Urban increment

Table 7 shows the urban increment (difference between regional and urban background level for a given city) of PM2.5 for the critical and non-critical areas. Large urban increments have been calculated for Sofia, Milan and northern Italy, Athens and southern Poland. A large urban increment may indicate adverse dispersion conditions and/or high local emission densities. 

PM 10 concentrations and constituents appear systematically higher at urban sites. Urban increments have been measured for most chemical constituents. Nearby (anthropogenic) sources and reduced dispersion in the urbanised areas are the main determining factors here. The observed increment for SIA is caused by more nitrate and sulphate. It is explained by depletion of chloride stabilising part of the nitrate and sulphate in the coarse mode. The question then arises how to assign the coarse mode nitrate (and sulphate) in the mass closure exercise as they replace the chloride. 

The urban increment observed for SIA systematically returns for each of its component (Fig. 3). 

Thorpe et al. [116] proposed the roadside incremental concentration of coarse particles above the urban background as a first estimate of the sum of source strength road dust resuspension and the coarse fraction of wear emissions. Other studies succeeded in separating different traffic emissions by means of multivariate receptor models applied to PM size distribution data ([84, 117]. 

The most simple and widely used spatial increment approach compares concentration levels measured in different environments, assuming that the actual level at a given site is the sum of emissions released on regional, urban, and local scales (cf. Fig. 2). Hence, by calculation of the spatial increments (e.g. traffic-urban background, urban background-rural background) basic assessments of the shares of emissions from the different source areas can be obtained. This approach constitutes the first step within a source apportionment method first proposed by Lenschow et al. [3]. 

The reason why SIA is higher in urban areas is less obvious as these are secondary aerosols. The observed increment is predominantly caused by more nitrate and sulphate. The reaction of nitric acid and sulphuric acid with the sea-salt aerosol in a marine urbanised environment follows an irreversible reaction scheme. In essence, the chloride depletion stabilises part of the nitrate and sulphate in the coarse mode and may partly explain part of the observed increment. However, it also raises the question how to assign the coarse mode nitrate in the mass closure. The sea salt and nitrate contributions cannot simply be added any more as nitrate replaces chloride. Reduction of NOx emissions may cause a reduction of coarse mode nitrate, which is partly compensated by the fact that chloride is not lost anymore. A reduction would yield a net result of ((N03-C1)/N03 = (62-35)/62=) 27/62 times the nitrate reduction (where the numbers are molar weights of the respective components), and this factor could be used to scale back the coarse nitrate fraction in the chemical mass balance. A similar reasoning may be valid for the anthropogenic sulphate in the coarse fraction. Corrections like these are uncommon in current mass closure studies, and consequences will have to be explored in more detail. 

Urban, F.S. Bernstein H. Gultz, Moisture Absorption of 105mm, M67, PropeUing Charge Increment Bags , PicArsn Product Assurance Directorate Rept No ASRSD-AQ-A-P-53-72 (1972) 54) H.L. Sayce, The Effect of Added ... 

TABLE 5.6 Calculated Incremental Reactivities and Kinetic and Mechanistic Reactivities for CO and Selected VOCs for Maximum Ozone Formation Conditions, Based on Scenarios for 12 Urban Areas in the United States... 

In general, the results of comparisons between the trace-element contents of plants grown in urban and rural soils confirm the conclusions already drawn from pot experiments (involving growing plants on soils to which incremental additions of trace elements were made) that boron and zinc are readily taken up by plants, while copper and lead are sparingly taken up at the levels of contamination typical of urban soils. 

Given the history of urban renewal and the national controversy over eminent domain, any attempt to compel those last homeowners, such as the Detroit home-owner in the last occupied home on her desolate block who told a reporter I refuse to move unless the Lord says so (Hackney, 2009), is doomed to failure. While there are undoubtedly many owners who feel trapped in houses without the possibility of sale, and lack the money to move, helping them move is likely to be a slow, incremental process, hindered by the city s chronic shortage of financial resources. No salient research exists that might help cities better manage such strategies. 

The order of the compounds determined by the magnitude of the mass emissions is not intended to reflect the relative influence of these compounds on atmospheric chemistry, greenhouse effect, or other atmospheric phenomena. To illustrate this point, compare in table I-C-5 the orders of importance of the top 31 species in mass emissions in table I-C-2 as estimated from a consideration of criteria other than mass emissions. We list the normalized relative values for molecular emissions (proportional to mass emissions/molecular weight), relative rate of attack by OH radicals (proportional to molecular emissions x oh), the relative rate of ozone generation as estimated for a polluted urban atmosphere (proportional to mass emissions x MIR, the maximum incremental reactivity factor Carter, 1998), and the relative number of CO2 molecules that atmospheric oxidation of this species will ultimately generate (proportional to molecular emissions x number of C-atoms molecule" ). The latter comparison... 

---