Wildfire-Derived Nitrogen Aerosols Threaten the Fragile Ecosystem in Himalayas and Tibetan Plateau

Abstract

Himalayas and Tibetan Plateau (HTP) is important for global biodiversity and regional sustainable development. While numerous studies have revealed that the ecosystem in this unique and pristine region is changing, their exact causes are still poorly understood. Here, we present a year-round (23 March 2017 to 19 March 2018) ground- and satellite-based atmospheric observation at the Qomolangma monitoring station (QOMS, 4276 m a.s.l.). Based on a comprehensive chemical and stable isotope (15N) analysis of nitrogen compounds and satellite observations, we provide unequivocal evidence that wildfire emissions in South Asia can come across the Himalayas and threaten the HTP’s ecosystem. Such wildfire episodes, mostly occurring in spring (March–April), not only substantially enhanced the aerosol nitrogen concentration but also altered its composition (i.e., rendering it more bioavailable). We estimated a nitrogen deposition flux at QOMS of ∼10 kg N ha–1 yr–1, which is approximately twice the lower value of the critical load range reported for the Alpine ecosystem. Such adverse impact is particularly concerning, given the anticipated increase of wildfire activities in the future under climate change.

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“…This implies that while TDP and TDN share the same origin, TDP and TP do not. This relationship between TDP and TDN is to be put in perspective with the recent findings of Bhattarai et al who demonstrated that atmospheric N at QOMS is mainly driven by BB emissions . Our results corroborate these findings as they confirm that BB not only significantly contributes to bioavailable nitrogen in the Himalayas but also shows that it is the main source of bioavailable phosphorus too.…”

“…Phosphorus and nitrogen are the key nutrients for any ecosystem and thus are likely to jointly influence the structure, functioning, and diversity of ecosystems, highlighting the importance of considering the N/P ratio for ecosystems. The concentrations of N species in aerosol at QOMS have been reported in our earlier publication . The TN/TP dry deposition ratios (i.e., F TN‑dry / F TN‑dry , calculated with F dry = C i V d , molar values) range from 0.905 to 79.0 with an average value of 15.5 ± 14.1 over QOMS.…”

“…TSP samples ( n = 75) were collected on precombusted (450 °C for 6 h) Whatman quartz fiber filters at a flow rate of 1.05 m 3 min –1 (standard conditions) from March 23, 2017, to March 19, 2018. Generally, 23 h samples were collected each week, except in April 2017, when they were taken daily as this period is expected to correspond to the peak in BB emissions from South Asia . After sampling, each filter was folded, wrapped with aluminum foil, and stored in a separate bag under 4 °C.…”

“…Major water-soluble ions (Na + , NH 4 + , Mg 2+ , Ca 2+ , Cl – , K + , etc.) and levoglucosan analysis were performed following the protocol described in ref . Briefly, a 4.02 cm 2 filter punch was ultrasonicated in 10 mL of Milli-Q ultrapure water for 30 min.…”

“…For TN, a 1.54 cm 2 filter punch was placed into a tin cup and oxidized at 1000 °C in a combustion chamber and then analyzed using an elemental analyzer (Flash 2000, Thermo Fisher Scientific). The measurement protocol details and data had already been reported in our earlier publication …”