Study on the Relationship of WSIS of PM_2.5 with NH₃ and Other Trace Gases over Delhi, India ^†

Abstract

The water soluble ionic species (WSIS) i.e., NH₄⁺, SO₄²⁻, NO₃⁻ and Cl⁻ of PM_2.5 and trace gases (NH₃, NO, NO₂, SO₂, HNO₃) were measured to study the relationship of ambient NH₃ in the formation of secondary inorganic aerosols in Delhi, India from January 2013–December 2018. During the study period, the average concentrations of NH₃, NO, NO₂, SO₂ and HNO₃ were 19.1 ± 3.8 ppb, 20.8 ± 4.3 ppb, 17.9 ± 4.2 ppb, 2.45 ± 0.47 ppb, 1.11 ± 0.35 ppb, respectively. The concentrations of trace gases were higher during post-monsoon whereas the concentrations of WSIS in PM_2.5 were estimated higher in winter. The correlation matrix of trace gases reveal that the ambient NH₃ neutralize the acid gases (NO, NO₂ and SO₂) at the monitoring site. Study reveals that the abundance of particulate NH₄⁺ at Delhi to neutralized the SO₄²⁻, NO₃⁻, Cl⁻ particles during all the seasons.

1. Introduction

The formation of secondary aerosols in the atmosphere influenced by reaction rate of NH₃ which depends on the favorable meteorological condition and availability of acid gases in the atmosphere [1,2]. Fine fraction of particulate matter (PM_2.5) is considered as one of the major pollutants having a negative impact on atmospheric chemistry [3,4]. Secondary aerosols contribute to a major fraction of PM_2.5 mass concentration which is mainly formed from NH₃ and its co-pollutants such as NO_x and SO_x [5]. NH₃ as a primary alkaline gas neutralizes the acid gases (HNO₃ and H₂SO₄) and form the secondary particulates (NH₄NO₃ and (NH₄)₂SO₄), which are the major fractions of airborne fine particles [6]. In recent past several studies on temporal and spatial changes of ambient NH₃, NO, NO₂, CO and SO₂ have been carried on short-term basis as well as year-long basis at the urban and sub-urban locations of India [7,8,9,10,11]. However, long-term study on seasonal basis as well gas-to-particle conversion is inadequate in Indian region. In this paper, we reported the annual and seasonal changes of ambient NH₃, NO, NO₂, SO₂ and PM_2.5 measured for the period of 2013–2018.

2. Materials and Methods

Ambient NH₃, NO, NO₂, and SO₂ were monitored at CSIR-National Physical Laboratory, New Delhi from January 2013 to December 2018. 24 h periodic sampling (2 samples/week) of PM_2.5 was also performed during this period on quartz filters. Ground based analyzers were used to continuous measurement of trace gases (NH₃, NO, NO₂ and SO₂) at 10 m height from the surface level [11]. The estimation of WSICs (Na⁺, NH₄⁺, K⁺, Ca²⁺, Mg²⁺, Cl⁻, NO₃⁻ and SO₄²⁻) of PM_2.5 were determined using Ion Chromatograph (DIONEX, Sunnyvale, CA, USA) with suppressed conductivity [12].

3. Results and Discussion

During the study period (2013–2018), the average levels of NH₃, NO, NO₂, SO₂ and HNO₃ were 19.1 ± 3.8 ppb, 20.8 ± 4.3 ppb, 17.9 ± 4.2 ppb, 2.45 ± 0.47 ppb, 1.11 ± 0.35 ppb, respectively whereas the levels of NH₄⁺, SO₄²⁻, NO₃⁻ and Cl⁻ of PM_2.5 were 9.1 ± 3.5 µg m⁻³, 12.3 ± 4.1 µg m⁻³, 10.8 ± 4.8 µg m⁻³ and 9.3 ± 3.2 µg m⁻³, respectively. Seasonal mixing ratios of NH₃, other trace gases (NO, NO₂ and SO₂) and concentrations of water soluble ionic components (WSICs) of PM_2.5 are depicted in

Table 1. Seasonal variation in trace gases (in ppb) in Delhi during 2013–2018.

Seasons	NH3	NO2	NO	SO2
Winter	20.9 ± 4.1	17.7 ± 4.5	18.1 ± 4.4	2.24 ± 0.37
Summer	19.4 ± 4.1	19.1 ± 4.3	21.4 ± 5.4	2.25 ± 0.43
Monsoon	14.0 ± 2.5	14.9 ± 3.7	20.4 ± 5.3	2.55 ± 0.26
Post-Monsoon	22.2 ± 3.9	20.0 ± 4.2	23.3 ± 4.5	2.77 ± 0.36
Average	19.1 ± 3.8	17.9 ± 4.2	20.8 ± 4.3	2.45 ± 0.47

and

Table 2. Seasonal variation of WSIC of PM_2.5 (in µg m⁻³) in Delhi during 2013–2018.

Seasons	PM2.5	Cl−	SO42+	NO3−	NH4+
Winter	190 ± 82	15.6 ± 8.9	19.6 ± 6.9	22.7 ± 9.5	17.5 ± 2.8
Summer	92 ± 30	7.5 ± 3.1	8.5 ± 2.2	5.0 ± 2.8	5.8 ± 3.5
Monsoon	86 ± 33	6.2 ± 2.1	9.9 ± 1.9	4.7 ± 2.4	3.9 ± 1.2
Post-Monsoon	171 ± 72	7.8 ± 3.0	11.3 ± 3.4	10.9 ± 3.8	9.3 ± 4.4
Average	135 ± 45	9.3 ± 3.2	12.3 ± 4.1	10.8 ± 4.8	9.1 ± 3.5

. The ambient NH₃ indicated significant seasonal variation with highest mixing ratio during post-monsoon season (22.2 ± 3.9 ppb) followed by winter (20.9 ± 4.1 ppb), summer (19.4 ± 4.1 ppb) and monsoon (14.0 ± 2.5 ppb) seasons.

The higher concentration of NH₄⁺ during winter season at the observational site of Delhi may be due to high (relative humidity) RH, low temperature and higher NH₃ mixing ratio influenced the NH₄⁺ formation [13]. In winter, nitrates availability was significant due to possible reduction in SO₂ oxidation rates in response to lower level of hydroxyl (OH) radical [14]. A relationship of particulate NH₄⁺ with SO₄²⁻, NO₃⁻ and Cl⁻ during all the seasons supports the hypothesis of gas-to-particle conversion. The highest average molar ratio of NH₄⁺ to the SO₄²⁻ during winter (4.86) followed by post-monsoon (4.38), summer (3.61) and monsoon (2.1) seasons indicated the complete neutralization of H₂SO₄, abundance of (NH₄)₂SO₄ and NH₃-rich condition during the winter season [11]. Since NH₃ is the only alkaline gas in the atmosphere with adequate level to neutralize a significant portion of SO₄²⁻, NO₃⁻ and Cl⁻ therefore the aerosol electro-neutrality relationship between NH₄⁺ and SO₄²⁻, NO₃⁻ and Cl⁻ ions can be computed [15].

4. Conclusions

The average levels of all trace gases (NH₃, NO, NO₂ and SO₂) were observed higher during post-monsoon season whereas the mass concentrations of WSICs of PM_2.5 were higher in winter seasons. The correlation matrix of trace gases demonstrated that the ambient NH₃ neutralize all the acid gases (NO, NO₂ and SO₂) at Delhi during the study period.