Impact of Sea Breeze on the Transport of Ship Emissions: A Comprehensive Study in the Bohai Rim Region, China

Abstract

Air pollutants from ship exhaust have a negative impact on air quality in coastal areas, which can be greatly exacerbated by sea breeze circulation. However, our understanding of this issue is still limited, especially in coastal areas with a complex topography and winding coastlines, such as the Bohai Rim region in China. In order to fill this knowledge gap, the Weather Research and Forecast model coupled with the chemistry (WRF/Chem) modeling system was employed to investigate the influence of sea breeze circulation on the transport of PM2.5 emitted by ships from April to September in 2014. The major findings are as follows: (1) The concentration of PM2.5 due to ship emissions was 2.94 μg/m3 on days with a sea breeze and 2.4 times higher than on days without a sea breeze in coastal cities in the region. (2) The difference in the contribution of ship emissions during days with a sea breeze and days without a sea breeze decreases with increasing distance from the coastline but remains non-negligible up to 50 km inland. (3) The shape of the coastline, the topographic height of the land area, and the latitude have a significant impact on sea breeze circulation and thus on the transport of ship emissions. (4) The differences in the contribution of ship emissions under days with a sea breeze versus days without a sea breeze were more evident than those under onshore versus alongshore and offshore winds, indicating that sea breeze circulation can cause cyclic accumulation of pollutants and thus reinforce the impact of ship emissions on coastal air quality more than by onshore winds. It should be emphasized that during the switching from sea breeze to a non-sea breeze, the pollutants that have been transported to the land area by sea breeze have not yet been carried back to sea, resulting in the ship contribution value still not significantly reduced even if the wind is a non-sea breeze at that moment. In addition, other factors e.g., emissions, precipitation, and chemistry can also play an important role in the observed trends in this study.

Impact of Ship Emissions on Pm2.5 in the Yangtze River Delta, China, With an Emphasis on the Onshore Airflow

“…We compared the WRF/Chem simulation results with the observational data using several statistical metrics, including correlation coefficient ( R ), mean absolute error (MAE), normalized mean bias (NMB), normalized mean error (NME), mean fractional bias (MFB), mean fractional error (MFE), mean bias (MB), and mean absolute error (MAE). The formulas for these metrics were provided in our previous publication (Ma et al, 2022).…”

“…The WRF/Chem model has been extensively applied to simulations of atmospheric compositions. The methods used in this study were similar to those in Ma et al (2022). Two scenarios (with and without ship emissions) were simulated to assess the impact of ship emissions on PM 2.5 in coastal cities under onshore wind conditions.…”

“…Based on a study of four sites, Lv et al (2018) found that during the onshore flow, ship emissions caused an average increase in PM 2.5 over land of 4.7%–17.1%, which was 1.8–2.7 times higher than during the remainder of the day. Our previous studies in the Bohai Rim showed that ship emissions contributed 3.17 µg m –3 to the PM 2.5 concentrations over the entire land area during onshore winds, while PM 2.5 concentrations were only 1.87 µg m –3 during non-onshore winds (Ma et al, 2022). However, these studies were only based on specific time scales and at specific stations.…”

“…Ship emissions are an important factor that influences the air quality in coastal areas (Ma et al, 2022; Zheng et al, 2023). The impact of ship emissions on air pollutants is not only concentrated in coastal areas, but it is also significant in inland areas.…”

“…The impact of ship emissions on air pollutants in coastal areas is more pronounced in the presence of onshore air currents. Previous studies have shown that onshore air currents can exacerbate the negative effects of ship emissions on coastal air quality (Liang et al, 2016; Liu et al, 2017; Lv et al, 2018; Ma et al, 2022). For example, a study by Liu et al (2017) found that onshore air currents significantly affected Shanghai's coastal and riverine areas within tens of kilometres of the shore, with the contributions of ship emissions to PM 2.5 ranging from 2 µg m –3 to 7 µg m –3 (20%–30%).…”