Isotopic Constraints on the Atmospheric Sources and Formation ofNitrogenous Species in Biomass-Burning-Influenced Clouds

Abstract. The interpretation of tropospheric cloud formation rests on understanding the sources and processes affecting aerosol constituents of the atmosphere that are preserved in cloudwater. However, this challenge is difficult to be quantitatively addressed based on the sole use of bulk chemical properties. Nitrogenous aerosols, mainly ammonium (NH 4 + ) and nitrate (NO 3 − ), play an important role in tropospheric cloud formation. Here we collected cloudwater samples at the summit of Mt. Tai (1545 m above sea level) in Eastern China during a long-lasting biomass burning (BB) event, and measured for the first time the isotopic compositions (mean ± 1σ) of cloudwater nitrogen species (δ 15 N-NH 4 + = −6.53 ± 4.96 ‰, δ 15 N-NO 3 − = −2.35 ± 2.00 ‰, δ 18 O-NO 3 − = 57.80 ± 4.23 ‰), allowing insights into their sources and potential transformation mechanism within the clouds. Large contributions of BB to the cloudwater NH 4 + (32.9 ± 4.6 %) and NO 3 − (28.2 ± 2.7 %) inventories were confirmed through a Bayesian isotopic mixing model, coupled with our newly-developed computational quantum chemistry module. Despite an overall reduction in total anthropogenic NO x emission due to effective emission control actions and stricter emission standards for vehicles, the observed cloud δ 15 N-NO 3 − values suggest that NO x emissions from transportation may have exceeded emissions from coal combustion. δ 18 O-NO 3 − values imply that the reaction of OH with NO 2 is the dominant pathway of NO 3 − formation (57 ± 11 %), yet the contribution of heterogeneous hydrolysis of dinitrogen pentoxide was almost as important (43 ± 11 %).