Composition and temporal behavior of ambient ions in the boreal forest

Abstract

A recently developed atmospheric pressure interface mass spectrometer (APi-TOF) measured the negative and positive ambient ion composition at a boreal forest site. As observed in previous studies, the negative ions were dominated by strong organic and inorganic acids (e.g. malonic, nitric and sulfuric acid), whereas the positive ions consisted of strong bases (e.g. alkyl pyridines and quinolines). Several new ions and clusters of ions were identified based on their exact masses, made possible by the high resolution, mass accuracy and sensitivity of the APi-TOF. Time series correlograms aided in peak identification and assigning the atomic compositions to molecules. Quantum chemical calculations of proton affinities and cluster stabilities were also used to confirm the plausibility of the assignments. Acids in the gas phase are predominantly formed by oxidation in the gas phase, and thus the concentrations are expected to vary strongly between day and night. This was also the case in this study, where the negative ions showed strong diurnal behavior, whereas the daily changes in the positive ions were considerably smaller. A special focus in this work was the changes in the ion distributions occurring during new particle formation events. We found that sulfuric acid, together with its clusters, dominated the negative ion spectrum during these events. The monomer (HSO<sub>4</sub><sup>−</sup>) was the largest peak, together with the dimer (H<sub>2</sub>SO<sub>4</sub>·HSO<sub>4</sub><sup>−</sup>) and trimer ((H<sub>2</sub>SO<sub>4</sub>)<sub>2</sub>·HSO<sub>4</sub><sup>−</sup>). SO<sub>5</sub><sup>−</sup> also tracked HSO<sub>4</sub><sup>−</sup> at around 20% of the HSO<sub>4</sub><sup>−</sup> concentration at all times. During the strongest events, also the tetramer and a cluster with the tetramer and ammonia were detected. Quantum chemical calculations predict that sulfuric acid clusters containing ammonia are much more stable when neutral, thus the detection of a single ion cluster implies that ammonia can be an important compound in the nucleation process. We also believe to have made the first observations of an organosulfate (glycolic acid sulfate) in the gas phase. This ion, and its cluster with sulfuric acid, correlates with the HSO<sub>4</sub><sup>−</sup>, but peaks in the early afternoon, some hours later than HSO<sub>4</sub><sup>−</sup> itself. A list of all identified ions is presented in the supplementary material, and also a list of all detected masses not yet identified

Proton affinities of candidates for positively charged ambient ions in boreal forests

“…The experimental measurements of ambient ions in boreal forests by Ehn et al (2010) included both positively and negatively charged ions. The molecules that were subjected to study were selected based on the molecular formula of the (uncharged) compounds identified in the measured positive mass spectra.…”

“…2. Largest calculated proton affinities (green squares) for each compound and the mean of 30 min average concentrations over the measurement period (black circles; Ehn et al, 2010). The value of the x axis is merely an index referring to the legend.…”

“…These methods were applied to several molecular ion species, which were selected based on the experimental measurements of ambient ions in boreal forests by Ehn et al (2010). Although the National Institute of Standards and Technology (NIST) WebBook contains data of over 40 000 compounds, several PAs with possible atmospheric relevance are missing.…”

Formation of Low Volatility Organic Compounds and Secondary Organic Aerosol from Isoprene Hydroxyhydroperoxide Low-NO Oxidation

“…The uncertainties in the fitted model parameters (m L2 = [k wall , γ SOA , Y] T ) were evaluated using the statistics of nonlinear regression. 21 We obtain 95% confidence limits on the model parameters by: 21 σ = 1.96 diag(Cov(m L2 )) 1/2 [6] We estimate the covariance matrix (inverse Hessian) for the fitted model parameters as: 21 Cov(m L2 ) ≈ s 2 (J(m L2 ) T J(m L2 )) −1 [7] S8 where Cov(m L2 ) is the covariance matrix for a the model results linearized about m L2 , s is the measurement standard deviation, and J(m L2 ) is the Jacobian matrix (dr/dm) of the residual vector r, calculated about the least-squares solution m L2 using finite differences.…”

“…5 The FIXCIT inlet was used in the calibrations to maintain the same f inlet values. Malonic acid (C 3 H 4 O 4 ) was the most sensitive organic compound tested and was used as the primary LVOC calibrant since it is thought to cluster at rates close to the collision limit 6 . The calibration factor calculated from Equation 3 using the malonic acid k ion measurements and FIXCIT f inlet of 7% is C = 7.9 x 10 10 molec.…”