An Investigation on the Status of Troposphere NO2 Over Iran During 2004 to 2012

Introduction
NO₂ is not only a potential air pollutant in urban and industrial environments but also a precursor for smog of urban environment that is another ozone pollutant as well. Some of the urban environments in Iran have high tropospheric and near surface NO₂ which requires research attentions especially in these years. That is the issue of concern in the present study using satellite data of OMI from 2004 to 2012. Here, we compare the trends of NO₂ for some cities of Iran while looking at the correlation between the near surface values of NO₂ and that of the tropospheric column values.
 
Material and methods
Spatial and temporal variations of the tropospheric column nitrogen dioxide over Iran are studied using the satellite data of OMI from 2004 to 2012. The surface data of NO₂ for Tehran is also considered.  The data were acquired from the air monitoring stations of the city for this period. Weather synoptic maps were also used for interpretation of the observed NO₂ variations. The monthly mean values of NO₂ for large cities were plotted for comparison; while long term trends for all this period were also presented for the large cities as well as the other less populated places for comparison. The maps of overall variations of column NO₂ for the Middle East were also obtained from the TEMIS database that have its monthly mean values as maps as well as the data for all places over the globe.
The synoptic maps were also obtained from the NOAA web site. This web site has the weather maps for different pressure levels of the globe, for various parameters of meteorology. Although, we only used the maps of 500 mb for the Middle East for the cold season months, as December. As will be observed, the higher values of NO₂ are found in colder months.
 
Results and discussions
Typical monthly averages of NO₂ for larg cities of Iran for 2010 show that depending on the size of the city the monthly average of NO₂ increases with city size. This is particularly so for cold months due to larger tropospheric column NO₂. Figure 1 compares the mean monthly values of NO₂ of tropospheric column with its near surface values for some of the locations in Iran for winter months. It shows a good correlation between these two for nominal winter months.  Such a correlation can be used to estimate the surface NO₂ from that of tropospheric column NO₂ measured by satellite. Larger values of NO₂ for winter months can be due to a lower temperature as well as a less sun shine that affect the NO₂ concentration in the photochemical smog processes over large city environments with large traffics. The long term trends for all large cities are on the rise, as an example Figure 2  shows such a trend for Tehran. The near surface NO₂ over Tehran also show that its concentration is the highest in the northern part of the city. This is mainly due to the anabatic winds due to mountain circulation that can transport air pollutants as NO₂ to the northern parts of Tehran. The valleys in the northern parts of the city from west to east also appears to have substantial concentrations of NO₂ indicating that up-valley winds may suck the polluted air from the urban environment. This process requires further monitoring of near surface circulations.
It is interesting that after a rain of substantial amount (typically larger than 10 mm), sometime (a couple of days) all the pollutants increase and reach the early levels except ozone related to photochemical smog in which NO₂ also have some contributions and increases very shortly after the rain period.
Results show that the maximum values of the tropospheric NO₂ occurs in the cold months including Novembers and Decembers with a typical value of 23.28×10¹⁵ molec/cm²;while its minimum occurs in summer (July) with a typical value of 5.06×10¹⁵ molec/ cm², with an annual mean of  8.7×10¹⁵ molec/cm².
 
Figure 1. Linear correlation between surface NO₂ and the tropospheric column NO₂ for some Iranian locations for November and December 2008-2010
 
Figure 2. Long term trends of monthly mean tropospheric column NO₂  for September for the Geophysics station of Tehran during 2004 and 2012
 
Table 1 shows the average values of NO₂ for some of the large cities of Iran indicating that as the cities get larger the concentrations of NO₂ consequently increase. Tehran as the largest city has the largest values of NO₂.
Table 1. Nine year averages of tropospheric NO₂ for December for some large cities of Iran (10¹⁵ molec/cm²)





station


Tajrish


Shahin shahr


Mashhad


Rasht


Marv dasht


Tabriz


Ahvaz




state


Tehran


Esfahan


Khorasan


Gilan


Fars


Azarbayejan


Khoozestan




NO₂


40.29


22.18


9.66


9.29


8.31


6.42


6.03





 
The synoptic patterns associated with larger concentrations of NO₂ in cold months show a strong ridge over the area in the middle troposphere. During these months, we have observed stronger near surface inversion based on the temperature data on the surface tower. The wind speed for these periods seems to be very low enhancing the levels of air pollutants especially the NO₂. Under such conditions, local circulations play a major role in advection of near surface pollutants towards some areas as for example towards the northern area of Tehran increasing the NO₂ to very large values that was also observed in the satellite data.
 
Conclusion
The monthly mean values of the tropospheric column NO₂ for the large cites of Iran show that its maximum occurred in the cold months; while the minimums were found for the summer months. The spatial concentrations of NO₂ over the area show larger values for large cities in proportion to the size of the cities with the largest concentration for Tehran. The trends of its concentrations for this area show that it is on increase for the large cities as Tehran, Esfahan, and Shiraz with no particular trends for the other places in this area. Also near surface values of the NO₂ for Tehran show that the northeastern part of the city has the largest concentrations indicating that the southeasterly anabatic wind that advects the pollutant towards this area may be responsible for this finding. The synoptic condition associated with high tropospheric column NO₂ concentration is the existence of a high pressure upper air ridge with low near surface winds and stronger stability. Often the near surface value of the NO₂ and the tropospheric column value have the same behavior. They show a good correlation which can be used to estimate the surface values from that of the satellite measurements.