Isoprene, a significant biogenic volatile organic compound (VOC), impacts global climate through cloud formation or inhibition. While terrestrial sources dominate, marine isoprene influences secondary organic aerosol (SOA) formation, particularly during phytoplankton blooms. Marine isoprene's biogenic origin stems primarily from phytoplankton, though heterotrophic bacteria and seaweeds also contribute. Emission estimates vary widely due to limited continuous in-situ measurements and incomplete understanding of production and loss mechanisms. Photochemical reactions in the sea surface microlayer (SML) may contribute to discrepancies. This study uses satellite observations and model simulations to investigate the spatial and temporal distribution of marine isoprene emissions, quantify emission trends, and assess the impact on air quality and climate, focusing on the western Pacific and eastern Indian Oceans.

Previous research on marine isoprene emissions has employed both bottom-up (e.g., using laboratory measurements and extrapolation) and top-down (e.g., atmospheric inversion techniques) approaches, yielding estimates differing by orders of magnitude. The paucity of continuous, in-situ measurements and the complex interplay of factors influencing isoprene production and loss (phytoplankton biomass, phytoplankton functional types, light, temperature, biological and chemical consumption in the water column, and air-sea gas exchange) have contributed to these discrepancies. While some studies have highlighted the role of photochemical reactions in the SML, others have found no significant correlation between isoprene fluxes and shortwave radiation, indicating the complexity of the phenomenon and the need for further investigation.

A marine isoprene emission model was developed, incorporating a PFT-specific isoprene production module dependent on light and temperature. The 3-D temperature structure in the euphotic zone was reconstructed from remotely sensed sea surface temperature (SST), sea surface height (SSH), and wind stress (WS). Dynamic mixed layer depth was derived using a temperature threshold method. Isoprene production in the mixed layer was quantified by integrating production at all depths within the mixed layer. SML isoprene flux was calculated by estimating photochemical isoprene production within the SML. Himawari-8 L3 data (chlorophyll-a (Chl-a) concentrations, SST, photosynthetically active radiation (PAR)) were used for model simulation. The Mann-Kendall (MK) test and Theil-Sen estimator were used for trend analysis. Sensitivity tests were conducted to assess the model's response to various driving parameters (Chl-a, SST, PAR, emission factor (EF), euphotic layer depth (Hmax), and mixed layer depth (DML)).

The study revealed significantly higher marine isoprene emission fluxes in the CEPO (7.4 nmol m⁻²h⁻¹) and the Tasman Sea (8.7 nmol m⁻²h⁻¹) compared to other regions. The mean marine isoprene emission flux was 6.5 nmol m⁻²h⁻¹, yielding a global estimate of 1.2 Tg C yr⁻¹. Remarkably, CEPO showed a significant increasing trend in marine isoprene emission flux (5.5 ± 0.1% yr⁻¹ from August 2015 to December 2020), with no significant trends in other open ocean areas. Positive correlations were found between isoprene emission flux and Chl-a concentration, PAR, and wind speed in CEPO, while a negative correlation existed with SST. Analysis of aerosol number size distributions and satellite-observed aerosol optical depth (AOD) showed positive correlations with isoprene fluxes, suggesting isoprene's importance in SOA formation. In CEPO, NO2 column densities appeared critical in controlling SOA yields from isoprene oxidation; higher NO2 led to faster AOD enhancements. Lower ozone abundances were observed when both high marine isoprene emission flux and AOD were observed in CEPO, suggesting suppression of ozone formation in NOx-limited conditions.

The findings highlight the previously unrecognized significance of marine isoprene emissions in CEPO and its substantial increasing trend. The positive correlations between isoprene emission flux and AOD suggest a substantial contribution of isoprene to SOA formation, particularly when NO2 is abundant. The negative correlation between isoprene flux and SST in tropical and subtropical regions, contrasted with positive correlations in high-latitude regions, suggests complex temperature-dependent mechanisms and likely variation in phytoplankton community composition. The observed decrease in tropospheric ozone in CEPO coincident with high isoprene emissions points to complex interactions in the atmospheric chemistry of this region. The strong El Niño event of 2015/16, which coincided with high isoprene emissions in CEPO, suggests a potential link between these events and the production of organic aerosols in the upper troposphere.

This study reveals unexpectedly high and increasing marine isoprene emissions in the CEPO, underscoring the need to refine global isoprene emission inventories. The critical role of NO2 in isoprene-derived aerosol formation is demonstrated. Future studies should expand measurements across broader tropical oceans to enhance our understanding of marine organic gas emissions' spatial distribution and climate impacts. Further investigation into the underlying mechanisms driving the observed trends in CEPO is warranted.

The study's reliance on satellite-derived data and model simulations introduces inherent uncertainties. The model's accuracy depends on the accuracy of input parameters such as chlorophyll-a concentration, SST, and PAR. Extrapolating regional findings to global estimates necessitates caution. The study's focus on a limited time period may not fully capture long-term trends. A more comprehensive understanding requires continuous in-situ measurements and improved quantification of isoprene production and loss processes across a range of environmental conditions.