What Factors Affect Concentration Span?

Background concentration (background concertration) refers to the concentration of pollutants caused by environmental winds transported from other regions and from various insignificant small pollution sources in the absence of obvious pollution sources or the discharge of pollutants although there are pollution sources . ^[1]

Background concentration (background concertration) is when there is no obvious

Background concentration is a factor in studying the diffusion of pollutants in the environment, so determining the background concentration (the original concentration in the environment) is also a study

Taking PM2.5 as an example, the research on background concentration mostly uses methods such as mathematical statistics, physical identification, chemical identification, and numerical simulation ^[2] , how to reasonably eliminate the contribution of local pollution sources, and at the same time give an objective evaluation of regional concentration levels, reflecting the The relative background concentration level is the key and difficult point. With the development of air quality numerical models, methods combining observational data and numerical simulation techniques have been developed, which usually include the following steps ^[6] :

Background concentration background time screening estimates

The background time screened by specific conditions should be representative in terms of wind speed and weather system identification. The concentration monitoring results of the corresponding time period can characterize the approximate level of the background concentration of the pollutant. The monitoring data of the physical monitoring station are matched one by one, and the statistical characteristics of the measured values and the background value range are analyzed. The method of cutting average is used to exclude the impact of higher and lower monitoring extremes, and preliminary estimates of different positions in a certain area Contaminant background concentration of the site. ^[6]

Background concentration mode verification and estimation

The scenario analysis method is used to simulate the background concentration of pollutants. The steps are as follows ^[6] :

Using the air quality model CMAQ to simulate the pollution pollution distribution in a complete year when all sources in the area and surrounding areas are not closed;

Using CMAQ to simulate the distribution of pollutant pollution in an entire year when anthropogenic sources in the region are closed, and the results represent the relative distribution of background concentration or the contribution of natural sources and surrounding sources in the region;

Calculate the ratio of the simulation results of step 2 to the simulation results of step 1 of all grids in the area to obtain the contribution rate of the surrounding sources of different grids;

Use the contribution rate calculated in step 3 to multiply the measured values at different monitoring points to obtain the absolute background density and its spatial distribution.

This method is widely used in calculating the contribution from sources outside a certain area, but it is less applied to simulate the background concentration in a certain area. ^[7]