Assessment of Critical loadslevels of S, N and surface O3 for

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Assessment of Critical loadslevels of S, N and surface O3 for




     *Gur Sumiran Satsangi, Renuka Saini, P.R. Kulshreshtha and Ajay Taneja

    Department of Chemistry, St. John’s College, Agra – 282 002, India


Critical load of S and N along with surface O exposure were computed for Neem (Azardirachta indica) 3

    tree. Critical load of S and N were calculated using a simple mass balance method taking into account of

    all acidity and alkalinity of wet and dry deposition data while and AOT40 concept was used to calculate

    the exceedance of O exposure level. Computed values of S and N indicated that present acids load (S = 3-1-1-1-1-1-1490 eq ha yr and N = 1500 eq ha yr) are lower than the critical load of S and N (S= 1881 eq ha yr -1-1-1-1and N = 1932 eq ha yr). Exceedance of critical load was found to be negative (Ex = - 1163eq ha yr)

    for S and N. While for O, calculated O exposure was found to be 4010 ppb.h which is much lower depdep33

    than the critical levels of O (10,000 ppb.h). All features indicated that the present levels of anthropogenic 3

    emissions in Agra, a semi-arid region do not pose any risk to vegetation.

Key words: Acidification, SO emission, NO emission, exceedance , AOT 40. 2x


Anthropogenic emissions of acidic precursor substances (SO and NO) are likely to rise with economic 2x

    -1development in the Asia. Escalation of regional emissions of SO, NO (at a rate about 4% yr) together 2x

    with ground level O concentration have been steadily increasing over the past decades. O is one of the 33

    most reactive pollutants in the atmosphere produced by photo-oxidation of CH, CO and NMHC (non 4

    methan hydrocarbons) in the presence of sufficient amount of NO. In case of an abundance of NOx in x

    atmosphere, the production of NO takes place by the reaction if NO with O or RO (peroxy radicals), 222

    which ultimately facilitate the O production. Higher levels of SO and NO caused the regional acidic 32x

    deposition while ground level O cause the photochemical pollution problems (Gorham, 1989). Economic 3

    projection indicates that large increase in emission may be occurring over the next few decades if current

    development pattern persists. If these happen, the impacts of SO, NO and O on ecosystem and public 2x3

    health would be experienced in near future (Pochanart et al., 2002; Lefohn et al., 1997).

     *Author for Correspondence: 22, Allora Enclave, Dayalbagh, Agra 282 005, India. email:,

    The rapidity with which the emissions are oxidized to acids and the effectiveness with they all are deposited and several of their deleterious effects on terrestrial ecosystems are urgently need to consider for saving the ecosystems. Political decisions on emissions reductions require scientific determination of

     and NO. Thus approach for controlling the emission and deposition of the deposition levels of SO2x

    acidifying pollutants is called critical loads. Critical load constituent is an important basis for the development and adaptation of acidification abatement policies. The critical load/levels concept is now widely used in developing emissions control policies in Europe and Asia. The UNECE has made it the basis for decision making underlying the control of trans-boundary air pollution; it has used it to develop legislation on controlling SO, NO emissions and recently also considered as scientific basis for control of 2x

    O and heavy metals. Therefore, the concept of critical load/levels of pollutants serves as a guide to 3

    environmental protection. Critical load was also established to evaluate the potential environmental risk of acid deposition, with the effects of pollutants being related to specific vulnerability of a particular sensitive ecosystem. Critical load is “quantitative estimates of an exposure to one or more pollutants

    below which significant harmful effects on specified elements of the environment do not occur according to present knowledge” (Nilsson and Grennfelt, 1988). Thus, critical loads are offered as thresholds values,

    if deposition is below the threshold, there are no problems; if it is above the threshold, harm is being done to the environment. The UNECE has also set critical levels for forest, crops and semi natural vegetation based on a cutoff concentration of 40 ppb (WHO, 1996; Mauzerall and Wang, 2001). This index is known

    as AOT40, which calculates O concentration over the threshold of 40 ppb. In the present communication 3

    the multi-stress risk assessment by the S, N and O exposure from its critical loads/levels for Neem 3

    (Azardirachta indica) in a semi-arid region of Agra (India) using the critical load approach and AOT 40 index has been made.

Material and methods

(i) Calculation of acidity by steady state mass balance method

Critical loads calculation for ecosystem is based on the method given by Hettelingh et al., 1995. The

    steady state mass balance (SSMB) method was predominantly used for the computation of critical loads of acidity. The method computes an equilibrium between acidification increasing processes (sulphur deposition and base cation uptake) and acidification decreasing processes (weathering rate and base cation deposition). This method assumes a time independent steady state of chemical interactions involving equilibrium between the soil solid phase and soil solutions. The SSMB is aimed to determine the long-term average source of acidity and alkalinity in the system and to determine the maximum acid input that will balance the system at a bio-geochemical safe limit.

    Critical load of acidity (CL(A)) includes the acidity contributed by both S and N compounds and for its evaluation, both pollutants are considered simultaneously. Critical load of acidity is computed as follows CL(A) = BC + HQ + AlQ ..…..(1) wcrit..crit.

     is base cation weathering rate, H is the critical hydrogen leaching, Al is critical aluminium where BCwcritcrit

    concentration and Q is runoff.

    The maximum allowable deposition of S, which does not lead to „harmful effects‟ in the case of zero N-

    deposition is given by

    CL(S) = BC - BC Cl+ CL(A) ..…..(2) maxdepudep

    where BC is the deposition of base cation, BC is net growth uptake of base cations, Clchloride depudep is

    deposition and CL(A) is acidity of critical load. Critical load of acidity is not directly applicable for evaluation of required S emission reduction, therefore the current UN/ECE protocols concentrate on a single compound either S or N. Therefore, the critical load of acidity has been divided between S and N assuming (a) the fraction of S deposition of total acid deposition is used as proxy of the part of the critical load of acidity attributed to S and, (b) N deposition contributes to acidification only when it is not taken up or immobilized by the ecosystem. Thus, a term sulphur fraction (S) has been introduced which is defined f


    S = S/(S + N N - N) for N>N+N (S = 1 otherwise) ..…..(3) fdepdepdepuidepuif

    Where S is the S deposition and N is the deposition of both oxidized and reduced N, N and N are depdepui

    net growth uptake and immobilized N. The critical deposition of S, for a given N deposition is given by CD(S) = S. CL(S) ..…..(4) fmax

    Critical deposition of S has been compared to S deposition in negotiating the sulphur protocol by computing the so-called exceedance of critical deposition. Exceedance of critical deposition is given as the difference between S deposition and critical deposition of S

    Ex(S) = S CD(S) ..…..(5) dep

    and Ex(S) 0 ensures that the ecosystem is protected.

    Beside S, the deposition of N also contributes to the acidification of ecosystems. The excess deposition of S and N can be written as:

    Ex (N, S) = S + (1 f).N (1- f) (N + N) CL(S) ..…..(6) depdepdepdedepdeuimax

    where f is denitrification factor, N is the deposition of N and CL(S) is given by eq. (9). If the dedepmax

    exceedance of acidity in above equation is negative or zero i.e. Ex(N, S) 0 for the given pair of depdep

    deposition (N, S) it can be said that critical loads are not exceeded. However, there are many values depdep

    of S and N deposition for which exceedance becomes zero i.e. unique critical load values cannot be defined. Furthermore, for

    N N + N = CL(N) ..…..(7) depuimin

    The maximum allowable deposition for S is given by CL(S); and the maximum „harmless‟ acidifying max

    deposition of N is obtained by inserting S = 0 in eq. (6) and solving Ex(N = 0): depdep

    CL(N) = CL(N) + CL(S)/(1-f) ..…..(8) maxminmaxde

(ii) Critical assessment of O by AOT 40 exposure index 3

The effect of O deposition on vegetation can be seen in measurements of plant canopy resistance to O 33

    uptake. These shows marked reduction in resistance during the daytime when plant stomata are open. Plants have evolved protective mechanisms to prevent damage caused by Oand natural oxidants. 3

    However, above the threshold limit, plants detoxicant processes can no longer exist and at this time damage can be visualized in plant species. To assess the damage to agricultural crops due to high O3

    concentration, relationships between crop loss and Oare required. AOT 40 (Accumulation exposure over 3

    threshold of 40 ppb) is an exposure-plants response index function set by the United Nations Economics Commission for Europe (UN-ECE) and US-EPA. It is calculated as the sum of differences between the hourly averaged Oconcentration and the threshold value of 40 ppb for each hour that the averaged O 3 3

    concentration exceeds 40 ppb. It is expressed mathematically as


    AOT 40 = ; ([O] 40)for [O] > 40 ppb ..…..(9) 3i 3

     i = 1

where [O] = hourly averaged O concentration 33

     40 = threshold value of O3

     -2An AOT 40 value of 10,000 ppb h for daylight hours (radiation > 50 W m) over a 6 month period has

    been established as a critical level for the protection of forests (WHO, 1996; Beck et al., 1998).

Data Source to calculate the critical load/levels

The critical load/level for S, N and O has been calculated for the Agra region of Northern India. Some of 3

    the required parameters were determined experimentally and others were derived from the literature as follows:

i. Base cation weathering rate (BC) w

    Weathering rate though an important parameter is not well known in the studied region. For the present study, weathering rate was determined from the correlation between observed weathering rate and total content of calcium and magnesium in soil as reported by Olsson and Melkerud (1990). The total Ca and Mg (XCa + XMg) were determined by soil analysis. Total number of analyzed composite soil samples was 25. The mean value of XCa + XMg is 494 and corresponding to this; the observed weathering rate was

    -1-1obtained as 1.62 Keq ha yr by the correlation graph.

ii. Runoff (Q)

    Runoff is defined as the flow rate through a system. In the soil system, the flow rate is precipitation minus evapo-transpiration minus surface runoff (Hettelingh et al., 1991). For calculating the runoff, the values of

precipitation, evapo-transpiration and surface runoff were taken as 766 mm, 1037 mm (Pandeya et al.,

    1977) and 73 mm (Biswas and Mukherjee, 1987), respectively. The calculated value of runoff (Q) was

     -1-1-3-3 ha yr. The value of H and Al were 0.09 eq m and 0.2 eq mtaken from found to be 3440 mcritcrit

    Hettelingh et al., (1991) for this study due to their unavailability from this region.

iii. Uptake data; BC and N uu

    The uptake rate depends upon the biomass productivity and the optimal internal concentration of plant species. The uptake data; BC and N of Neem (Azardirachta indica ) tree was taken from literature (ICAR, uu

    -1-1 -1-11987) and the values are 658.4 eq ha yrand 187.3 eq ha yr, respectively.

iv. Nitrogen immobilization

    Nitrogen immobilization (N) is defined as “the acceptable annual level of N immobilization in soil organic i

    matter (including the forest floor) at N inputs equal to the critical load, at which adverse ecosystem change will not take place” (UBA, 1996). Nitrification rate in some soils are very low and nitrogen may accumulate in the soil organic matter. The rate of immobilization of stable organic N compounds in the

    -1-1soil of this region is very low. The value of nitrogen immobilzation is taken as 0.009 eq ha yr (ICAR,


v. Deposition and exposure data

    The data on wet and dry deposition fluxes for the various ionic components and O exposure were taken 3

    from our previous published results (Saxena et al., 1996; 1997; Satsangi et al., 2003; 2004).

vi. Receptor

    India is a tropical country where texture of soil, climate and species composition varies spatially, so it is necessary to calculate the critical load on a local basis considering individual plant receptors because tolerance of acidity is species dependent. Therefore, in the present study we have considered Neem (Azardirachta indica) tree because this plant is uniformly distributed in the study area.

Results and discussion

Calculated data of critical loads of S and N and its exceedance for Neem (Azardirachta indica) were

    -1 presented in Table 1. The calculated value of critical load of actual acidity was found to be 622 eq ha-1yr. Relationship between N and S deposition and the exceedance of the critical loads of S and N are

    -1 -1illustrated in Fig. 1. CL(S) and CL(N) were found to be 1881 and 1932 eq hayr, respectively. In maxmax

    the figure critical deposition is represented by thick line, which embody no „harmful effects‟ and this function may be termed the critical load function of the ecosystems. It is also evident from the figure that the estimated deposition load of S and N which was calculated from the deposition data (represented by dash line ( ) in the figure) are much lower than the critical loads of S and N (represented by dark line

    ( ) in the figure). Exceedance of acidity has also been calculated for soil with respect to Neem tree.

    -1 -1 yr(by eq. 6) is consistent Calculated exceedance of critical deposition was found to be 1163 eq ha

    with the exceedance value mapped by sensitivity approach (Kuylenstierna et al., 2001). Negative

    exceedance of critical loads suggested that the ecosystem is protected from the impact of the current levels of acidification to the ecosystem. This result further confirms our earlier findings that the acidity produced during the atmospheric reaction, is neutralized by the alkaline substances derived from the wind blown soil (Satsangi et al., 1998, 2002).

    Further, indirect effects of acidic deposition are also related to soil chemistry. Soil systems are considered to be sensitive to acidification if they have low buffering capacity or acid neutralizing capacity (ANC). The ANC normally attains a value of zero at pH values in the range of 4.6 5.6, and may thus

    have positive or negative values. Positive ANC is called alkalinity and negative is called acidity. The ANC can be calculated as

    2+2+++-+2--ANC = [Ca] + [Mg] + [Na] + [K] + [NH] [SO] [NO] [Cl] ……(10) 443

     (Reuss et al., 1986) (all concentrations are in equivalent)

    3-1and found to be +33.3 x 10 neq g Positive value of ANC and pH of soil (7.5 and 8.2) of this region .

    indicate that associated acidity of deposition is neutralized by the soil particles. Fig. 1 also demonstrate how can one plan a path to reach non exceedance levels if in future the deposition levels lie above the critical loads, say at a point P. By reducing N deposition substantially one reaches the point A and 11

    therefore non exccedance without reducing S deposition; on the other hand one can again reach non exceedance by only reducing S deposition till reaching A and finally with a smaller reduction of both S 2

    deposition and N deposition one can reach non exceedance at point A. In practice however, external 3

    factors, like costs of emission reductions and feasibility would determine the path to be followed to reach zero exceedance.

    For the preliminary evaluation of the trees protection from O exposure, the AOT 40 was calculated. 3

    Since there is no O threshold exposure index established for Asian countries, we have applied the AOT 3

    40 exposure index. We have observed high concentrations of O more than 40 ppb during winter and 3

    summer seasons in contrast to April and Sept. as stated by the UNECE, which would be sensitive for trees. Therefore, in present calculation we have calculated AOT 40 for Sept. to June. Figure 2 shows the day to day increased in AOT40 value for the photosensitive period (during daytime). It is apparent from the that there is substantial month to month variation of O exposure, mainly due to the variation in local 3

    emissions of precursors and the inter-seasonal variation of meteorological conditions (Satsangi et al., 2004). The calculated value of AOT for Neem tree is 4010 ppb.h, which is lower by 40% from the critical level of O (10,000 ppb.h). This implies that at present risk to trees damage during these months by O 33

    exposure is not serious. But due to hasty urbanization and industrialization in India AOT 40 scenario is also likely to increase. Thus there is need to implement effective control strategies and technology for controlling the emissions of precursors of O. 3


    The present study of multi-stress risk assessment with respect to Neem (Azardiracta indica) has revealed

    -1-1-1-1that the present load of S and N and O (490 eq ha yr, 1500 eq ha yr and 4010 ppb.h ) are much 3

    -1-1-1-1lower than the critical load of S, N and O (1881 eq ha yr, 1932 eq ha yr and 10,000 ppb.h). The 3

    current critical load exceedance value for soil further indicates that the ecosystem is protected. It is however important to mention that although the current state of scientific knowledge does not pose any damage to ecosystems, the fact that the growth in SO emissions associated with the envisaged evolution 2

    of energy gives reasons for serious concern about maintaining sustainable conditions for natural and agricultural ecosystems. The simple approach used here helps to identify areas where exceedance is more likely.


    Dr. G.S. Satsangi is indebted to CSIR, New Delhi for assisting the Research Associateship. Authors also like to thank Dr. Ashok Kumar, Head, Department of Chemistry, St. John‟s College, Agra for providing

    necessary facilities for analysis.


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    Table 1: Computed data of the study with respect to Neem (Azardirachta indica) as a receptor

    Parameters Calculated values Used Eq.

    -1-1 yr Eq. 1 Critical load of actual acidity (CL (A)) 622 eq ha

    -1-1Maximum allowable deposition of S (CLmax (S) 1881 eq ha yr Eq. 2

    -1-1Maximum allowable deposition of N (CLmax (N) 1932 eq ha yr Eq. 8

    -1-1Critical deposition of S (CD(S)) 677 eq ha yr Eq. 4

    -1-1Exceedance of Acidity (EX S, N) - 1163 eq ha yr Eq. 6 depdep

    AOT 40 for O 4010 ppb.h Eq. 9 3

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