Source variability and atmospheric processing of NH_x using δ¹⁵N: Linkages with WSOC fluorescence indices

Area of Science:

• Atmospheric Chemistry
• Environmental Science
• Geochemistry

Background:

• Reactive nitrogen species, including ammonia gas (NH3) and particulate ammonium (NH4+), significantly impact atmospheric chemistry, air quality, and ecosystem health.
• The Indo-Gangetic Plain (IGP) is a global hotspot for NH3 emissions, yet detailed source investigations and partitioning studies are limited.
• Understanding NHx dynamics is crucial for air quality management, especially during periods of poor air quality like post-monsoon crop-residue burning.

Purpose of the Study:

• To identify the major sources of atmospheric ammonia (NH3) in the northwestern Indo-Gangetic Plain.
• To investigate the factors controlling the partitioning of NH3 to particulate ammonium (NH4+).
• To examine the concurrent aging of inorganic and organic aerosols and their relationship with NH3 sources.

Main Methods:

• Utilized stable isotope analysis (δ15N) to trace nitrogen sources.
• Employed the MixSIAR mixing model for source apportionment of NH3.
• Coupled δ15N-NH4+ data with fluorescence indices of water-soluble organic carbon (WSOC) to assess aerosol aging.

Main Results:

• Biomass burning (37%) and agricultural emissions (33%) were identified as the dominant NH3 sources, followed by coal-fired power plants (21%) and vehicular emissions (10%).
• Source contributions varied with wind speed; calmer conditions favored local combustion, while windier periods increased agricultural contributions.
• Higher relative humidity (RH) promoted NH3 to NH4+ partitioning under calmer conditions, an effect masked by atmospheric mixing during windier periods.
• A significant correlation between δ15N-NH4+ and WSOC fluorescence indices suggested common sources and simultaneous aging of NH4+ and organic aerosols.

Conclusions:

• Biomass burning and agricultural activities are critical sources of NH3 in the IGP, significantly affecting air quality.
• Meteorological conditions, particularly RH and wind speed, play a vital role in NH3-NH4+ partitioning and atmospheric transport.
• The study highlights the interconnectedness of NHx dynamics, aerosol aging, and secondary organic aerosol formation, essential for targeted mitigation strategies in emission hotspots.