Variability of physical meteorology in urban areas at different scales: implications for air quality.

Air quality in cities is influenced not only by emissions and chemical transformations but also by the physical state of the atmosphere which varies both temporally and spatially. Increasingly, tall buildings (TB) are common features of the urban landscape, yet their impact on urban air flow and dispersion is not well understood, and their effects are not appropriately captured in parameterisation schemes. Here, hardware models of areas within two global mega-cities (London and Beijing) are used to analyse the impact of TB on flow and transport in isolated and cluster settings. Results show that TB generate strong updrafts and downdrafts that affect street-level flow fields. Velocity differences do not decay monotonically with distance from the TB, especially in the near-wake region where the flow is characterised by recirculating winds and jets. Lateral distance from an isolated TB centreline is crucial, and flow is still strongly impacted at longitudinal distances of several TB heights. Evaluation of a wake-flow scheme (ADMS-Build) in the isolated TB case indicates important characteristics are not captured. There is better agreement for a slender, shorter TB than a taller non-cuboidal TB. Better prediction of flow occurs horizontally further away and vertically further from the surface. TB clusters modify the shape of pollutant plumes. Strong updrafts generated by the overlapping wakes of TB clusters lift pollutants out of the canopy, causing a much deeper tracer plume in the lee of the cluster, and an elevated plume centreline with maximum concentrations around the TB mean height. Enhanced vertical spread of the pollutants in the near-wake of the cluster results in overall lower maximum concentrations, but higher concentrations above the mean TB height. These results have important implications for interpreting observations in areas with TB. Using real world ceilometer observations in two mega-cities (Beijing and Paris), we assess the diurnal seasonal variability of the urban boundary layer and evaluate a mixed layer height (MLH) empirical model with parameters derived from a third mega-city (London). The MLH model works well in central Beijing but less well in suburban Paris. The variability of the physical meteorology across different vertical scales discussed in this paper provides additional context for interpreting air quality observations.

Wood burning: A major source of Volatile Organic Compounds during wintertime in the Paris region.

Wood burning is widely used for domestic heating and has been identified as a ubiquitous pollution source in urban areas, especially during cold months. The present study is based on a three and a half winter months field campaign in the Paris region measuring Volatile Organic Compounds (VOCs) by Proton Transfer Reaction Mass Spectrometry (PTR-MS) in addition to Black Carbon (BC). Several VOCs were identified as strongly wood burning-influenced (e.g., acetic acid, furfural), or traffic-influenced (e.g., toluene, C8-aromatics). Methylbutenone, benzenediol and butandione were identified for the first time as wood burning-related in ambient air. A Positive Matrix Factorization (PMF) analysis highlighted that wood burning is the most important source of VOCs during the winter season. (47%). Traffic was found to account for about 22% of the measured VOCs during the same period, whereas solvent use plus background accounted altogether for the remaining fraction. The comparison with the regional emission inventory showed good consistency for benzene and xylenes but revisions of the inventory should be considered for several VOCs such as acetic acid, C9-aromatics and methanol. Finally, complementary measurements acquired simultaneously at other sites in Île-de-France (the Paris region) enabled evaluation of spatial variabilities. The influence of traffic emissions on investigated pollutants displayed a clear negative gradient from roadside to suburban stations, whereas wood burning pollution was found to be fairly homogeneous over the region.

Projected changes in clear-sky erythemal and vitamin D effective UV doses for Europe over the period 2006 to 2100.

The benefits and the harmful effects of solar ultraviolet radiation (UVR) exposure have been well discussed. Most studies show concern for the solar overexposure in the tropics and low latitude sites and its scarcity at higher latitudes. Both cases are of concern, the first for diseases such as skin cancer and the second for the lack of vitamin D production in the skin. In this study, we evaluate the influence of climate change scenarios on the total ozone content (TOC) and typical aerosol properties, such as the optical depth (AOD) and single scattering albedo (SSA), over Europe. From these parameters, we estimate the erythemal and the vitamin D effective UVR doses from 2006 to 2100. Our results indicate a small reduction of the UVR daily doses caused by the ozone layer recovery and partially compensated by an AOD diminution through this century. The attenuation will be larger at high latitudes, during the springtime and for more polluted scenarios during this century. However, this diminution should not be sufficient to provide a protection condition for erythema. On the other hand, at higher latitudes, it possibly contributes to a relevant increase in the exposure time necessary for the synthesis of vitamin D, mainly during autumn and spring seasons.

Sunscreen use and melanocytic nevi in children: a systematic review.

We conducted a systematic review of the association between melanocytic nevi (MN) in childhood and sunscreen use. A bibliographic search was conducted between November 2008 and January 2009 using the following key words on MEDLINE and EMBASE: child*, in combination with naevi, nevi, naevus, nevus and sunscreen, sun protection. We also used Medical Subject Headings [sunscreening agents], or [radiation protection] with [nevus, pigmented]. A first screening was done on title and abstract reading. Randomized trials and cohort and cross-sectional studies analyzing the relationship between the use of sunscreen and MN in children were selected. Three reviewers abstracted data from each article. The three sets of results were compared for concordance and rereviewed if necessary. Fifteen articles were included (20,743 children). The studies were not consistent in terms of the ages of the children, MN count methods, or sunscreen use assessment. Owing to this heterogeneity, we were unable to pool the studies and conduct a meta-analysis. Twelve studies did not report that the use of sunscreen had a protective effect against MN development. Three studies reported a lower MN count when sunscreen was applied. This systematic review underlines the methodologic differences between studies. Eight of 15 studies reported a positive association between sunscreen application and MN count. Differences in MN counts, overexposure to sun, and inadequate sunscreen application on fair-skinned children could explain the disparity in the results. There is still no evidence of a protective effect of sunscreen against MN development in children.

Comparison between UV index measurements performed by research-grade and consumer-products instruments.

Ultraviolet radiation (UVR) exposure, skin cancer and other related diseases are not just subjects of scientific literature. Nowadays, these themes are also discussed on television, newspapers and magazines for the general public. Consequently, the interest in prevention of sun overexposure is increasing, as the knowledge of photoprotection methods and UVR levels. The ultraviolet index (UVI) is a well-known tool recommended by the World Health Organization to avoid harmful effects of UV sunlight. UVI forecasts are provided by many national meteorological services, but local UVI measurements can provide a more realistic and appropriate evaluation of UVR levels. Indeed, as scientific instruments are very expensive and difficult to manipulate, several manufacturers and retail shops offer cheap and simple non-scientific instruments for UVI measurements, sometimes included in objects of everyday life, such as watches, outfits and hand-held instruments. In this work, we compare measurements provided by several commercial non-scientific instruments with data provided by a Bentham spectrometer, a very accurate sensor used for UV measurements. Results show that only a few of the instruments analyzed provide trustworthy UVI measurements.

Improved retrievals of the optical properties of cirrus clouds by a combination of lidar methods.

We focus on improvement of the retrieval of optical properties of cirrus clouds by combining two lidar methods. We retrieve the cloud's optical depth by using independently the molecular backscattering profile below and above the cloud [molecular integration (MI) method] and the backscattering profile inside the cloud with an a priori effective lidar ratio [particle integration (PI) method]. When the MI method is reliable, the combined MI-PI method allows us to retrieve the optimal effective lidar ratio. We compare these results with Raman lidar retrievals. We then use the derived optimal effective lidar ratio for retrieval with the PI method for situations in which the MI method cannot be applied.