Elsevier

Chemosphere

Volume 55, Issue 8, May 2004, Pages 1127-1142
Chemosphere

Assessment of the photochemistry of OH and NO3 on Jeju Island during the Asian-dust-storm period in the spring of 2001

https://doi.org/10.1016/j.chemosphere.2003.10.003Get rights and content

Abstract

In this study, we examined the influence of the long-range transport of dust particles and air pollutants on the photochemistry of OH and NO3 on Jeju Island, Korea (33.17°N, 126.10°E) during the Asian-dust-storm (ADS) period of April 2001. Three ADS events were observed during the periods of April 10–12, 13–14, and 25–26. Average concentration levels of daytime OH and nighttime NO3 on Jeju Island during the ADS period were estimated to be about 1 × 106 and 2 × 108 molecules cm−3 (∼9 pptv), respectively. OH levels during the ADS period were lower than those during the non-Asian-dust-storm (NADS) period by a factor of 1.5. This was likely to result from higher CO levels and the significant loading of dust particles, reducing the photolysis frequencies of ozone. Decreases in NO3 levels during the ADS period was likely to be determined mainly by the enhancement of the N2O5 heterogeneous reaction on dust aerosol surfaces. Averaged over 24 h, the reaction between HO2 and NO was the most important source of OH during the study period, followed by ozone photolysis, which contributed more than 95% of the total source. The reactions with CO, NO2, and non-methane hydrocarbons (NMHCs) during the study period were major sinks for OH. The reaction of N2O5 on aerosol surfaces was a more important sink for nighttime NO3 during the ADS due to the significant loading of dust particles. The reaction of NO3 with NMHCs and the gas-phase reaction of N2O5 with water vapor were both significant loss mechanisms during the study period, especially during the NADS. However, dry deposition of these oxidized nitrogen species and a heterogeneous reaction of NO3 were of no importance.

Introduction

Both hydroxyl (OH) and hydroperoxyl (HO2) radicals can play a significant role in the chemistry of the daytime troposphere (Wayne, 1991). The former attacks hydrocarbons and most of the reduced sulfur- and nitrogen-containing compounds. The latter plays a pivotal role in the production or destruction of the tropospheric ozone. In relatively clean air, daytime OH is produced primarily by the reaction of the O(1D) radical, which is produced by the photolysis of ozone at wavelengths shorter than 340 nm, with water vapor:O3+hν→O(1D)+O2(λ⩽340nm)H2O+O(1D)→2OHA recent field study indicated that in the polluted air mass, the photolysis of nitrous acid (HONO) was an important source of OH in the early morning hours (Alicke et al., 2002):HONO+hν→OH+NO(λ⩽400nm)Reactions of HO2 with NO and O3 are important secondary sources of OH:HO2+NOOH+NO2HO2+O3OH+2O2The HO2 radical is produced primarily by the reactions of CO and hydrocarbons with OH as well as photolysis of aldehydes (HCHO, CH3CHO, etc.):CO+OHCO2+HH+O2+MHO2+MMeanwhile, a number of ground-based field campaigns have been performed including OH and/or HO2 radical measurements in rural or even remote environments (Atlas and Ridley, 1996; Armerding et al., 1997; Mount and Williams, 1997; Fischer et al., 1998; Plass-Dülmer et al., 1998; Carslaw et al., 1999; Kanaya et al., 2000; Brauers et al., 2001; Cantrell et al., 2003; Geyer et al., 2003; Holland et al., 2003; Mauldin et al., 2003). Peak daytime OH radical concentrations at rural sites were about 1 × 107 molecules cm−3 at the surface in the mid-latitudes during the summer (Creasey et al., 2001; Holland et al., 2003). Hydroxyl radical concentrations in the marine boundary layer (MBL) during the summer showed similar magnitude (about 2 × 106 molecule cm−3) at Mace Head (Carslaw et al., 1999), over the Southern Pacific Ocean (Mauldin et al., 1998) and at Hawaii (Eisele et al., 1996). A comparison of observations with model calculations showed relatively good agreement for HOx (OH + HO2) under high NOx (NO + NO2) conditions, whereas the model overestimated the measurements by about 50% under low NOx conditions (Mauldin et al., 1998; Carslaw et al., 1999). Mauldin et al. (1998) speculated that model overestimation might be in part due to a lack of heterogeneous losses of the HOx species in the model. If the heterogeneous losses of HOx can play a significant role in the OH removal processes, its concentration can be influenced during dust-storms because of the enhancement of dust particle loading.

The nitrate radical (NO3) plays an important role in the chemistry of the nighttime troposphere, serving as an oxidizing agent for various hydrocarbons and as a medium for the removal of NOx in the form of nitric acid (Wayne, 1991; Wayne et al., 1991). The nitrate radical is produced by the reaction of NO2 with O3:NO2+O3NO3+O2Nitrate radical levels in polluted MBL are generally higher compared to those in the clean MBL. For example, Allan et al. (2000) observed at locations in Ireland and Tenerife, that NO3 ranges from 1 to 5 pptv in clean marine air while it increases up to 40 pptv in semi-polluted continental air masses. In the case of polluted air mass, nitrate radicals were mainly removed indirectly through reactions of N2O5, either in the gas phase or on aerosol surfaces. Reactions of NO3 with unsaturated hydrocarbons also contributed significantly to its loss (Carslaw et al., 1997; Allan et al., 1999; Geyer et al., 2001b). In clean marine air masses, dimethyl sulfide (DMS) was the dominant scavenger of NO3 (Carslaw et al., 1997; Allan et al., 2000).

It is speculated that the atmospheric photochemistry of chemical species in remote MBL can be affected by the long-range transport of dust particles accompanied by significantly high loadings of anthropogenic pollutants. In East Asia, dust-storms tend to originate in desert areas of northwestern China during the spring; their impact has been observed not only in nearby Asian countries (e.g., Korea and Japan) but also as far away as the Aleutians and North America (Jaffee et al., 1999). SO2 and NO levels on Jeju Island (formerly known as Cheju Island) were found to be as high as a few ppbv in the atmospheric boundary layer (ABL) during the Asian-dust-storm (ADS) period when the continental outflow of air pollutants from polluted Asian regions occurs (Kim et al., 1998). Photochemical oxidation of atmospheric species can be facilitated during the ADS period because of the enhancement of nitrogen oxide, CO, SO2, and particulate load. DMS oxidation by NO3 can be significant during the ADS period, compared to the remote MBL where OH mainly regulates its oxidation.

In this paper, we present a stepwise analysis of the photochemistry of OH and NO3 using ground-based measurement data sets obtained on Jeju Island (see Fig. 1) during the ACE-Asia. First, we summarize the observational data sets used in this study. Secondly, detailed descriptions of the model used for the calculations of OH and NO3 concentrations are presented. Thirdly, the issues of OH and NO3 chemistry in the case of high NOx environments are explored in relation to the impact of the ADS on their photochemistry. Finally, we also attempt to account for the factors governing their photochemistry in the ABL and to determine the contribution of the ADS on OH and NO3 levels. To our knowledge, this study is the first attempt to elucidate the photochemistry of OH and NO3 on Jeju Island in relation to ADS events.

Section snippets

Observational data

This study is based on the measurements of several atmospheric trace gases and relevant meteorological parameters made during an intensive ground-based field study for the Asian Pacific Regional Aerosol Characterization Experiment (ACE-Asia). As part of ACE-Asia, an intensive field campaign took place off the coast of Gosan (formerly known as Kosan), Jeju (33.17°N, 126.10°E) in the month of April 2001 (Kim et al., 2004).

In this field program, we measured trace gases such as O3, NO, NO2, CO, SO2

Model description

A photochemical box model (PCBM) was employed to generate diurnal profiles of OH and NO3. For the PCBM, a pseudo-steady-state approximation (PSSA) was used to calculate concentrations of relatively short-lived species (OH, NO, HO2, CH3O2, etc.). This approach was expected to lead to a reasonable representation of the short-lived species. The system of equations representing each species in the model was solved iteratively using a Gauss–Seidel method (Burden and Faires, 1989). The PCBM of a full

Comparison of the chemical composition of the air between ADS and NADS periods

The time series of O3, NO2, DMS, and SO2 in the ABL in the month of April 2001 are given in Fig. 2. A discussion on CO was not considered due to the instability of its measurements as mentioned in Section 2. Fig. 3 and Table 3 show diurnal variations of these species and wind speed/direction and observed concentrations of these species during the ADS events and NADS period, respectively. Three ADS events were observed during our study period which include: (1) 1306 LST (local sun time) 10 April

Conclusions

The photostationary state method was used to examine the diurnal variation of OH and NO3 under conditions affected by the long-range transport of air pollutants as well as dust particles from the East Asian continent during ADS events. Three ADS events were observed at our study site in Gosan, Jeju Island during the ACE-Asia experiment. Short-term (21 days) observations of O3, NO2, DMS, and SO2 in the MBL indicated that DMS and SO2 concentrations were different between the ADS and NADS periods,

Acknowledgements

This research was supported by a Korean Science and Engineering Foundation (KOSEF) Grant (R05-2002-000-00017-0). The authors would like to thank NOAA/ARL and NOAA/CMDL for providing the HYSPLIT model, meteorological data, and solar radiation radiometer data. The authors would like to thank Dr. Han at NIER, Korea for providing data on the ionic composition of aerosols. The authors would like to thank Dr. Swan at AGAL, Australia for providing DMS data. The authors would like to thank Dr. Chuang

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