Deviance explained by variables related to PM_2.5 concentrations in Xi'an.

<p>Deviance explained by variables related to PM_2.5 concentrations in Xi'an.</p

Scatter plots between PM_2.5 concentration (μg/m³) and the variables gas-phase concentrations (SO₂, NO₂, CO, and O₃, μg/m³), AOT values, and Temperature (°C).

<p>Scatter plots between PM_2.5 concentration (μg/m³) and the variables gas-phase concentrations (SO₂, NO₂, CO, and O₃, μg/m³), AOT values, and Temperature (°C).</p

Evaluations of the model: (a) histogram of residuals and (b) fitted PM_2.5 concentrations vs. observations.

<p>Evaluations of the model: (a) histogram of residuals and (b) fitted PM_2.5 concentrations vs. observations.</p

The location of Xi'an City in China and the distribution of 13 observation stations.

<p>The location of Xi'an City in China and the distribution of 13 observation stations.</p

Number of days with the same wind directions in each season (a. spring; b. summer; c. autumn; d. winter) and the whole year (e.).

<p>Number of days with the same wind directions in each season (a. spring; b. summer; c. autumn; d. winter) and the whole year (e.).</p

Schematic overview of the estimation experiment.

<p>Schematic overview of the estimation experiment.</p

Bivariate smoothing functions of fitted components for log(PM_2.5) in the generalized additive model.

<p>(<b>a</b>) Bivariate smoothing function for the wind components and the corresponding fitted values. (<b>b</b>) Bivariate smoothing function for longitude and latitude and the corresponding fitted values. The black dots represent the 13 observation stations that monitor PM_2.5 concentrations.</p

Spatial distribution of annual mean AOT in 2013 for Xi'an and its surrounding cities.

<p>Spatial distribution of annual mean AOT in 2013 for Xi'an and its surrounding cities.</p

Categories of air quality and corresponding PM_2.5 concentrations.

<p>Categories of air quality and corresponding PM_2.5 concentrations.</p

Summary of data (except for wind direction) used to model the PM_2.5 concentrations.

<p>Summary of data (except for wind direction) used to model the PM_2.5 concentrations.</p