Dataset: Western Pacific Aerosol Data

We measured the concentrations of total nitrogen (TN), nitrate isotopes (¹⁵N), ammonium isotopes (¹⁵N), oxygen isotope (¹⁷O and ¹⁸O), and a suite of ion concentrations (calcium (Ca²⁺), magnesium (Mg²⁺), sodium (Na⁺), and potassium (K⁺), Chloride (Cl^-) and sulfate (SO₄^2-)). TN measurements were conducted at the Center for Isotope Geochemistry and Geochronology (CIGG) at the Qingdao National Laboratory for Marine Science and Technology (QNLM) in Qingdao, China. All other measurements were conducted at the Department of Earth, Environmental, and Planetary Sciences at Brown University in Providence, RI (USA).

TN was measured using an elemental analyzer (Elementar vario Isotope select) with an analytical standard deviation of ± 0.04 % for TN (n = 6). After the frozen samples were received at Brown University, a 5 × 5 cm square was cut from the center of the filter at room temperature and extracted in a pre-cleaned bottle with ~100 mL of MQ water (exact amounts were recorded via weight) and then sonicated for 1 h. After sonication, the filters were removed, and the samples were frozen at 20 ◦C until further analyses.

 The concentration of ammonium, nitrate, and nitrite were determined colorimetrically (indophenol blue) using a discrete UV–Vis analyzer (Westco Smartchem 200). U.S. EPA Compliant Methods 350.1 (O’Dell, 1993), 353.2 (Revision 2.0), and 354.1 were followed for ammonium, nitrate, and nitrite, respectively. Standard lab protocols were followed, including calibration to standards as well as blanks and in-house quality control measurements. The pooled standard deviation (1σ) for ammonium was 0.6 μmol L ^-1, and 0.3 μmol L ^-1 for nitrate and nitrite.

The denitrifier method (Casciotti et al., 2002; Sigman et al., 2001) was used to complete nitrate (¹⁵N /¹⁴N) and oxygen (¹⁸O /¹⁶O) isotope analyses on a continuous flow isotope ratio mass spectrometer. Values were reported in ‰ relative to standards. The azide method (Zhang et al., 2007) was used to complete ammonium (¹⁵N /¹⁴N) isotope analyses on a continuous flow isotope ratio mass spectrometer. Values were reported in ‰ relative to standards (IAEA-N₂ and USGS-25) and the pooled standard deviation for duplicates(n = 13 pairs) and replicates (n = 20) was 0.3 ‰ and 0.5 ‰, respectively.

Concentrations of calcium, magnesium, sodium, and potassium were measured on an Inductively Coupled Plasma (ICP) atomic emission spectrometer. Samples were filtered with 0.45-μm filters and an aliquot of each sample solution was made 2% acidic with nitric acid. A commercial standard (IV-4) for each specific element was run roughly every 10 samples. The relative standard deviation for the standards was ≤3.3% for all four elements. Chloride and sulfate concentrations were measured on an Ion Chromatography (IC) instrument (Dionex Integrion HPIC) using suppressed conductivity detection. Anions were determined using a Dionex AS19-4 μm guard (4 × 50 mm) and analytical column (4× 250 mm) with 20 mM KOH as eluent with a flow rate of 1 mL min^-1. All samples were filtered with 0.45-μm filters before being analyzed. In-house quality control standards were run approximately every six samples. The pooled standard deviation for duplicates was 2.0 μmol for both chloride and sulfate (n = 13 pairs and 12 pairs, respectively). The relative standard deviation for the low and high concentration QCs were ≤6.4% and <3.4%, respectively.

Instruments:

High-volume aerosol sampler (model 2031, Qingdao Laoying Environmental Technology Co) operating at 1.0 m³ min ^-1 with 25 × 20 cm filters.

Elemental analyzer: the concentrations of TN were measured using an elemental analyzer (Elementar vario Isotope select).

Discrete UV–Vis analyzer (Westco Smartchem 200): for ammonium, nitrate, and nitrite concentrations.

Continuous Flow Isotope Ratio Mass Spectrometer: for ammonium, nitrate, and oxygen isotope values.

Inductively Coupled Plasma (ICP) atomic emission spectrometer: to measure concentrations of calcium, magnesium, sodium, and potassium.

Ion Chromatography (IC) (Dionex Integrion HPIC): to measure chloride and sulfate concentrations.