Averaging time, pollutant concentration

The averaging time of the rapid-response record [Fig. 4-1 (a)] is an inherent characteristic of the instrument and the data acquisition system. It can become almost an instantaneous record of concentration at the receptor. However, in most cases this is not desirable, because such an instantaneous record cannot be put to any practical air pollution control use. What such a record reveals is something of the turbulent structure of the atmosphere, and thus it has some utility in meteorological research. In communications... 

A very useful format in which to display air quality data for analysis is that of Fig. 4-8, which has as its abscissa averaging time expressed in two different time units and, as its ordinate, concentration of the pollutant at the receptor. This type of chart is called an arrowhead chart and includes enough information to characterize fully the variability of concentration at the receptor. 

In general, air quality data are classified as a function of time, location, and magnitude. Several statistical parameters may be used to characterize a group of air pollution concentrations, including the arithmetic mean, the median, and the geometric mean. These parameters may be determined over averaging times of up to 1 year. In addition to these three parameters, a measure of the variability of a data set, such as the standard deviation... 

In Northern Europe the concentrations of particulate matter are typically quite small compared with the more polluted regions in Europe [5, 6]. In Northern Europe annual average PM2.5 concentrations are 10 pg m-3 in urban areas, whereas in Eastern and Southern Europe the concentrations are two or three times larger [7]. In areas with low local emissions the composition and the mass concentrations of fine particles are substantially affected by the long-range transport (LRT) from other areas, even from thousands of kilometers away. For example in the urban areas of Helsinki, Finland, 64—76% of the PM2 5 mass was estimated to originate from the LRT in 1998-2002 [8]. Regarding special pollution episodes, a total of 27 pollution episodes were attributed to LRT in Southern Finland in 2001-2007 [9]. Half of those episodes was caused by smokes from wildfires in Eastern Europe, and the total duration of the episodes was 26 days in 7 years time period. 

A composite sample consists of a series of smaller samples collected at regular intervals over a period of time and deposited in the same container. To some extent, composite samples represent the average characteristics of the source during the sampling period—they are useful in determining average pollutant concentrations or pollutant loads. The laboratory analysis of such samples is more cost-effective but generally yields less information. 

The results of a 1987 field test of vehicle emissions in Los Angeles, in which measurements of pollutant concentrations in the air were used to compute actual on-the-road vehicle emissions, surprised the technical community (Pierson et al., 1990). Concentrations of CO and HCs averaged 2.7 and 3.8 times higher, respectively, than predicted by the emission inventory models that are the basis for predictions of mobile source emissions. More recent studies show that the measured values of HCs and CO are about 2 times the predicted concentrations. The remote-sensing and road side data show that about 50% of the CO and HC emissions come from 10% of the vehicles. What was not expected were the high emission rates detected in the worst 20% of more recent model cars (Calvert et al., 1963). 

Threshold level Time-weighted average pollutant concentration values, exposure beyond which is likely to adversely affect human health. See Environmental exposure)... 

Flux measurements to collector surfaces demonstrated that removal rates can be very rapid. In Table I, characteristic times have been calculated for deposition during dense fog. These values were determined from the total solute fluxes, mixing heights, and average pollutant concentrations measured during the individual events. The removal times were calculated to be 6 to 12 h for these periods with the exception of N(V) in non-acidic fogs. Between the occurrences of fog, aerosol deposition was substantially reduced ... 

Pollutant Fluxes. Hourly deposition velocities were multiplied by hourly pollutant concentrations to get hourly pollutant fluxes. These were summed over exposure periods for hours of wetness with different critical relative humidity criteria (75 to 90% in 5% intervals). Average fluxes were then calculated by dividing by the time-of-wetness. The results were compared with fluxes calculated by multiplying average deposition velocities for a period by the average pollutant concentration during times of wetness. The values by the two methods were fairly consistent. 

Some operating permits may be written on a not-to-exceed basis, which means that a given pollutant concentration cannot exceed a given value. In that case, only the maximum value over a given time period may be of interest. Some permits are written so that the average concentration cannot exceed a specific value, so it is important to know how the permit is written. 

Dulal P, Sinha DK (1992) A dispersion model on air pollution chemical kinetic approach for conversion of pollutants. Int J Environ Stud 40(l) 55-66 Larsen RI (1969) A new mathematical model of air pollutant concentration averaging time and frequency. JAPACA 19(Nl) 24-30... 

Millions of tons of soot, dust, and smoke particles are emitted into the atmosphere of the United States each year. The average suspended particulate concentrations in the United States vary from about 0.00001 g/m of air in rural areas to about six times as much in urban locations. In heavily polluted areas, concentrations of particulates may increase to 0.002 g/m . 

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