Mohammad Aslam Khan Khalil

Core chemical theory is combined with transport processes and local emissions to study the validity of commonly made assumptions regarding steady states to interpret urban and regional ozone data. It is shown that photo-stationary states of NO and NO₂ cannot exist in urban areas or polluted regions in addition to the lack of such a state for ozone. Calculations in published papers, which make the assumption of a photo-stationary state for NO and NO₂, or ozone, are likely to be inaccurate by unacceptable amounts. The Leighton Ratio is re-interpreted to show how it incorporates both the peroxy radical and transport effects on ozone, separating local chemical production from its transport. To illustrate the concepts and test the assumptions, hourly observations of ozone and its precursors from Yanbu in Saudi Arabia are analyzed. The major role of local transport processes and a large import of ozone at night, probably from aloft, are uncovered as a by-product.

Yanbu, on the Red Sea, is an affluent Saudi Arabian industrial city of modest size. Substantial effort has been spent to balance environmental quality, especially air pollution, and industrial development. We have analyzed six years of observations of criteria pollutants O₃, SO₂, particles (PM_2.5 and PM₁₀) and the known ozone precursors—volatile organic compounds (VOCs) and nitrogen oxides (NO_x). The results suggest frequent VOC-limited conditions in which ozone concentrations increase with decreasing NO_x and with increasing VOCs when NO_x is plentiful. For the remaining circumstances ozone has a complex non-linear relationship with the VOCs. The interactions between these factors at Yanbu cause measurable impacts on air pollution including the weekend effect in which ozone concentrations stay the same or even increase despite significantly lower emissions of the precursors on the weekends. Air pollution was lower during the Eids (al-Fitr and al-Adha), Ramadan and the Hajj periods. During Ramadan, there were substantial night time emissions as the cycle everyday living is almost reversed between night and day. The exceedances of air pollution standards were evaluated using criteria from the U.S. Environmental Protection Agency (EPA), World Health Organization (WHO), the Saudi Presidency of Meteorology and Environment (PME) and the Royal Commission Environmental Regulations (RCER). The latter are stricter standards set just for Yanbu and Jubail. For the fine particles (PM_2.5), an analysis of the winds showed a major impact from desert dust. This effect had to be taken into account but still left many occasions when standards were exceeded. Fewer exceedances were found for SO₂, and fewer still for ozone. The paper presents a comprehensive view of air quality at this isolated desert urban environment. Implications: Frequent VOC-limited conditions are found at Yanbu in Saudi Arabia that increase ozone pollution if NO_x is are reduced. In this desert environment, increased nightlife produces the highest levels of VOCs and NO_x at night rather than the day. The effects increase during Ramadan. Fine particles peak twice a day—the morning peak is caused by traffic and increases with decreasing wind, potentially representing health concerns, but the larger afternoon peak is caused by the wind, and it increases with increasing wind speeds. These features suggest that exposure to pollutants must be redefined for such an environment.

It is observed that side by side plots in rice fields, managed by the same practices, produce methane emissions that are often different by factors of 2 to 4. Similarly on a given day when many plots are sampled, the emissions from one plot may differ from another by factors of 3–4 on average. These large variations must be taken into account if these data are extrapolated to larger scales such as countries or the world. In this paper we analyze and delineate the nature of this variability. We distinguish temporal and spatial variability and examine the effects of both on the observed seasonal average emission rates. While temporal variability is managed well in most experiments because frequent measurements are taken, the spatial variability remains a potential major uncertainty in many experiments. Using the characteristics of the observed variability we use simulations to show that with a proper and realistic sampling strategy, it is possible to get to within 15%–20% of the true flux for most cases of interest. We calculate how many plots are needed and how often they should be sampled during the growing season to obtain a seasonal average flux within prescribed limits relative to the true flux.

For more than 20 years the global emissions and the lifetime of methane have probably been constant, so the buildup of methane in the atmosphere has been slowing down for as long. During this time, there have been periodic events occurring every seven to eight years, when global methane concentrations increased by some 10 ppb and later fell back, in some cases due to temporary increases of emissions from the northern tropics that spread to the global scale. These conclusions are derived from the accumulated global observations that now span 23 years and define the role of human activities in the recent cycle of atmospheric methane.

Collectively, man-made emissions of a few greenhouse gases may cause about the same amount of global warming as increasing carbon dioxide. Among the most potent of these non-CO₂ greenhouse gases are the perfluorocarbons that have extraordinarily long atmospheric lifetimes of 10 000 to more than 50 000 yr. We report atmospheric concentrations over two decades, between 1978 and 1997, of the three most abundant perfluorocarbonsCF₄, C₂F₆, and C₃F₈and delineate the sources that account for the present abundances and trends. We show that C₂F₆ and C₃F₈ are present at only 2.9 and 0.2 pptv, respectively. CF₄ is the most abundant perfluorocarbon at 74 pptv (in 1997) of which about 40 pptv are from natural emissions, 33 pptv from aluminum manufacturing, and 1 pptv from the semiconductor industry. The increasing trend of CF₄ has slowed in recent years due to the major reductions in the emission rate per ton of aluminum produced. The effect of the falling emission factor is partially offset by increased production and increasing use by the semiconductor industry.