Synergistic Uptake by Acidic Sulfate Particles of Gaseous Mixtures of Glyoxal and Pinanediol

Development of an enhanced method for atmospheric carbonyls and characterizing their roles in photochemistry in subtropical Hong Kong

Carbonyl compounds are ubiquitous and play vital roles in tropospheric photochemistry and oxidation capacity, particularly affecting radical cycling and ozone formation. An enhanced method based on ultra-high-performance liquid chromatography and electrospray ionization tandem mass spectrometry was developed to simultaneously quantify 47 carbonyl compounds with carbon (C) numbers from 1 to 13. Applying this new method to samples collected at ground and ship-borne field campaigns, we investigated the abundance, characteristic distribution, and photochemical activity of carbonyl species in the coastal atmosphere of Hong Kong. The total concentration of detected carbonyls ranged from 9.1 to 32.7 ppbv and exhibited distinct spatial variability. In addition to the usual abundant carbonyl species (formaldehyde, acetaldehyde, and acetone), aliphatic saturated aldehydes with C ≥ 5 (particularly hexaldehyde and nonanaldehyde), and di‑carbonyls, exhibit significant abundance and photochemical reactivity in the coastal site and on the sea. The measured carbonyls could contribute to an estimated peroxyl radical formation rate of 1.88-8.43 ppb/h via OH oxidation and photolysis, greatly enhancing oxidation capacity and radical cycling. The ozone formation potential (OFP) estimated from the maximum incremental reactivity (MIR) was dominated (69 %-82 %) by formaldehyde and acetaldehyde, with significant contribution (4 %-13 %) from di‑carbonyls. Furthermore, another dozens of long-chain carbonyls without MIR values, which were typically below detection or not included in the conventional analytical method, would increase the ozone formation rate by an additional 2 %-33 %. Additionally, the glyoxal, methylglyoxal, benzaldehyde and other α, β-unsaturated aldehydes also exhibited considerable contribution to secondary organic aerosol (SOA) formation potential. This study highlights the importance of various reactive carbonyls in the atmospheric chemistry of urban and coastal regions. The newly developed method can effectively characterize more carbonyl compounds and advance our understanding of their roles in photochemical air pollution.

Humidity Dependence of the Condensational Growth of α-Pinene Secondary Organic Aerosol Particles

The influence of relative humidity (RH) on the condensational growth of organic aerosol particles remains incompletely understood. Herein, the RH dependence was investigated via a series of experiments for α-pinene ozonolysis in a continuously mixed flow chamber in which recurring cycles of particle growth occurred every 7 to 8 h at a given RH. In 5 h, the mean increase in the particle mode diameter was 15 nm at 0% RH and 110 nm at 75% RH. The corresponding particle growth coefficients, representing a combination of the thermodynamic driving force and the kinetic resistance to mass transfer, increased from 0.35 to 2.3 nm² s^-1. The chemical composition, characterized by O:C and H:C atomic ratios of 0.52 and 1.48, respectively, and determined by mass spectrometry, did not depend on RH. The Model for Simulating Aerosol Interactions and Chemistry (MOSAIC) was applied to reproduce the observed size- and RH-dependent particle growth by optimizing the diffusivities D_b within the particles of the condensing molecules. The D_b values increased from 5 α^-1 × 10^-16 at 0% RH to 2 α^-1 × 10^-12 cm^-2 s^-1 at 75% RH for mass accommodation coefficients α of 0.1 to 1.0, highlighting the importance of particle-phase properties in modeling the growth of atmospheric aerosol particles.