Search

Despite the implications of winter precipitation for socioeconomic activities and transportation services, the influence of cities on winter precipitation is less studied compared to that on summer precipitation. Here we investigated the statistical relations between precipitation, temperature, and impervious surface fraction in 12 major cities across the contiguous United States. The results showed negative correlations between snowfall intensity and impervious surface fraction. The correlations depend on latitude and the distance to complex terrain features (water bodies or topography), with stronger correlations for inland cities than coastal/lakeside cities. We further selected Kansas City for modeling analyses based on the Weather Research and Forecasting model. Simulation results indicated that the heating effect of urban land occurs in the near-surface atmosphere during the precipitation period, leading to changes of different hydrometers and an overall tendency of reducing snowfall but increasing rainfall. • Statistical analyses quantifying the influence of urbanization on winter precipitation across 12 U.S. cities over 10 years. • Modeling simulation elucidating the physical processes through which urban land influences winter precipitation. • Snow only, rain only and mixed precipitation eventsand a season-long simulation conducted over Kansas City. • The urban-induced temperature results in reduced snowfall and increased rainfall especially during mixed-precipitation event. [ABSTRACT FROM AUTHOR]

Urban areas are vulnerable to climate and weather extremes, including severe convective events. However, large cities can themselves intensify such events, although the magnitude of this intensification remains uncertain. In this study, we quantify the influence of Moscow on severe convective events by using simulations with the convection-permitting COSMO-CLM model with 1-km resolution for multiple summers for 2007–2016 and analyzing the difference between simulations with and without the urban canopy parameterization (URB and noURB, respectively). We found a significant difference between these simulations for extreme precipitation and wind characteristics. Within the city boundary, the difference is about 20% for 30 mm daily precipitation events and 70% for 50 mm events. Strong wind events are twice as frequent in the URB simulation. The urban canopy significantly increases the magnitude and variability of extreme quantiles for vertical velocities. We found more than a 50% increase in compound events with both wind and precipitation exceeding their 0.99 quantiles and in events with the 2–5 km updraft helicity exceeding critical values, which serve as a good proxy for mesocyclone formation. Our findings may help to refine the population bias and can be used to specify the predicted changes in convective hazards in mid-latitude cities. • High-resolution simulations show significant impact of Moscow on convective events. • Intense daily precipitation increases over Moscow and downwind northeastern areas. • The city leads to a 50% increase in compound extreme wind and precipitation events. • Moscow significantly raises updraft helicity and vertical velocity in the city limits. • Population bias in convective reports for large cities is likely smaller than thought. [ABSTRACT FROM AUTHOR]

The question of how much cities are the sources of their own air pollution is not only theoretical as it is critical to the design of effective strategies for urban air quality planning. In this work, we assess the validity of the commonly used incremental approach to estimate the likely impact of cities on their air pollution. With the incremental approach, the city impact (i.e. the concentration change generated by the city emissions) is estimated as the concentration difference between a rural background and an urban background location, also known as the urban increment. We show that the city impact is in reality made up of the urban increment and two additional components and consequently two assumptions need to be fulfilled for the urban increment to be representative of the urban impact. The first assumption is that the rural background location is not influenced by emissions from within the city whereas the second requires that background concentration levels, obtained with zero city emissions, are equal at both locations. Because the urban impact is not measurable, the SHERPA modelling approach, based on a full air quality modelling system, is used in this work to assess the validity of these assumptions for some European cities. Results indicate that for PM 2.5 , these two assumptions are far from being fulfilled for many large or medium city sizes. For this type of cities, urban increments are largely underestimating city impacts. Although results are in better agreement for NO 2 , similar issues are met. In many situations the incremental approach is therefore not an adequate estimate of the urban impact on air pollution. This poses issues in terms of interpretation when these increments are used to define strategic options in terms of air quality planning. We finally illustrate the interest of comparing modelled and measured increments to improve our confidence in the model results. [ABSTRACT FROM AUTHOR]

In order to assess the role of urea in the Gulf of Trieste, oceanographic data collected from 2002 to 2011 were analyzed together with ancillary ambient information and compared to past studies. The recent levels of urea found in these coastal waters (median = 1.1 µM N, maximum value = 19.7 µM N) are often high and similar to those reported in the early 1980s. A preliminary estimate of the external inputs indicated that this enrichment in urea is mainly due to emissions from urban sewage systems, whereas the contributions of rivers and atmospheric deposition are scarce. As a consequence, urea appears to be a reliable tracer of the diffusion of wastewaters in the coastal marine environment, more specific and sensitive than other nutrients, with a behavior that also reflects the technology of the treatment plants. The stability of urea levels over the last three decades suggests that the upgrade of wastewater treatment technologies was probably balanced by the concomitant increase of the anthropogenic pressure in the area (477,000 to 1,300,000 inhabitant equivalent). Budget estimates on the gulf-wide scale indicate that urea (177-530 t N) is not negligible compared to dissolved inorganic nitrogen (409-919 t N) and that it can constitute up to 56% of the nitrogen available for plankton growth. A large accumulation of urea can occur during summer periods characterized by stable weather conditions and weak circulation, whereas a biologically mediated degradation to ammonium is observed in autumn in concomitance to a strong shift of the marine ecosystem toward heterotrophic conditions. These processes, together with a potential competition between phytoplankton and bacteria for the utilization of this nitrogen form, suggest that the biogeochemical role of urea should be better investigated in mid-latitude coastal zones subjected to highly variable ambient conditions and to overloads of this compound. [ABSTRACT FROM AUTHOR]

PM10 levels of the mineral components Si, Al, Fe, Ca, Mg and some trace metals were measured at three different sites in the urban area of Vienna (Austria). Observed trace metal concentrations varied between less than 0.1ngm−3 (Cd) and approximately 200ngm−3 (Zn), mineral components showed enhanced concentrations ranging from 0.01μgm−3 (Ca) to 16.3μgm−3 (Si). The contribution of the respective mineral oxides to PM10 mass concentrations accounted on average for 26.4±16% (n=1090) of the PM10 mass, with enhanced rates in spring and autumn (monthly averages of up to 40%) and decreased contributions in the cold season (monthly averages below 10%). The atmospheric occurrence of Al, Ti and Sr could be assigned to crustal sources, whereas for the elements Ba, Ca, Fe, Mg, Mn and V an increased contribution of non-crustal origin was observed. PM10 levels of As, Cd, Co, Cr, Cu, Ni, Pb, Sb, Sn and Zn were predominantly derived from man-made emissions. Intersite comparison indicated that urban PM10 mass concentrations and PM10 levels of As, Pb and Zn were predominantly influenced from the transport of aerosols from outside into the city, whereas for the elements Ba, Mg, Ca, Cu and Fe a distinctly increased impact of local emissions was observed. The contribution of these urban emissions to total PM10 concentrations was estimated by calculating the so-called “urban impact”, which was found to be 32.7±18% (n=392) in the case of PM10 mass concentrations. The investigated elements accounted on average for 31.3±19% (n=392) of the observed PM10 mass increase. The mean values for the “urban impacts” of individual elements varied between 25.5% (As) and 77.0% (Ba). [Copyright &y& Elsevier]

The measurements of PM2.5 and PM10 at two sites—an urban site in Vienna (AUPHEP-1) and a rural site considered local background (AUPHEP-2)—indicated only low aerosol generation activity in the city on an annual basis. Defining the term “urban impact” as the difference between observations at the urban and the local background site we find an annually averaged urban impact for PM2.5 of 3.4 μg m−3 and for PMC of 3.3 μg m−3 (the coarse fraction PMC=PM10−PM2.5). The relative increase of the particulate matter (PM) concentration at the urban site compared to the background site (AUPHEP-2) is annually averaged only 19% for PM2.5, but 60% for PMC. The chemical main constituents of the PM2.5 urban impact are black carbon (BC), organic carbon (OC), and sulfate; the main constituents of the PMC urban impact are OC and indicators for mineralic aerosol (Fe, Ca, Mg, Na, K). The BC/TC ratio of the PM2.5 urban impact is typical as for combustion sources, e.g. automotive traffic, oil or coal combustion. Urban coarse OC is considered to originate from non-pyrogenic sources. From the trace metals investigated (As, Cd, Co, Cu, Cr, Mn, Ni, Pb, V, Zn) only Cd, Ni, Pb, and Zn exhibited a slight cold season enrichment in the urban airshed. From the weak signal of a seasonality of oil or coal combustion indicators we conclude that local domestic heating sources are using “clean fuels”. [Copyright &y& Elsevier]