Introduction

One of major problems in large and industrial cities, such as Arak, Iran, is reduction of air quality. Variations in the Boundary Level Height (BLH) have a significant impact on air quality. The height and thickness of boundary layer changes at different hours of day and night and also on different days of year. Daily variations in temperature, humidity, wind, pollution and contamination, and turbulence and thickness of the boundary layer occur due to daily warming and nocturnal cooling of the Earth's surface, and a very stable layer with a temperature inversion always plays the role of the boundary layer cap. The present study aims at examining the variations in BLH different times of day over the months and seasons of the statistical term, and the effect of these changes on the reduction or increase of pollutants in Arak city.

Methodology

Arak boundary layer daily and monthly data for the Different hours with a spatial resolution of 0.125degree arc from 1979 until the end of 2018, was collected from European Centre for Medium-Range Weather Forecasts. Variations in the Arak BLH over the statistical term were assessed and estimated using linear regression, non-linear regression, and Mann-Kendall statistical test. The air pollutant raw data of three air pollution stations across Arak city were obtained from General Directorate for Environmental Protection of the Markazi Province. After collecting the preliminary data, the air quality index (AQI) relation was calculated for five major air pollutants, namely suspended particulate matter, nitrogen dioxide, surface ozone, carbon monoxide, and sulfur dioxide, and then the responsible pollutant, i.e. the pollutant with the most AQI of other pollutants, was identified. To determine the influence of BLH variations on air quality, Pearson correlation test was administered between the BLH data and the five major air pollutant data.



Results and discussion

The results showed that the BLH has an increasing trend in spring, summer and winter at all hours and it does not vary significantly in the autumn. Significant variations in the BLH of Arak city are different at night time and during daylight hours. In spring, on average the BLH increases by 9%, the highest increase being 18h. In this season, the BLH increases more at night than in daylight hours. In summer, on average the BLH increases by 6%, with the highest increase at 24h and the lowest increase at 12h. In this season, The BLH increases more at night than at daylight. The highest BLH occurs in winter, with an average increase of 34%, the highest being at 15h with an increase of 48%. Unlike both spring and summer, in winter the BLH increases more during the day than at night both. Overall, the BLH of the fourth decade increases by 9% in proportion to the average BLH of the whole yearly term, with the highest increase being 18% at 24h. To determine whether fossil fuels increase and land use and land cover changes of Arak city increase the BLH or whether global warming and macro-scale climate change increases the BLH of the cities including Arak, the BLH of four other points, besides those of Arak city center, were examined. The increase in Arak BLH is not much different from that of the rest, that is during the forty-year statistical term, the BLH increases to the same extent for both Arak city and its surrounding lands including the plain and agricultural areas, the mountainous areas, and the suburb. Therefore, it can be inferred that climate change and global warming have a greater impact on variations in the BLH.As stated in the data and methodology sections, the air quality index (AQI) relation was calculated for the five major air pollutants, namely particulate matter, nitrogen dioxide, surface ozone, carbon monoxide, and sulfur dioxide; The pollutants responsible for each days were determined for each station and particulate matter was found to be the pollutant responsible for most days at all stations in the Arak city.



Conclusion

The Arak BLH increases significantly in all seasons except autumn. This increase was not the same during daylight hours and months of the year, with the highest increase being related to winter, and the autumn of no significant change. The Arak BLH variations are in line with temperature changes in Arak city and it can be judged that the reason behind the Arak BLH increase is the increase in temperature of this city in the wake of global warming. The fossil fuel consumption increase and land use change of Arak have a minor effect on the city BLH.

The pollutant responsible in Arak city is particulate matter most of the days, originating in automotive fuel, industries in and around the city, and dust particles. In the seasons when the dust masses do not enter the city, the BLH is negatively correlated with the suspended particles and on the days when the dust masses enter the city, the BLH relationship is positive and significant.

The BLH increase in Arak city does not help much to reduce the concentration of pollutants in Arak city, as the BLH increase in some seasons is positively associated with ozone increase and particulate matter, and the mean of BLH is highly lower in winter, autumn, and other season’s nights so that it can naturally become polluted in a short time. As Arak is growing, it is increasingly being polluted and climate change is also occurring, and as urban adjustment and adaptation was repeatedly underlined in the Fifth Intergovernmental Panel on Climate Change. it is expected that the officials and policymakers develop and implement the urban development plans in accordance with natural and climatic features, and with adjustment and adaptation to climate change. The results showed that the correlation of the BLH with carbon monoxide and sulfur dioxide is negative, and with ozone and the suspended particles 2.5 and 10 microns is positive



Keywords: Boundary Layer Height (BLH), Air quality, linear and non-linear regression, Arak city.