.1 Diurnal and Monthly variation of O3 , NO, NO2, NOx, OX and CO:
The hourly averaged concentrations of O3, NO, NO2, NOx, OX and CO at all the five study locations i.e. IMD Lodi Road, IGI Airport Palam, NCMRWF Noida, CRRI Mathura road and CV Raman Dheerpur from 2013 to 2019 are shown in Fig. 3. Mean hourly values of concentrations of different parameters along with standard deviation, maximum and minimum values at all the sites are given in Table 1. During the study period, the hourly averaged concentration of surface O3, NO, NO2, CO, NOx (NO + NO2) and OX(NO2 + O3) for all five locations have been found in the range from 32.44 ppb to 36.57 ppb, 19.46 to 28.09 ppb,20.83 to 26.89 ppb,1.67 to 1.89 ppm,43.04 to 54.99 ppb and 54.06 to 60.99 ppb respectively. Typically, the concentration of surface O3 starts increasing gradually after sunrise (between 0630-0700h) attains a maximum around noontime (between 1300- 1500h), and starts declining after this peak. (Fig. 3) The high concentration between 13:00h and 15:00h implies photochemical production of surface O3 and transport of photochemical build-up of surface O3 to the measuring site. [20] The decrease in night-time O3 concentrations is mainly due to titration of O3 by surface emission of NO and ground-level destruction of O3 in a shallow boundary layer. The dry deposition directly onto the earth’s surface can significantly reduce the level of O3. In highly polluted regions, where the level of NO is high, the loss of O3 may occur due to the following titration reaction:
Table 1
Hourly maximum, minimum, and average values of NO, NO2, NOx, O3, and OX during 2013–2019.
STATIONS
|
(ppb)
|
NO
|
NO2
|
O3
|
NOx(NO + NO2 )
|
OX(NO2 + O3)
|
CO(ppm)
|
IMD LODI ROAD
|
Average
|
26.05 ± 13.4
|
20.83 ± 7.5
|
33.22 ± 15.08
|
46.88 ± 17.4
|
54.06 ± 15.9
|
1.91 ± 0.5
|
|
Minimum
|
7.83
|
10.84
|
14.11
|
19.62
|
30.25
|
0.87
|
|
Maximum
|
58.77
|
39.67
|
70.37
|
98.12
|
85.89
|
3.10
|
IGI AIRPORT PALAM
|
Average
|
20.0 ± 10
|
23.44 ± 9.7
|
32.47 ± 11.7
|
43.45 ± 19.6
|
55.91 ± 13.07
|
1.74 ± 0.8
|
|
Minimum
|
10.40
|
7.73
|
14.71
|
19.62
|
33.39
|
0.85
|
|
Maximum
|
50.13
|
41.84
|
62.56
|
82.86
|
96.92
|
2.90
|
CV RAMAN DHEERPUR
|
Average
|
19.46 ± 7.4
|
23.58 ± 8.3
|
36.55 ± 9.19.2
|
43.04 ± 14.9
|
60.13 ± 11.6
|
1.72 ± 0.6
|
|
Minimum
|
9.36
|
11.61
|
22.01
|
23.19
|
41.53
|
0.96
|
|
Maximum
|
36.38
|
36.84
|
59.15
|
70.33
|
84.88
|
3.22
|
CRRI MATHURA ROAD
|
Average
|
28.09 ± 15.38
|
26.89 ± 9.7
|
34.09 ± 8.79
|
54.99 ± 22.7
|
60.99 ± 13.69
|
1.67 ± 0.41
|
|
Minimum
|
10.84
|
13.06
|
17.53
|
25.18
|
31.12
|
0.84
|
|
Maximum
|
57.83
|
43.12
|
52.97
|
100.95
|
87.30
|
2.76
|
NCMRWF NOIDA
|
Average
|
20.58 ± 6.5
|
21.3 ± 7.14
|
33.28 ± 8.73
|
41.88 ± 10.64
|
54.58 ± 12.5
|
1.83 ± 0.63
|
|
Minimum
|
8.70
|
10.57
|
19.06
|
24.37
|
32.22
|
0.76
|
|
Maximum
|
36.09
|
36.42
|
59.11
|
58.13
|
85.99
|
3.20
|
*± Standard Deviation
|
|
|
|
|
|
|
|
O3 + NO → NO2 + O2 Eq. (1)
Both NOx and CO have shown build-up during the morning (0700–0900 h) and late evening/night hours (1900–2400 h) during the study period which is different from the variations in ozone (Fig. 3). Higher levels of NOx and CO during morning and late evening hours at all the sites used in this study are due to the combinations of anthropogenic emissions, boundary layer processes, chemistry as well as local surface wind patterns. During night hours, the boundary layer descends and remains low till early morning, thereby resisting the mixing of the anthropogenic emissions with the upper layer. Hence, pollutants get trapped due to shallow nocturnal boundary layer depth resisting the mixing of local emissions with the free tropospheric air. It is important to note that the major anthropogenic source for CO and NOx in an urban region like New Delhi (mainly NO) is fossil fuel burning (combustion in motor vehicles).[21] Fig. 4 shows the monthly variation of NO, NO2, O3, and CO at all the monitoring stations during the study period. The concentrations of NOx have been found highest in post-monsoon months (October, November and December) and the lowest value in monsoon months (July, August and September). The concentrations of CO have been found highest in winter months (December, January and February) and lowest value in monsoon months (July, August and September) (Table 2 and Table 3). This seasonal pattern may be due to a combined effect of large near-surface anthropogenic emissions, boundary layer processes, retarded photochemical loss owing to lower solar intensity, as well as local surface wind patterns. In contrast, O3 peaked during the summer months (March, April, May and June) (Fig. 4, Table 2 and Table 3), clearly due to its direct linear relationship with incoming solar radiation.
Table 2
Monthly average Concentrations of NO, NO2, O3 and CO in Delhi.
Monthly Average
|
|
|
|
|
Months
|
NO
|
NO2
|
O3
|
CO
|
January
|
25.53
|
20.16
|
25.91
|
2.07
|
February
|
19.42
|
20.67
|
29.64
|
1.81
|
March
|
21.92
|
25.50
|
33.50
|
1.95
|
April
|
23.89
|
23.86
|
39.80
|
1.81
|
May
|
22.56
|
26.58
|
41.53
|
1.71
|
June
|
20.47
|
24.37
|
40.24
|
1.56
|
July
|
20.06
|
23.05
|
35.16
|
1.81
|
August
|
21.74
|
22.19
|
28.68
|
1.52
|
September
|
19.86
|
22.48
|
31.91
|
1.35
|
October
|
23.13
|
23.25
|
38.70
|
1.76
|
November
|
27.75
|
22.99
|
33.52
|
2.08
|
December
|
28.73
|
26.84
|
31.47
|
1.87
|
New Delhi(5 Station avg*)
|
|
|
|
|
Table 3
Average seasonal variation of NO, NO2, NOx, O3 and CO in Delhi.
Seasons
|
NO
|
NO2
|
NOx
|
O3
|
CO
|
Winter Season
|
24.56
|
22.56
|
47.12
|
29.01
|
1.92
|
Summer Season
|
22.21
|
25.08
|
47.29
|
38.77
|
1.76
|
Monsoon Season
|
20.55
|
22.57
|
43.13
|
31.92
|
1.56
|
Post Monsoon Season
|
26.54
|
24.36
|
50.90
|
34.57
|
1.90
|
The high solar radiation intensity (i.e. temperature) has a direct influence on chemical kinetic rates and the mechanism pathways for the O3 production. [22] The photoxidation of CO, CH4 and NMHCs in presence of a sufficient amount of NOx and sunlight during the summer months (March-June) leads to the formation of O3. The concentrations of O3 showed the highest values in the summer season and the lowest value in the winter season (Table 3). O3 concentrations were comparable to those reported at other urban locations in India, [18, 21, 23–26) and higher compared to rural and high altitude sites in India. [27, 28]
.2 Variation in rate of change of Surface O3 at the Urban site:
Figure 5 shows the variation of hourly averaged rate of change of O3 concentrations (d [O3]/dt) at all the five sites used in this study during the study period. The rate of change of O3 can be used as an indicator of urban and rural chemical environments. [29] Urban environments commonly show similar morning and evening rates of change in O3, while rural sites are characterized by asymmetric diurnal patterns i.e. higher build-up rates in the morning and lower loss rates in the evening hours.[29] This can be because, in morning time O3 formation is strongly dependent on the available amount of precursors emitted from morning vehicular traffic (0700-1100h) and sudden change in boundary layer height with the sunrise, while the evening time loss rate (1700-2200h) largely depends on nitrous oxide (NO) (conversion of NO2 to NO in the evening) concentration which participates in O3 titration processes Eq. (1). The average rates of change of ozone (d [O3]/dt) during morning hours (0800h-1100h) and in the evening and late evening hours (1700-1900h) at Delhi (5 station average in our study) have been estimated at 4.16 ppbh− 1 and − 4.48 ppbh− 1, respectively (Table 4). The mean rate of change of O3 at all the sites used in the present study was found similar to that of other urban locations like Agra, Kanpur, Ahmedabad, Pune and also at an urban site in New Delhi India. [15, 30–34]The night-time rate of change was found almost steady and slightly negative, perhaps due to O3 loss to surface deposition and also due to fast titration of O3 in the evening at these experimental sites.
Table 4
Observed rate of change of Surface Ozone (dO3/dt) in different cities of India.
Site
|
Type of Site
|
Rate of change of O3 (ppb/h)
|
References
|
|
|
(0800h-1100h)
|
(1700-1900h)
|
|
Delhi*
|
Urban
|
4.2
|
-4.48
|
(This study)*
|
Delhi
|
Urban
|
4.7
|
-5.5
|
[33]
|
Delhi
|
Urban
|
4.5
|
−5.3
|
[32]
|
Kanpur
|
Urban
|
3.3
|
−2.6
|
[30]
|
Agra
|
Urban
|
2.5
|
−2.4
|
[15]
|
Ahmedabad
|
Urban
|
5.9
|
−6.4
|
[31]
|
Pune
|
Urban
|
4.8
|
−2.6
|
[34]
|
Anantapur
|
Rural
|
4.6
|
−2.5
|
[27]
|
Gadanki
|
Rural
|
4.6
|
−2.6
|
[29]
|
Thumba
|
Coastal (rural)
|
5.5
|
−1.4
|
[35]
|
Kannur
|
Coastal (rural)
|
4.9
|
−6.4
|
[36]
|
Dayalbagh
|
Rural
|
2.2
|
-2.3
|
[15]
|
Pantnagar
|
Hilly
|
5.6
|
-8.5
|
[37]
|
Mohal
|
Hilly
|
7.3
|
-5.9
|
[38]
|
* 5 station avg.
|
|
|
|
|
3.3 Chemical coupling of O3 and its precursors:
Leighton [39] has demonstrated the fact that in the troposphere the photochemical inter-conversion of O3, NO and NO2 is generally controlled by the following reactions:
NO2 + hλ→ NO + O* Eq. (2)
O* + O2 + M →O3 + M Eq. (3)
NO + O3 →NO2 + O2 Eq. (4)
Where M (usually N2 or O2) represents a molecule that absorbs the excess vibrational energy and thereby stabilizes the O3 molecule formed, hλ represents the energy of a photon (with wavelength λ = 424 mm) and O* is the active mono-atomic oxygen. The above reactions constitute a reversible cycle i.e. the overall effect of Eqs. (2) and (3) are exactly opposite and canceled by the effect of Eq. 4. It represents a null cycle where there is no net production of O3. [40, 41] Therefore, these reactions as described in Leighton [40] represents a closed energy system in which oxides of nitrogen (NOx) is partitioned between its constituents, NO and NO2 (primary NO2), and oxidant (OX) is partitioned between its constituents NO2 and O3.This state is also defined as a photo stationary state (PSS). During daylight hours (as these reactions are driven by sunlight) NO, NO2 and O3 are at an equilibrium state for a few minutes. [40, 41] Hence the concentrations of the above species during PSS can be defined by the following equation: [39]
Where j1 is the NO2 photolysis rate and k3 is the rate coefficient for the reaction between NO and O3, according to Eq. (4). j1 depends on the intensity of solar radiation. Rate coefficient k3 for Eq. (4) is temperature-dependent. [55–57] It can be computed using the following equation: [42]
k3 (ppm− 1 min− 1) = 3:23*103 exp (−1430/T) Eq. (6)
Figure 6 shows the diurnally averaged values of rate constant k3 (ppm− 1min− 1) with an ambient air temperature of IMD Lodi Road station for the year 2019. As discussed earlier, k3 is directly proportional to temperature; therefore the maximum value of k3 coincides with the value of ambient air temperature (°C) during the noon period. The values of k3 rise after 0700 h in the morning as the sun rises, and attains its maximum value of 27.37 ppm− 1min− 1 at around 1400 h when the ambient air temperature is maximum,i.e. 26.74°C. The rate of photolysis j1 for IMD Lodi Road station is calculated using the value of obtained rate constant k3 and the hourly averaged concentrations (in ppm) of NO, NO2, and O3. The value was calculated in the range of 0.58 min− 1 to 1.87 min− 1 with an average value of 1.02 min− 1. Figure 7 describes the relationship between O3, NO, NO2 with NOx as described in the Eqs. (3) and (4). The figure shows the variations in the daylight(the period of sunlight) concentrations of NO, NO2 and O3 versus NOx .The daylight concentrations were determined using the sunrise and sunset values obtained from the US astronomical site (http://aa.usno.navy.mil/data/docs/RS_OneDay.php), which generally varied between 0500–1900h during summers and 0630–1800h during winters. [40, 41] Polynomial fit curves for NO, NO2 and O3 were drawn that adequately described the interaction between the three species.
-
As seen in Fig. 7, it can be observed that surface O3 concentrations decrease with increasing NOx concentrations while NO and NO2 concentrations increase with increasing NOx concentrations for all the stations used in this study.
-
[NO] concentrations dominate as compared to [NO2] and [O3] concentrations at higher values of [NOx] whereas at lower values of [NOx], [O3] dominates over [NO] and [NO2] for all the sites as can be seen in Fig. 7. [O3] and [NO] curves cross over with [NOx] at values ranging between 75.77 ppb to 78.89 ppb, maximum being for CRRI Mathura Road station. This can be due to higher precursor gases concentrations as this is a heavily polluted area as described in the above section. (Sect. 2). When [NOx] increases [O3] values diminish but values of [NO] increase as can be seen in Fig. 7 for all the sites.
-
[NO2] and [O3] curves intersect with [NOx] at values ranging between 83.067 to 85.97 ppb, maximum being for CRRI Mathura Road station as can be observed in Fig. 7. When [NOx] is < 85.97 ppb [O3] is in dominant form.
Figure 7 clearly describes the chemical coupling of the three species as well the interaction and relationship of O3 with its precursors. The above figure establishes the fact that at higher concentrations of NOx, O3 is destroyed and NO2 remains the predominant form of the oxidant [OX]. Similar behavior has been reported earlier by Clapp and Jenkin [43] and Mazzeo et al. [40] for urban and rural sites in the UK and Buenos Aires, respectively. Similar behavior has also been reported by Tiwari et al. [41] for an urban site in New Delhi, India
Figure 8 presents the variation in daytime O3 concentration (1-hour average) as a function of the NO2/NO ratio (sample interval: 1hour) at all sites during the whole study period. The daylight concentrations were determined using the sunrise and sunset values obtained from the US astronomical site (http://aa.usno.navy.mil/data/docs/RS_OneDay.php) which generally varied between 0500–1900h during summers and 0630–1800h during winters. [40, 41] It was observed that for all the sites, the ambient level of hourly averaged O3 increases with an increase in the ratio of [NO2]/ [NO]. According to Fig. 8, concentrations of hourly averaged ambient O3 at all the 5 sites, increase rapidly at small values of [NO2]/ [NO], this may be implicated that when O3 was at low levels, the reactions of production of O3 was a dominated reaction. When O3 concentration reached about 35.98 ppb (5 site average), it became relatively stable. This shows that at an average value of 35.98 ppb, O3 concentration is close to reaching a photo stationary state. Similar behavior was reported by Han et al. [44] for Tianjin in China.
3.3.1 Relationship between [NO2] and [NOx]
Figure 9 presents the variation of the [NO2]/[NOx] ratio at all the locations for the whole study period(averaging time:60 min) in the function of [NOx]. It has been observed that the [NO2]/[NOx] ratio decreases with the increase of the [NOx] levels. According to the data available, it has been calculated that for New Delhi (5 station average) when [NOx] = 100 ppb, [NO2]/[NOx] ratio was found to be ≈ 0.296. It has been observed that mean [NO2] concentrations for all the locations used in the study were not very high(≈ 23.24 ppb). In these cases, the average [NO2]/[NOx] ratio for New Delhi (5 station average) was calculated to be ≈ 0.51, which is significantly higher than the ratio generally found in vehicular NOx emissions. The highest observed values of [NO2]/[NOx] ratio at all the locations of our study for the whole study period can be explained by an additional oxidation process for NO to NO2, possibly that of the reaction of NO with oxygen to form NO2:[40, 43]
2NO + O2 → 2NO2 Eq. (5)
3.4 Relationship between [OX] and [NOx], [NOx] dependent and [NOx] independent contribution to [OX]
Figure 10 depicts the relationship between daylight averaged (60 min average) concentrations of [OX] with respect to [NOx] at all sites during the whole study period. It is seen from the figure that the concentrations of [OX] increase with the increasing concentration of [NOx] at all the sites of this study. As seen from Fig. 10, at all the sites the curve fits the linear regression curve[y = mx + c]. A non-significant positive correlation has been observed at all the sites during the study period (r2 value ranges between 0.08 to 0.16 at P = 0.05). With help of the slope of the curve obtained for all sites, we can divide it into an [NOx] independent contribution and [NOx] dependent contribution for the concentration of oxidants at all the sites of an urban location.[40, 41, 43] The NOx-dependent contribution can be contributed to local contribution sources of oxidants and can also be correlated to primary pollutant sources. [40, 41, 43] It is called as [NOx] dependant contribution because an increase/decrease in [NOx] affects the concentration of oxidant sources at all the sites. [40, 41] On the other hand, we can say that NOx independent contributing sources of oxidants at all the sites can be called as more regional contributing sources of oxidants as an increase/decrease in [NOx] values does not cause an effect on oxidant concentration at all sites. The local sources depict the prevalent photochemistry at the urban site. As reported by earlier studies at urban sites the major contributor to NOx is NO2, and in turn, the major contributor of NO2 is the process of combustion of fuels (diesel trucks, cars, motor-generator). Nagpure et al.[45] and Badrinath et al.[46] have first reported that NO2 concentrations in Delhi are unusually high due to increased traffic emissions by diesel trucks, even at night and also due to crop residue burning in adjoining areas of New Delhi during post monsoon and winter season (October to January). NO being a highly unstable compound gets converted to NO2 as soon as it is produced in the troposphere.[47–48] VOC’s and CO initiated chain reactions with hydroxyl and peroxy radicals present in an urban atmosphere are also the major contributors of oxidant and ozone formation at an urban site like New Delhi. These chain reactions in the troposphere catalyzes the conversion of NO and NO2 (Eqs. (6) and (7)) and act as major contributors to the accumulation of oxidants at the site.
HO2 + NO→NO2 + HO Eq. (6)
RO2 + NO→NO2 + RO Eq. (7)
(R: Organic functional group).
Due to a lack in the consistency of VOC’s data we were not able to further investigate the above relationship. However, we aim to further analyze this relationship in our next study.
3.5 Diurnal and monthly variation of Oxidants [OX]
The above sections of this study have established the fact that the production and accumulation of oxidants at the site are generally governed by photochemical processes. To investigate further into this relationship we have plotted averaged diurnal and monthly variation of the oxidants at all the locations of this study during the period of 2013–2019 (Fig. 10 and Fig. 11). Large variability was seen in the diurnal curve during the study period at all the sites of the study with concentrations of oxidants [OX] increasing after 0800h in the morning as the sun rises and attaining its maximum peak at noon (1200h to 1400h), and decreasing thereafter as the sun sets (after 1700hrs) (Fig. 11). This is because during noon rate of photochemical production is high as the intensity of sunlight is maximum during the noon hours in Delhi (1200h to 1400hr). Figure 12 shows the average monthly variation of oxidants at all the sites during the whole study period. It clearly shows that the maximum concentration of [OX] is observed during summer months (71.88 ppb during the month of June at CV Raman Dheerpur station) and minimum concentration is observed during winter months (37.86 ppb during the month of January at IMD Lodi Road station) (Fig. 12). Maximum values during the summer season depict enhanced photochemical activity because of high-temperature values at all the sites due to intense solar radiation and hence an increase in the number of sunny days. Minimum values during the winter season may be due to fewer sunny days which may be attributed to cloudy skies and high humidity because of frequent rainfall that may be due to frequent western disturbances occurring during this season in New Delhi during the study period, hence resulting in washout of pollutants (Fig. 16). Tiwari et al. [41] have reported similar behavior based on an observational study (SAFAR data) at a single site in New Delhi which showed maximum ozone concentrations during the summer in Delhi due to favorable meteorological conditions such as high solar intensity, clear skies, and low relative humidity. Ghude et al. [16] also reported similar behavior based on a model study. Earlier studies and this study have established that the regional and global contribution of oxidants at a site is governed by photochemistry as well as the prevailing meteorological conditions. In lieu of the above, the relationship of oxidants with temperature, humidity, wind speed and wind direction have been studied during the year 2018 and 2019 for New Delhi (5 station average) (Fig. 13, Fig. 14 and Fig. 15). Figure 13 establishes the fact that higher temperature and lower humidity values increase [OX] concentrations. Pollution rose for the years 2018 and 2019 was drawn to study [OX] concentrations. We have separated wind directions into four groups; (i) 0–90°, (ii) 90–180°, (iii) 180–270°, and (iv) 270–360° and the corresponding [OX] values were found to be 47.14 ppb, 47.19 ppb, 49.15 ppb and 50.05 respectively. Therefore it suggests that major contributors of [OX] are winds coming from the west direction into New Delhi. (Fig. 14 and Fig. 15) The sources of oxidants may be biomass burning, crop residue burning during early winter and winter months and some proportion is vehicular exhaust. When conditions were calm (wind speed ≤ 0.5 m/s) [OX] concentrations were found to be 57.84 ppb (averaged). Stagnant wind conditions do not allow the mixing of trace gases by decreasing the boundary layer height and pollutants are trapped near the surface causing a surge in the rise of ground level oxidants at all the different urban sites used in the present study.
3.6 Ozone Generation at the urban site-New Delhi-Relationship with NOx and Temperature (5 station average)
Although the average maximum mean daytime concentration at New Delhi (5 site average) was found to be in the range of 20–30 ppb, but about ~ 2.5 % times hourly mean daytime surface ozone concentration exceeded 90 ppb (1 hourly average)marking severe pollution events in New Delhi (5 station average) under favorable conditions.
It is observed from Fig. 18 that ground-level concentration of ozone decreases with increasing NOx concentration which is typical behavior for VOC sensitive urban conditions.[49] In the NOx-saturated or VOC-sensitive regime, O3 decreases with increasing NOx and increases with increasing VOC.[50, 51]