Transpacific Transport of Asian Peroxyacetyl Nitrate (PAN) Observed from Satellite: Implications for Ozone

Peroxyacetyl nitrate (PAN) is produced in the atmosphere by photochemical oxidation of non-methane volatile organic compounds in the presence of nitrogen oxides (NOx), and it can be transported over long distances at cold temperatures before decomposing thermally to release NOx in the remote troposphere. It is both a tracer and a precursor for transpacific ozone pollution transported from East Asia to North America. Here, we directly demonstrate this transport with PAN satellite observations from the infrared atmospheric sounding interferometer (IASI). We reprocess the IASI PAN retrievals by replacing the constant prior vertical profile with vertical shape factors from the GEOS-Chem model that capture the contrasting shapes observed from aircraft over South Korea (KORUS-AQ) and the North Pacific (ATom). The reprocessed IASI PAN observations show maximum transpacific transport of East Asian pollution in spring, with events over the Northeast Pacific offshore from the Western US associated in GEOS-Chem with elevated ozone in the lower free troposphere. However, these events increase surface ozone in the US by less than 1 ppbv because the East Asian pollution mainly remains offshore as it circulates the Pacific High.

Ship-plume sulfur chemistry: ITCT 2K2 case study

The ship-plume sulfur chemistry was investigated for the ITCT 2K2 (Intercontinental Transport and Chemical Transformation 2002) ship-plume experiment, using the ship-plume photochemical/dynamic model developed in this study. In order to evaluate the performance of the model, the model-predicted mixing ratios of SO2 and H2SO4 were compared with those observed. From these comparisons, it was found that the model-predicted levels were in reasonable agreements with those observed (0.56≤R≤0.71), when the pH of sea-salt particles (pHss) was ≤~6.5. The ship-plume equivalent lifetimes of SO2 (τ(eq)(SO(2))) were also estimated/investigated for this particular ship-plume case. The magnitudes of τ(eq)(SO(2)) were found to be controlled by two main factors: (i) the mixing ratios of in-plume hydroxyl radicals (OH) and (ii) pHss. The former is governed primarily by stability conditions of the marine boundary layer (MBL), when the ship NOx emission rate is fixed. The latter determines if the heterogeneous oxidation of dissolved SO2 occurs via reaction with hydrogen peroxide (H2O2, when pHss<6.5) or with ozone (O3, when pHss>6.5). According to the multiple ship-plume photochemical/dynamic model simulations, the estimated τ(eq)(SO(2)) over the entire ship plumes ranged from 10.32 to 14.32 h under moderately stable (E) to stable (F) MBL conditions. These values were clearly shorter than the background SO2 lifetime (τ(b)(SO(2))) of 15.18-23.20 h. In contrast, τ(eq)(SO(2)) was estimated to be 0.33 h when the pHss remained at ~8.0 (a rather unlikely case). In addition, the SO2 loss budget was further analyzed to estimate the influences of the two main factors on the ship-plume sulfur chemistry. The changes in the loss budget with pHss clearly showed a shift in the dominant SO2 loss processes from heterogeneous SO2 conversion (when pHss>~6.5) to the gas-phase oxidation of SO2 by OH (when pHss<~6.5).

Establishing policy relevant background (PRB) ozone concentrations in the United States

Policy Relevant Background (PRB) ozone concentrations are defined by the United States (U.S.) Environmental Protection Agency (EPA) as those concentrations that would occur in the U.S. in the absence of anthropogenic emissions in continental North America (i.e., the U.S, Canada, and Mexico). Estimates of PRB ozone have had an important role historically in the EPA's human health and welfare risk analyses used in establishing National Ambient Air Quality Standards (NAAQS). The margin of safety for the protection of public health in the ozone rulemaking process has been established from human health risks calculated based on PRB ozone estimates. Sensitivity analyses conducted by the EPA have illustrated that changing estimates of PRB ozone concentrations have a progressively greater impact on estimates of mortality risk as more stringent standards are considered. As defined by the EPA, PRB ozone is a model construct, but it is informed by measurements at relatively remote monitoring sites (RRMS). This review examines the current understanding of PRB ozone, based on both model predictions and measurements at RRMS, and provides recommendations for improving the definition and determination of PRB ozone.

Intercontinental transport of aerosols and photochemical oxidants from Asia and its consequences

The intercontinental transport of aerosols and photochemical oxidants from Asia is a crucial issue for air quality concerns in countries downwind of the significant emissions and concentrations of pollutants occurring in this important region of the world. Since the lifetimes of some important pollutants are long enough to be transported over long distance in the troposphere, regional control strategies for air pollution in downwind countries might be ineffective without considering the effects of long-range transport of pollutants from Asia. Field campaigns provide strong evidence for the intercontinental transport of Asian pollutants. They, together with ground-based observations and model simulations, show that the air quality over parts of North America is being affected by the pollutants transported from Asia. This paper examines the current understanding of the intercontinental transport of gases and aerosols from Asia and resulting effects on air quality, and on the regional and global climate system.