Sulfuric acid, tropospheric sources

In the last 150 years the anthropogenic emission of sulfur has increased dramatically, primarily due to combustion processes [1]. In the 1950s anthropogenic emission surpassed natural emission and the atmospheric sulfur cycle is one of the most perturbed biogeochemical cycles [1,2]. The oceans are the largest natural source of atmospheric sulfur emissions, where sulfur is emitted in a reduced form, predominantly as dimethyl sulfide (DMS) and to a much lesser extent carbonyl sulfide (OCS) and carbon disulfide (CS2) [3]. Ocean emitted DMS and CS2 are initially oxidised to OCS, which diffuses through the troposphere into the stratosphere where further oxidation to sulfur dioxide (SO2), sulfur trioxide (SO3) and finally sulfuric acid (H2SO4) occurs [1-4]. 

Carbonyl sulfide is also the most abundant reduced sulfur gas in Earth s troposphere, but for completely different reasons. Volcanic sources of OCS are negligible by comparison with biogenic emissions, which are important sources of several reduced sulfur gases (e.g., OCS, H2S, (CH3)2S, (CH3)2S2, and CH3SH) in the terrestrial troposphere. Many of these gases are ultimately converted into sulfate aerosols in the troposphere, but OCS is mainly lost by transport into the stratosphere, where it is photochemically oxidized to SO2 and then to sulfuric acid aerosols, which form the Junge layer at —20 km in Earth s stratosphere. 

Aerosols (suspended particles) can be natural in origin or related to human activity such as combustion (of fossil fuels) or biomass burning. They can be, for example, sea salt, mineral dust, soot, dilute sulfuric acid droplets, and their existence will depend on the proximity of sources and suitable conditions (e.g., windspeed, humidity) for their formation and transport. Aerosols, which can both scatter and absorb, are most concentrated in the lower troposphere (planetary boundary layer) and decrease quickly with altitude. High altitude aerosols are usually insignificant in terms of UV transmission, except in unusual circumstances, such as immediately after a large volcanic eruption such as Mount Pinatubo in 1991. 

The stratospheric aerosol is composed of an aqueous sulfuric acid solution of 60-80% sulfuric acid for temperatures from — 80 to — 45°C, respectively (Shen et al. 1995). The source of the globally distributed, unperturbed background stratospheric aerosol is oxidation of carbonyl sulfide (OCS), which has its sources at the Earth s surface. OCS is chemically inert and water insoluble and has a long tropospheric lifetime. It diffuses into the stratosphere where it dissociates by solar ultraviolet radiation to eventually form sulfuric acid, the primary component of the natural stratospheric aerosol. Other surface-emitted sulfur-containing species, for example, S02, DMS, and CS2, do not persist long enough in the troposphere to be transported to the stratosphere. 

Organic emissions from mobile sources (cars, trucks, planes, and so on) become oxidized in the troposphere and can then assist the formation of particulate secondary organic aerosols. Such small particulate matter can penetrate deep into our lungs and cause acute irritations. It has been reported that carboxylic acids, such as benzoic acid, can form stable complexes with sulfuric acid (H2SO4) in a similar manner that carboxylic acids can form dimers (Sec. 10.2). Suggest a structure for a stable complex between benzoic... 

Although most of the atmospheric particulate mass is confined to the troposphere (region below an altitude of 11 km), the stratospheric aerosol can have significant effects on climate. This subject has been reviewed by Pueschel (1996). The primary source of particulate in the stratosphere (altitude fix>m 11 to SO km) is the formation of sulfuric acid droplets by gas-to-particle conversion of SO2 injected into the stratosphere by major volcanic eruptions. These droplets are formed by homogeneous nucleation involving photochemical reactions of SOj and water vapor. They spread widely over the hemisphere (north or south) in which they originated. 

Both heterogeneous and homogeneous nucleation appear to play roles in ice formation in the upper troposphere. Liquid droplets several microns in radius can be found at temperatures down to about T —35°C [38] (Fig. 2). These are probably concentrated solutions of sulfuric acid, ammonium sulphate, and possibly other nitrogen compounds formed on deliquescent aerosols. Some of the aerosols on which the ice particles nucleate are formed aloft by gas-to-particle conversion in the clear air surrounding clouds ([39]) some originate from volcanos, and some arise from gas and/or particle anthropogenic sources. 

Acid rain arises from the oxidation of S02 and N02 in the troposphere to form sulfuric and nitric acids, as well as other species, which are subsequently deposited at the earth s surface, either in precipitation (wet deposition) or in dry form (dry deposition). The contribution of organic acids has also been recognized recently (see Chapter 8). These oxidation and deposition processes can occur over relatively short distances from the primary pollutant sources or at distances of a fOOO km or more. Thus both short-range and long-range transport must be considered. 

Once the importance of DMS to the global sulfur cycle was established, numerous measurements of DMS concentrations in the marine atmosphere have been conducted. The average DMS mixing ratio in the marine boundary layer (MBL) is in the range of 80-1 lOppt but can reach values as high as 1 ppb over entrophic (e.g., coastal, upwelling) waters. DMS mixing ratios fall rapidly with altitude to a few parts per trillion in the free troposphere. After transfer across the air-sea interface into the atmosphere, DMS reacts predominantly with the hydroxyl radical and also with the nitrate (N03) radical. Oxidation of DMS is the exclusive source of methane sulfonic acid (MSA) in the atmosphere, and the dominant source of S02 in the marine atmosphere. We will return to the atmospheric chemistry of DMS in Chapter 6. 

Acid rain (acidic deposition) Rain or snow or dry particulate matter with a high enough concentration of nitrogen and sulfur oxides to be acid (caused by absorption of these oxides from plumes from terrestrial sources and subsequent chemical reactions in the troposphere. 

---