Understanding Marine Boundary Layer Aerosol Budget using Airborne Flux Measurement

Description

According to the IPCC Sixth Assessment Report (AR6), aerosol-induced cloud adjustments remain one of the largest uncertainties in climate modeling. Cloud properties are most sensitive to particle addition when background concentrations are low, making the sources, sinks, and abundance of cloud condensation nuclei (CCN) over marine regions a critical uncertainty in aerosol-cloud systems. Understanding the relative contribution of different sources to the MBL aerosol budget is crucial, as uncertainties in background marine aerosol characterization significantly impact anthropogenic aerosol effect predictions.

The Azores archipelago provides an ideal location to study MBL aerosol and CCN number budgets due to its remote marine environment and frequent long-range aerosol transport. Data were collected during the Aerosol and Cloud Experiments in the Eastern North Atlantic (ACE-ENA) aircraft campaign. The ARM site provided airborne measurements of aerosol distribution and cloud microphysics during two intensive observation periods (June-July 2017 and January-February 2018). Two Condensation Particle Counters (CPCs) measured particle concentrations greater than 3 nm and 10 nm in diameter, respectively. The 3-10 nm particle concentration was calculated as the difference between these measurements. The Aircraft Integrated Meteorological Measurement System probe provided vertical wind velocity data. Additional particle size distribution measurements (PSD) were also collected.

We used the ogive optimization method to analyze airplane data for 3-10 nm particle fluxes.The ogives were created for each horizontal lag through cumulative summation of co-spectral energy from the highest frequencies. Only fluxes that captured both high and low-frequency ends of the turbulent energy spectrum were used to estimate the aerosol number budget. Flux values (both magnitude and sign) at different heights helped identify source locations of 3-10 nm particles within the MBL. The average positive fluxes of 3-10 nm particles were 1560.1 ± 1992.7 cm-2 s-1, while the average negative flux of 3-10 nm particles was -762.04 ±  513.2 cm-2  s-1. Box model results showed these particles contributed 20-50% to the MBL aerosol number budget during the campaign. Back trajectory analysis identified North America as the probable source of aerosol precursor gases, advancing our understanding of the MBL aerosol budget.