Impact of International Shipping Emissions on Ozone and PM 2.5 : The Important Role of HONO and ClNO 2

Abstract. Ocean-going ships emit large amounts of air pollutants such as nitrogen oxide (NOx) and particulate matter. Ship-released NOx can be converted to nitrous acid (HONO) and nitryl chloride (ClNO2), which produce hydroxyl (OH) and chlorine (Cl) radicals and recycle NOx, thus affecting the oxidative capacity and production of secondary pollutants. However, these effects have not been quantified in previous investigations of the impacts of ship emissions. In this study, a regional transport model (WRF–Chem) revised to incorporate the latest HONO and ClNO2 processes was used to investigate their effects on the concentrations of ROx (RO2+HO2+OH) radicals, O3, and fine particulate matter (PM2.5) in Asia during summer. The results show that the ship-derived HONO and ClNO2 increased the concentration of ROx radicals by approximately two to three times in the marine boundary layer. The enhanced radicals then increased the O3 and PM2.5 concentrations in marine areas, with the ship contributions increasing from 9 % to 21 % and from 7 % to 10 %, respectively. The largest ROx enhancement was simulated over the remote ocean with the ship contribution increasing from 29 % to 50 %, which led to increases in ship-contributed O3 and PM2.5 from 21 % to 38 % and from 13 % to 19 %, respectively. In coastal cities, the enhanced levels of radicals also increased the maximum O3 and averaged PM2.5 concentrations from 5 % to 11 % and from 4 % to 8 % to 4 % to 12 %, respectively. These findings indicate that modeling studies without considering HONO and ClNO2 can significantly underestimate the impact of ship emissions on radicals and secondary pollutants. It is therefore important that these nitrogen compounds be included in future models of the impact of ship emissions on air quality.


; ; n is the number of hour.

Figure S1 .
Figure S1.Measurement sites.(a) Blue dots denote the surface weather stations in NCDC in July 2018.(b) Red dots denote the available surface air-quality monitoring stations operated by MEE in July 2018.

Figure S2 .
Figure S2.(a) Observations of O3 at China MEE station and two marine sites in Japan and modeled O3 mixing ratios with (b) default chemistry and (c) integrated HONO and chlorine chemistry (Unit: ppbv).The red circles in (b) and (c) highlight the O3 simulation in coastal areas.

Figure S3 .
Figure S3.(a) Spatial distribution of averaged NO2 (Unit: ppbv) at the surface.Arrows present the simulated wind vectors from the BASE case.

Figure S4 .
Figure S4.Vertical distribution of averaged (a)(b) HONO (Unit: ppbv) and (c)(d) nighttime ClNO2 (Unit: pptv) at cross-section by 113°E and 31°N, respectively in July 2018 from the BASE case.Arrows and the gray shadow present the topography high from the BASE case.We also highlight the South China Sea (SCS) and Pearl River Delta (PRD) in (a) and Yangzi River Delta (YRD) and East China Sea (ECS) in (b).

Figure S5 .
Figure S5.Spatial distribution of averaged N2O5 (Unit: pptv) during nighttime (18:00-06:00 LST) at the from the BASE case in July 2018.Arrows present the simulated wind vectors from the BASE case.

Figure S6 .
Figure S6.Spatial distribution of averaged fine particulate chloride (Unit: μg m −3 ) at the surface from the BASE case in July 2018.Arrows present the simulated wind vectors from the BASE case.

Figure S8 .
Figure S8.Vertical profiles of O3 variations (Unit: ppbv) from different chemistry in nine regions.

Figure S11 .
Figure S11.Contribution of ship emission to the concentration of detailed aerosol species in oceanic areas and coastal cities. PSO4, PNO3, PNH4 and PPM represent the fine particulate sulfate, fine particulate nitrate, fine particulate ammonia, and primary particulate matter, respectively.Def_ship and ReNOM_ship represents Def-Def_noship and BASE-BASE_noship, respectively.

Table S2 .
Statistics of meteorological variables and air pollutants.
a SIM is the simulated data, OBS is the observational data.bMean Bias (MB) = ∑ (  −  )

Table S3 .
Observational data was obtained from China's Ministry of Ecology and Environment (MEE).Statistics of model performance in different cases for hourly O3 measurements at MEE station and monthly O3 in three marine sites (Unit: ppbv).
c Observational data was obtained from the National Climate Data Center (NCDC).WS10: wind speed at 10 m; T2: temperature at 2 m; RH: specific humidity at 2 m. d a North Central Plain (NCP), b Yangzi River Delta (YRD), c Pearl River Delta (PRD), d Hok Tsui (HT), e Yonagunijima (Yona).

Table S4 .
Comparison between simulated and observed HONO, N2O5, and ClNO2 in coastal and marine sites.
a Observed data was filtered with one-day backward trajectory from ocean.