF is considered (by about 60 %). We further showed that the impact on extreme (95th percentile) air pollution is much stronger, and the modulation of UEI is also larger for such situations. Finally, we evaluated the contribution of the urbanization-induced modifications of vertical eddy diffusion to the modulation of UEI and found that it alone is able to explain the modeled decrease in the urban emission impact if the effects of UCMF are considered. In summary, our results showed that the meteorological changes resulting from urbanization have to be included in regional model studies if they intend to quantify the regional footprint of urban emissions. Ignoring these meteorological changes can lead to the strong overestimation of UEI.

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The regional impact of urban emissions on air quality in Europe: the role of the urban canopy effects

Abstract. Wind-blown dust (WBD) emitted by the Earth’s surface due to sandblasting can potentially have important effects on both climate and human health via interaction with solar and thermal radiation, reducing air quality. Apart from the main dust “centres” around the world, like deserts, dust can be emitted from partly vegetated mid- and high-latitude areas like Europe if certain conditions are suitable (strong winds, bare soil, reduced soil moisture, etc.). Using a wind-blown dust model (WBDUST) along with a chemical transport model (Comprehensive Air-quality model with Extensions, CAMx) coupled to a regional climate model (Weather Research and Forecasting, WRF), this study is one of the first to provide a model-based estimate of such emissions over Europe as well as the long-term impact of WBD emissions on the total particulate matter (PM) concentrations for the 2007–2016 period. We estimated average WBD emissions of about 0.5 and 1.5 Mgyr-1km-2 in fine and coarse modes. Maximum emissions occur over Germany, where the average seasonal fine- and coarse-mode emission flux can reach 0.5 and 1 gs-1km-2, respectively. Large variability is seen in the averaged daily emissions with values of up to 2 gs-1km-2 for the coarse-mode aerosol on selected days. The WBD emissions increased the modelled winter PM2.5 and PM10 concentrations by up to 10 and 20 µg m−3, respectively, especially over Germany, where the highest emissions occur. The impact on other seasons is lower. Much higher impacts are modelled, however, on selected days when occasionally the urban PM2.5 and PM10 concentrations are increased by more than 50 and 100 µg m−3. The comparison with measurements revealed that if WBD is considered, the summer biases are reduced; however, the winter PM is overestimated even more greatly (so the bias increases). We identified a strong overestimation of the modelled wind speed (the maximum daily wind is almost 2 times higher in WRF than the measured ones) suggesting that WBD emissions are also overestimated – hence the enhanced winter PM biases. Moreover, we investigated the secondary impacts of the crustal composition of fine WBD particles on secondary inorganic aerosol (SIA): sulfates (PSO4), nitrates (PNO3) and ammonium (PNH4). Because the water pH value, and thus the uptake of the gaseous precursors of SIA, is perturbed and because the increased aerosol surface serves as an oxidation site, we modelled seasonal PSO4 and PNO3 concentrations increased by up to 0.1 µg m−3 and PNH4 ones decreased by up to −0.05 µg m−3, especially during winter. In terms of average daily impact, these numbers can, however, reach much larger values of up to 1–2 µg m−3 for sulfates and nitrates, while the decrease in ammonium due to WBD can reach −1 µg m−3 on selected days. The sensitivity test on the choice of the inorganic equilibrium model (ISORROPIA vs. EQuilibrium Simplified Aerosol Model V4, EQSAM) showed that if EQSAM is used, the impact on SIA is slightly stronger (by a few 10 %) due to larger number of cations considered for water pH in EQSAM. Our results have to be regarded as a first estimate of the long-term WBD emissions and the related effects on PM over Europe. Due to the strong positive wind bias and hence strong WBD emissions, we should consider these results as an upper bound. More sensitivity studies involving the impact of the driving meteorological fields, WBD model choice and the input data used to describe the land surface need to be carried out in future to better constrain these emissions.


Lukas Bartik

Papers

The regional impact of urban emissions on air quality in Europe: the role of the urban canopy effects

Modelling the European wind-blown dust emissions and their impact on particulate matter (PM) concentrations

Impact of urbanization on gas-phase pollutant concentrations: a regional-scale, model-based analysis of the contributing factors