High Solubility of so 2 Printer-friendly Version Interactive Discussion High Solubility of so 2 : Evidence in an Intensive Fog Event Measured in the Ncp Region, China Acpd High Solubility of so 2 Printer-friendly Version Interactive Discussion

Introduction Conclusions References Tables Figures Back Close Full Screen / Esc Atmospheric Chemistry and Physics Discussions This discussion paper is/has been under review for the journal Atmospheric Chemistry and Physics (ACP). Please refer to the corresponding final paper in ACP if available. Abstract Introduction Conclusions References Tables Figures Back Close Full Screen / Esc Abstract A field experiment was conducted in a heavy SO 2 pollution area located in north China plain (NCP). During the experiment, SO 2 and other air pollutants, liquid water content (LWC) of fog droplets, and basic meteorological parameters were measured. During the experiment, an intensive fog event occurred between 5 and 8 November 2009. 5 During the fog period, the concentrations of SO 2 showed a strong variability, and the variability was closely correlated to the appearances of fogs and LWC. For example, the averaged concentration of SO 2 during the non-fog period was about 25 ppbv. By contrast, during the fog period, the concentration of SO 2 reduced to about 4–7 ppbv. The large reduction of SO 2 suggests that a majority of SO 2 (about 70–80%) had been 10 converted from gas-phase to aqueous-phase, showing a high solubility of SO 2. However , according to the value of Henry Law constant, the solubility of SO 2 is modest, which cannot explain the measured large reduction of SO 2. This study highlights that the aqueous reactions of SO 2 in the droplets of fogs play important roles to enhance the solubility of SO 2. To account for the effect of aqueous reactions on the solubility 15 of SO 2 , an " effective " Henry Law constant of SO 2 is proposed in this study. The study shows that without considering aqueous reactions of SO 2 in fog droplets, the estimate of the partitioning of SO 2 in droplets is significantly lower than the measured values. By contrast, when the " effective " Henry Law constant is applied in the calculation, the calculated SO 2 concentrations are significantly improved, showing that the aqueous 20 reactions of SO 2 play important roles in controlling the solubility of SO 2 , and should be considered in model calculations.


Introduction
Sulfur dioxide (SO 2 ) can be released to the atmosphere due to both natural and anthropogenic emissions.The natural emissions include volcanic eruptions and biomass burnings.The anthropogenic emissions are mainly due to coal and oil burning Figures processes, accounting for more than 75% of global emissions (Chin et al., 2000).Coal accounts for about 70% of China's total energy consumption, and the consumption of coal increases rapidly in the recent years due to the aggressive economical development in the country, with the consumption of coal of 2.01 billion metric tons in 2009 (IEA, 2010).The large consumption of coal in China leads to high SO 2 emissions, resulting in the high concentrations of SO 2 (Kawamoto et al., 2004).One of the major fates of SO 2 gas is to form sulfate particles, and understanding the formation of sulfate particles in the troposphere is critical because of their effects on human health and climate (Zhao et al., 2006;Tie et al., 2003Tie et al., , 2009)).Thus, studying the budget (such as emission, transformation, and deposition) of SO 2 becomes a crucial environmental issue in China.Sulfur dioxide is a soluble gas in water, and can be oxidized within cloud and fog droplets, producing sulfuric acid.In this study, a field experiment was conducted in a heavy SO 2 pollution area which is located in north China plain (NCP).During the experiment, SO 2 and other air pollutants, liquid water content (LWC) of fog droplets, and basic meteorological parameters were measured.A large variability of SO 2 and liquid water content of fog droplets were observed, providing a good opportunity to study the solubility of SO 2 in the in-situ field measurement.The study of solubility of SO 2 has important implication for understanding the aqueous oxidation of SO 2 in fog droplets, which significantly influence the acidity in fogs.Because the acidity in clouds, precipitation, and fogs is a central environmental focus in China due to its rapidly economical development, the direct measurement of solubility of SO 2 can provide important information for the environmental issues mentioned above.
The paper is organized as the following way.In Sect.2, we describe the field experiment, including the instruments and the data.In Sect.

Information of the experiment
The field experiment took placed in the north edge of Tianjin City (39.4 • N in latitude and 117.05 • E in longitude) at a surface site.The surface site is about 30 km from the center of the city.The city of Tianjin is a mega city with a population of 10 million.
The city is located in the heart of North China Plain (NCP), and it is one of the populated and polluted regions in China (see Fig. 1).During the recent years, the rapid increase of economical development results in heavy atmospheric pollution, especially SO 2 pollution in this region (Tie and Cao, 2009;Zhang et al., 2010).
Figure 1 shows the horizontal distribution of SO 2 concentrations measured by the SCIAMACY (SCanning Imaging Absorption spectroMeter for Atmospheric Cartogra-pHY), and the horizontal distribution of fogs measured by MODIS (Moderate Resolution Imaging Spectroradiometer).Figure 1 shows that the fog formation is closely correlated to the high concentrations of SO 2 in the NCP region.The coherence between the high SO 2 concentrations and the frequent fog formation provides a good opportunity to study SO 2 solubility by analyzing in-situ measurements of SO 2 concentrations and fog droplets in the NCP region.Thus, an intensive field measurement was conducted during a period of the fog season (from 5 to 8 November 2009) to study the variability of SO 2 and its correlation to LWC during the formation of fogs.Several instruments were deployed during the field experiment, including; (1) Fog Droplet Measurement Technology (FDMT) for fog droplet measurement, with diameters ranging from 2 to 32 µm (15 size bins), (2) Scan Mobility Particle Sizer (SMPS) for aerosol particle measurement, with particle diameters ranging from 10 to 662 nm (71 size bins), (3) the instruments of air pollutants (O 3 , CO, NO x , and SO 2 ), and (4) the instruments for basic meteorological fields (such as temperature, relative humidity, air pressure, winds, etc).
These integrated instruments provide necessary information to study the solubility of SO 2 in fog droplets.In order to insure the quality of the instruments (QA/QC issue), all the instruments were calibrated before the measurement.The detailed information of calibration is described by Zhang et al. (2010).2934 Introduction

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Background of meteorological conditions during experiment
During the experiment, an intensive fog event occurred between 5 and 8 November 2009.During the fog event, the NCP region was under the effect of a weak low pressure system, with calm winds.The averaged surface wind speed was only 1.1 m/s during the event.The averaged surface temperature was low (6.8 • C) and humidity was high with a large variation.These weather and meteorological conditions were very favorable for the formation of fogs.After the fog event (the late of 8 November), the low pressure system moved out of the NCP region.As a result, the averaged surface wind speed rapidly increased to about 4 m/s, and the averaged surface temperature increased to about 15 During the fog event, a periodic formation of fog occurred.Based on the duration of fog occurrences in the event, three fog periods are defined according to the combined characteristics of observed relative humidity (RH), ambient temperature (T ), fog liquid water content (LWC), and the range of visibility (VIS).The first fog period (Fog-1) occurred from 03:45 to 10:42, 6 November.The second fog period (Fog-2) was appeared from 19:10, 6 November to 12:27, 7 November, and the third fog period (Fog-3) occurred from 16:29, 7 November to 05:40, 8 November (see Fig. 2).The detailed analysis of the effects of meteorological conditions on the formation of fogs is shown in Quan et al. (2011).

Measurement of air pollutants
One of the important information for occurrence of fogs is the appearance of liquid water.The measurement shows that liquid water appeared during the 3 fog periods, with the averaged liquid water content (LWC) of 0.23, 0.35, and 0.35 g/m 3 , respectively (as indicated in Fig. 2).before, during, and after the 3 fog appearances.During the three fog periods, there are several important aspects which are highlighted as follows.
(1) The visibility range rapidly changed during the occurrence of fogs.For example, the mean visibility range was about 870 m during the non-fog periods, which was relatively low, indicating that the concentrations of aerosols were high during the entire measurement (a heavy haze event).During the 3 fog periods, the visibility ranges rapidly reduced to 20-50 m, suggesting that the fog occurrence resulted in an extremely low visibility event.Because the occurrence of extremely low visibility events was closely correlated to the formation of fogs, it can be used as an indirect evidence for the occurrence of fogs in the NCP region.
(2) The measured SO 2 concentrations showed a strong variability, and the variability was closely correlated to the appearance of fogs and LWC, which shows the strong effect of LWC on the soluble gas of SO 2 .For example, the averaged concentration of SO 2 during the non-fog period was about 25 ppbv.By contrast, the concentrations of SO 2 during the 3 fog periods were rapidly reduced to 6.8, 4.4, 7.2 ppbv, respectively.
(3) In order to better understand the causes of the large reduction of SO 2 concentrations during the appearance of fogs and LWC, the measured variability of CO is analyzed (in the middle panel of Figure 2).The main difference between CO and SO 2 is that CO is not a soluble gas in water, while SO 2 is a soluble gas.Unlike the variability of SO 2 , there was no clear evidence that CO variability was correlated to the occurrence of fogs and LWC.For example, the averaged concentration of CO during the non-fog period was 5.2 ppmv.Unlike the concentrations of SO 2 , during the 3 fog periods, the changes in the concentrations of CO were not significant, with the mean concentrations of 4.8, 5.9, 7.6 ppmv, respectively.In addition, there was also an indication that the variability of CO concentrations was more sensitive to wind speeds rather than the formation of fogs.For example, after the fog period, the averaged wind speed increased from 1.1 to 4.1 m/s, leading to a sharp decrease of CO concentration (from 10 to 2 ppmv in a few hours).The complete different behaviors between the variability of CO and SO 2 during the fog periods suggest that SO 2 significantly reduced by dissolving gas-phase SO 2 in fog droplets during the formation of fogs.Introduction

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Full More quantitative analysis of the effect of fog water on SO 2 concentrations is shown in Fig. 3.It shows that during the 3 fog periods, the visibility ranges reduced by 94, 96, and 96%, respectively.There were no consistent changes in CO concentrations, with −8, +13, and +46% variations during the 3 fog periods, respectively.By contrast, the concentrations of SO 2 consistently reduced by −72, −82, and −71% during the 3 fog periods, respectively.The large reduction of SO 2 indicated that a majority of SO 2 (about 70-80%) had been converted from gas-phase to aqueous phase, showing a high solubility of SO 2 .However, according to the value of Henry Law constant (the measure of solubility) of SO 2 , the solubility of SO 2 is modest (with the Henry Law constant of 10 2 -10 3 dependent on pH values), which is much less than the highly soluble species such as H 2 O 2 (the Henry Law constant is 2.4 × 10 5 ).As a result, the measured high solubility of SO 2 during the fog periods cannot be explained by the Henry Law constant.Other processes (such as aqueous reactions of SO 2 in water) could play important roles in controlling the solubility of SO 2 , which need to be carefully studied.

SO 2 solubility
As suggested by Ravishankara (1997), the actual or effective solubility of chemical species in liquid water of clouds/fogs are determined by the following factors; (1) the diffusion rate (ka) for transport gas-phase molecules into water, which is the first step to determine the solubility of chemical species; (2) the disassociation rate of a chemical species in droplets, which is a factor to enhance the solubility of chemical species; and (3) the rate of aqueous reaction of chemical species in droplets, which can further increase the solubility of chemical species.
Figure 4 shows the schematic description of the enhancement of solubility of SO 2 due to aqueous phase reactions.The Henry Law constant of chemical species is a combination result of the diffusion rate (ka) and composition of a chemical species, which is modest for SO 2 .The Henry Law constant combined with aqueous phase reactions can enhance the solubility of SO 2 , which can be referred as an "Effective 2937 Figures

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Full Henry Law constant" as indicated in Fig. 4. Figure 4 shows that when the molecules of SO 2 are transported (diffused) into a liquid droplet, aqueous reactions convert SO 2 into other chemical species.As a result, more molecules of SO 2 are transported into water droplets, and less molecules of SO 2 are returned back to the atmosphere, leading to the enhancement of solubility of SO 2 .The quantitative calculation of the solubility of SO 2 with the consideration of aqueous phase reaction is described by the following method.
If no aqueous reaction is considered, the solubility of SO 2 is determined by the Henry Law constant, and the partitioning between gas and aqueous phase concentrations is expressed by the following equation; Where SO 2 (g) and SO 2 (aq) represent SO 2 concentrations in gas and aqueous phases, respectively.However, if the aqueous phase reactions of SO 2 in droplets are considered, the calculation of solubility of SO 2 becomes to be complex.According to the study by Seinfeld and Pandis (1998), there are two important aqueous-phase reactions for SO 2 .After SO 2 resolved in water, it can either react with H 2 O 2 or O 3 .Because O 3 is a very low soluble species (with the Henry Law constant of 0.02), the aqueous-phase reaction with O 3 is not significant compared to the reaction with H 2 O 2 .Thus, the calculation of solubility of SO 2 is mainly determined by the following expressions; Where H 2 O 2 (g) and H 2 O 2 (aq) represent H 2 O 2 concentrations for gas and aqueous phase, respectively.These expressions suggest that the solubility of SO (k) between SO 2 (aq) and H 2 O 2 (aq).Because the rate of the aqueous reaction between SO 2 (aq) and H 2 O 2 (aq) is very fast (Seinfeld and Pandis, 1998), it requires more SO 2 (g) to be converted to SO 2 (aq), and the value of SO 2 (aq) is strongly dependent upon the availability of H 2 O 2 (aq).As a result, the solubility of SO 2 is enhanced and the "effective" Henry Law constant of SO 2 can be expressed; Where H e '(SO 2 ) represents the "effective" Henry Law constant of SO 2 , and H e (H 2 O 2 ) represents the Henry Law constant of H 2 O 2 .
The hypothesis of the "effective" Henry Law constant of SO 2 is tested by comparing the calculation with measurement of the aqueous phase partitioning of SO 2 in fog droplets.For the calculation, the measured values of LWC are used (see Fig. 2).The initial values of SO 2 are set to be the measured values before the occurrences of the fogs.During the fog periods, the concentration of SO 2 (g) is set to be 5.0 ppbv which is similar to the measured SO 2 concentrations during the 3 fog periods, and the concentration of H 2 O 2 (g) is set to be 1.0 ppbv which is often measured in the atmosphere.
With the above considerations, the partitioning between gas and aqueous phase concentrations of SO 2 are calculated.The calculation shows that without the consideration of aqueous phase reaction, the calculated concentrations of SO 2 are significantly overestimated the measured values (as indicated in the red lines of Fig. 5).For example, the measured concentrations of SO 2 during the 3 fog periods are about 5 ppbv, while the calculated concentrations are about 20 ppbv that is about 4 times higher than measured values.By contrast, when the "effective" Henry Law constant is applied in the calculation, the calculated SO 2 concentrations are significantly improved.For example, the calculated minimum concentrations of SO 2 are close to the measured values (as indicated in the blue lines of Fig. 5), showing significant reduction compared to the case when the aqueous phase reaction is not considered.Although the calculation with the "effective" Henry Law constant considerably improves the calculated concentrations of SO 2 during fog periods, there are still some discrepancies between the calculation and measurement.For instant, there is a tendency that the calculated SO 2 concentrations 2939 Introduction

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Full have an earlier increase (with a time lag of 2 h) compared to the measured values.In the calculation, the reduction of SO 2 concentration is mainly due to the appearance of LWC.Thus, any uncertainty of measured LWC could cause the errors in the calculation of SO 2 concentration during fog periods.

Summary
Sulfur dioxide (SO 2 ) can be released to the atmosphere due to both natural and anthropogenic emissions.The anthropogenic emissions are mainly due to coal and oil burning processes, and coal accounts for about 70% of China's total energy consumption.The large consumption of coal in China results in high concentrations of SO 2 in the north China plain (NCP) region, leading to severely environmental problems in the region.Thus, studying the budget (such as emission, transformation, and deposition) of SO 2 becomes a crucial environmental issue in China.Sulfur dioxide is a soluble gas in water, and can be oxidized within cloud and fog water droplets, producing sulfuric acid.In this study, a field experiment was conducted in a heavy SO 2 pollution area in NCP.During the experiment, gas-phase SO 2 and other gas-phase air pollutants, water content of fog droplets, and basic meteorological parameters were measured.A large variability of SO 2 and liquid water content (LWC) of fog droplets were observed, providing a good opportunity to study the solubility of SO 2 in the field measurement.
The measurement shows that during 5 to 8 November 2009, there were 3 fog periods, with the averaged liquid water content (LWC) of 0.23, 0.35, and 0.35 g/m 3 , respectively.During the fog periods, the measured SO 2 concentrations showed a strong variability and the variability was closely correlated to the appearance of fogs and LWC.For example, the averaged concentration of SO 2 during the non-fog period was about 25 ppbv.By contrast, the concentrations of SO 2 during the 3 fog periods were rapidly reduced to 6.8, 4.4, 7.2 ppbv, respectively.The large reduction of SO 2 concentrations suggests that SO 2 was significantly reduced by dissolving gas-phase SO 2 into fog droplets during the formation of fogs.The large reduction of SO 2 (about 70-80%) Introduction

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Full during the fog periods also suggests that the solubility of SO 2 is considerably high.However, according to the value of Henry Law constant, the solubility of SO 2 is modest, which cannot explain the large reduction of SO 2 .This measurement result suggests that the aqueous reactions of SO 2 play important roles to enhance the solubility of SO 2 .Thus, an "effective" Henry Law constant of SO 2 is proposed in this study, which accounts for the aqueous reactions in the solubility of SO 2. The study shows that the calculated concentrations of SO 2 are significantly overestimated the measured values without considering the aqueous reactions.By contrast, when the "effective" Henry Law constant is applied in the calculation, the calculated SO 2 concentrations are significantly improved, showing that the aqueous reactions of SO 2 play important roles in controlling the solubility of SO 2 , and should be considered in model calculations.Introduction

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Full Discussion Paper | Discussion Paper | Discussion Paper | 3, we analyze of the result of the experiment to study solubility of SO 2 in fog droplets.
Discussion Paper | Discussion Paper | Discussion Paper | Figure 2 also shows the variability of visibility, CO, and SO 2 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 2 depends on three quantities; (1) the aqueous concentrations of SO 2 (aq) in droplets, (2) the aqueous concentrations of H 2 O 2 (aq) in droplets, and (3) the rate of aqueous reaction Introduction Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper |

Fig. 2 .
Fig. 2. The measured fog liquid water content (red lines in upper panel), the range of visibility (black lines in upper panel), the concentrations of CO (middle panel), and the concentrations of SO 2 (lower panel).The green boxes indicate the 3 fog periods.

Fig. 3 .
Fig. 3.The effects of fog formation on visibility (panel 1), LWC (panel 2), CO concentrations (panel 3), and SO 2 concentrations (panel 4).The dark blue columns show the mean values during the 3 fog periods, and the light blue columns show the mean values during non fog periods.

Fig. 5 .
Fig. 5.The measured SO 2 concentrations (black lines during the non-fog periods, and green lines during the 3 fog periods), and the calculated SO 2 concentrations (red lines -without considering the aqueous phase reactions; blue lines -with considering the aqueous phase reactions).