Variation characteristics of carbon monoxide and ozone over the course of the 2014 Chinese National Arctic Research Expedition

LI Bokun, BIAN Lingen*, ZHENG Xiangdong, DING Minghu &XIE Zhouqing*

1 Institute of Polar Environment, University of Science and Technology of China, Hefei 230026, China;

2 Chinese Academy of Meteorological Sciences, Beijing 100081, China

1 Introduction

Carbon monoxide is a major pollutant in the atmosphere.Over-exposure to carbon monoxide can directly cause adverse health effects. It is also one of the indirect greenhouse gases in the atmosphere[1]. The carbon monoxide in the atmosphere originates via emissions arising from the combustion of fossil fuels and biomasses as well as the photochemical transformation of certain hydrocarbons (e.g.methane)[2-3]. The removal of carbon monoxide from the atmosphere relies on a reaction with a hydroxyl radical (OH)[4].Thus, by changing the concentration of hydroxyl radicals in the troposphere, natural or anthropogenic disturbances of tropospheric carbon monoxide can influence the chemical properties of the troposphere[5-6]. The lifespan of tropospheric carbon monoxide varies with latitude and season, ranging from several weeks to over one year. The lifespan is mostly determined by the tropospheric hydroxyl radical concentration[7-9].

Ozone is also one of the pollutants in the troposphere as well as a major greenhouse gas[10-12]. The tropospheric ozone derives from both natural inputs via the stratosphere and photochemical processes involving hydrocarbons and nitrogen oxides (NOx) in the troposphere[13]. The nitrogen oxides and volatile organic matters from vehicle exhaust and industrial emission are the main precursors of ozone generated from chemical reactions in the troposphere[14]. Because of the high oxidizability of ozone, it is closely connected with many chemical processes, especially photochemical reactions with greenhouse gases[15-16].

Continuous measurements of the concentrations of carbon monoxide and ozone in the marine boundary layer were carried out aboard the R/VXUE LONGicebreaker.The measurements may provide the latest data on the spatial variations of carbon monoxide and ozone in the marine boundary layer from mid-latitude seas to the northern highlatitude sea including the Arctic Ocean. Moreover, these data can aid in the understanding of the relationship between changes of the tropospheric ozone in the Arctic region and the climate change at mid- and high-latitudes[17].

2 Experimental methods

2.1 The expedition

The R/VXUE LONGicebreaker, the R/V of the 6th Chinese National Arctic Research Expedition, departed from Shanghai harbor on July 11th 2014 and returned on September 22nd 2014. Various scientific investigations were carried out during the expedition. The course of the journey is shown in Figure 1. The cruise path covered the East China Sea, the Sea of Japan, the Sea of Okhotsk, the Bering Sea,the Chukchi Sea, the Beaufort Sea, and the Arctic Ocean.

2.2 In-situ measurements

The carbon monoxide measurements were performed using an internet-enabled automatic infrared carbon monoxide analyzer (model EC9830T, Ecotech Inc., Australia). The ozone measurements were performed using an internetenabled automatic ozone analyzer (model EC9810B, Ecotech Inc., Australia). The analyzers had been calibrated before departure based on the NIST standard system.

2.3 Air mass back trajectory

Air mass back trajectory was performed according to the transmission diffusion model of HYSPLIT (Hybrid Single-Particle Lagrangian Integrated Trajectory) developed by the Air Resources Laboratory of the United States National Oceanic and Atmospheric Administration. These calculations were carried out to identify the source of the air masses[18-19].

3 Results and discussion

As the analyzers were installed at the bow of R/VXUE LONGicebreaker and the engine of R/VXUE LONGicebreaker was at the stern, the measurements were not affected by the icebreaker’s emission on most occasions. However the data collected when wind was coming from the stern were removed in order to eliminate the pollution coming from the stack.

3.1 The temporal and spatial variations of carbon monoxide

Carbon monoxide concentration measured during the 6th Chinese National Arctic Research Expedition ranged between 47.00 and 528.52 ppbv with an average of 103.59 ±40.37 ppbv. The time series is shown in Figure 2.

The whole cruise is divided into six sections, and the average, maximum, and minimum carbon monoxide concentration of each section is shown in Table 1.

As shown in Table 1, the concentrations of carbon monoxide were high over the East China Sea and the Sea of Japan. The maximum (528.52 ppbv) of the expedition was observed on the northward course over the East China Sea;for the southward leg of the journey, the maximum (220.12 ppbv) was also observed over the East China Sea. Besides the East China Sea and the Sea of Japan, observations of carbon monoxide concentrations in the other sections, such as the Sea of Okhotsk, the Bering Sea, and the Arctic Ocean, were at a relatively low level. The minimum (47.00 ppbv) was observed over the Arctic Ocean.

The concentration of carbon monoxide in the atmosphere is greatly affected by human activities. Consequently, the concentrations were much higher over the East China Sea and the Sea of Japan than other sections, such as the Sea of Okhotsk, the Bering Sea, and the Arctic Ocean. After R/VXUE LONGicebreaker left the Sea of Japan for the highlatitude seas, the concentrations sharply decreased to very low levels and were relatively stable (Figure 3).

In order to investigate the potential influence due to the air mass, 7-day back trajectories of the respective carbon monoxide maximum observed over the northward and the southward legs of the journey were performed using the HYSPLIT model (Figure 4). As shown in Figure 4, the back trajectories imply that the air masses passed through Northeast Asia. These regions have intensive industrial activities. Industrial and vehicle exhaust emissions may have contributed greatly to the maximum observed over both the northward and the southward courses. For comparison,another back trajectory was performed (Figure 5), where the minimum was observed. The result indicates that the air mass mainly passed through the Arctic Ocean and the Northwest Pacific Ocean. This represents the marine backgroud. As a result, the concentration observed at this site was close to the average over this section.

The diurnal variation of carbon monoxide concentration over different sections is shown in Figure 6. The amplitude of the variation was relatively low except for values collected over the East China Sea. The amplitude over the East China Sea reached up to 400 ppbv. This may be due to the relatively small number of samples and the intense anthropogenic emission. The amplitude over the Sea of Japan was as high as 60 ppbv. While this is higher than the other sections, it is far lower than over the East China Sea. If the data from the East China Sea is excluded, the amplitudes of the remaining sections were all lower than 20 ppbv. The amplitude over the Arctic Ocean was only 10 ppbv. The carbon monoxide concentrations in most sections reach their maximum around midnight. This may be the result of reduced sunlight which leads to a decrease in the photochemical reactions that remove carbon monoxide in the atmosphere. No double-peak was found over the pelagic seas as reported in a previous study of urban atmospheric carbon monoxide[20]. This discrepancy is probably due to a lack of anthropogenic emission. The departure and the return of R/VXUE LONGicebreaker were both near noontime, and the associated sampling at noon may have resulted in the maximum observation over the East China Sea.

3.2 The temporal and spatial variations of ozone

Ozone concentration measured during the 6th Chinese National Arctic Research Expedition ranged between 3.27 and 77.82 ppbv with an average of 29.47±10.48 ppbv. The time series is shown in Figure 7.

The whole course is divided into six sections according to latitudes and distaces to the coasts. The average, maximum,and minimum ozone concentration of each section is shown in Table 2.

Similar to the concentrations of carbon monoxide, the concentrations of ozone were very high over the East China Sea and the Sea of Japan (Table 2). The maximum (77.82 ppbv) of the expedition was observed on the northward course over the East China Sea. For the southward leg of the journey,the maximum (74.01 ppbv) was also observed over the East China Sea. Besides the East China Sea and the Sea of Japan,observed ozone concentrations in the other sections, such as the Sea of Okhotsk, the Bering Sea, and the Arctic Ocean,were relatively low. The minimum (3.27 ppbv) was observed over the Arctic Ocean.

Table 2 Ozone concentration in different sections

The concentration of ozone in the atmosphere is not directly affected by human activities, but it is affected by the carbon monoxide, precursors of ozone (e.g. nitrogen oxides), sunlight, and the oxidative ability of the atmosphere.However, the relatively high concentrations of ozone observed over the East China Sea and the Sea of Japan could still indicate the impact of intensive human activity on the concentration of ozone. The 7-day back trajectories imply that the high values were affected by continental air masses (Figure 4). There is an evident decrease in the ozone concentration with increasing latitude over the mid-latitude seas. When the R/VXUE LONGicebreaker entered high-latitude seas, there was a reduction in the rate of decrease (Figure 8).

The diurnal variation of ozone concentration over different sections is shown in Figure 9. Similar to carbon monoxide, the amplitude of the variation was relatively low except for values over the East China Sea. The amplitude over the East China Sea reached up to 60 ppbv, which may be due to the relatively small number of samples and intensive anthropogenic emission. If the data from the East China Sea is excluded, the amplitudes of the other sections were all lower than 10 ppbv. The amplitude was only 5 ppbv over the Arctic Ocean. The ozone concentration in most sections started to decrease immediately after the sunrise and reaching their minimum around noon. The daytime ozone losses in the marine boundary layer over these sections maybe due to not only the photochemistry involving NOXcycles but also by the sunrise ozone destruction which seems to be caused by the halogen species, such as Br. The halogen species in the marine boundary layer may be released from sea-salt particles reacting with pollutants. If the Br2released from sea-salts is accumulated in the marine boundary layer in the nighttime, Br radical which destructs ozone catalytically is easily formed by the feeble solar radiation just after sunrise[12].The observed maximum over the East China Sea was at noon because that was the time the R/VXUE LONGicebreaker departed from and returned to the harbor with its high levels of anthropogenic emission.

3.3 The correlation between carbon monoxide and ozone

The analysis of the relationship between carbon monoxide and ozone could provide information about the source of the air masses[21-22]. Figure 10 shows the correlation between carbon monoxide and ozone.

In most sections, such as the East China Sea, the Sea of Japan, the Sea of Okhotsk, and the Arctic Ocean, there were positive correlations between carbon monoxide and ozone(r= 0.34-0.60). This indicates that the ozone observed during the expedition likely originated from photochemical reactions of carbon monoxide[23-24]. The regression slopes for the East China Sea and the Sea of Japan are relatively lower than those of the Sea of Okhotsk and the Arctic Ocean due to the anthropogenic emissions in Northeastern Asia[25].

However, the positive correlation was weak (r= 0.09) in the Bering Sea section. This suggests an air mass source quite distant from the surface CO emission sources. In addition,the tropopause heights are relatively low in the high latitude regions. This indicates a possible downward transport of the lowermost stratospheric ozone into the troposphere[26].

4 Conclusion

The concentrations of carbon monoxide and ozone in the marine boundary layer were measured aboard the R/VXUE LONGicebreaker using internet-enabled analyzers. Samples were taken during the 6th Chinese National Arctic Research Expedition from July to September 2014.

Carbon monoxide concentration measured during the journey ranged between 47.00 and 528.52 ppbv with an average of 103.59 ± 40.37 ppbv. The concentrations over midlatitude and coastal seas were much higher than over highlatitude and pelagic seas, probably owing to anthropogenic emission. The maximum was observed over the East China Sea, and almost all the extremely high values were observed over coastal seas. The concentration over pelagic seas was much lower than over the East China Sea and the Sea of Japan, and the amplitudes of the diurnal variations were also smaller. Ozone concentration measured during the cruise ranged between 3.27 and 77.82 ppbv with an average of 29.46± 10.48 ppbv. There was a gradient of ozone concentration outside the Arctic Ocean which decreased sharply with the increasing latitude (during both the northward and the southward courses). However, no significant variation was observed over the Arctic Ocean. Similar to carbon monoxide,the diurnal variation over high-latitude seas and the Arctic Ocean was very small. The positive correlation between carbon monoxide and ozone in most sections suggests that the ozone in the marine boundary layer mainly originated from photochemical reactions involving carbon monoxide.However, the weak correlation over the Bering Sea may be affected by the input of stratospheric ozone.

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