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Abstract This study reports simultaneous measurements of inorganic Nr trace gases (NH3, NOx, HNO3) and the corresponding particulates (NH4+, NO3−) for elucidating photochemical interactions that are involved in their spatial… Click to show full abstract
Abstract This study reports simultaneous measurements of inorganic Nr trace gases (NH3, NOx, HNO3) and the corresponding particulates (NH4+, NO3−) for elucidating photochemical interactions that are involved in their spatial and diurnal evolution along the north west – south transect of an urban NCR. Three sites were chosen with different land use characteristics where the spatial gradient in Nr precursor gases were observed as Faridabad (92.33 μg m−3) > Delhi (56.01 μg m−3) > Rohtak (24.84 μg m−3). Their percentage fractions in the total Nr composition were estimated as 25.45% of NOx, 28.91% of HNO3, 40.05% of NH3 at industrial Faridabad site respectively; 27.59% of NOx, 8.36% of HNO3, 61.61% of NH3 respectively at urban representative Delhi site and 22.37% of NOx, 15.36% of HNO3, 53.53% of NH3 respectively at background Rohtak site. Particulate Nr concentrations, on the other hand, exhibited insignificant inter site variability where its percentage contribution to the total Nr had the sequential order as Rohtak (3.56%) > Delhi (1.69%) > Faridabad (0.22%) for NH4+ fractions whereas NO3− fractions has the order as Faridabad (5.37%) > Rohtak (5.18%) > Delhi (0.75%). Ionic composition of these particulates was characterized by the dominance of Ca2+ as the major base cation in which the role of photochemical reactions becomes limited. Hence, such Nr distribution patterns were further evaluated for their diurnal behaviour at each site where D/N > 1 ratios were observed for NOx (1.52), HNO3 (1.02), NH3 (1.72), NO3− (1.01) at Rohtak as well as for NH4+ (3.02) and NO3− (2.60) at Delhi and Faridabad site respectively. However, their transformation extents in the photochemical reactions were observed to be independent of diurnal phases where the incomplete phase conversions were characterized by the molar ratios of NOx/NOy > 0.8 at Delhi and NH3/NHx > 0.5 at all the sites. The correlation matrix of these parameters confirmed their gas phase oxidation occurring independent of their meteorological condition of temperature and relative humidity. Such observations were thermodynamically evaluated by the calculation of their Km and Kp values that were further plotted for their ambient temperature and below deliquesce relative humidity levels for ascertaining the favourable conditions that are existing for the NH4NO3 aerosol formation over the study region.