William C. Keene

Evidence of inorganic chlorine gases other than hydrogen chloride in marine surface air

We report the first measurements of inorganic chlorine gases in the marine atmosphere using a new tandem mist chamber method. Surface air was sampled during four days including one diel cycle in January, 1992, at Virginia Key, Florida. Concentrations of HCl* (including HCl, ClNO3, ClNO2, and NOCl) were in the range 40 to 268 pptv and concentrations of Cl2* (including Cl2 and any HOCl not trapped in the acidic mist chamber) were in the range <26 to 254 pptv Cl. Concentrations of Cl2* increased during the night, and decreased after sunrise as HCl* concentrations increased by similar amounts. The measurements suggest an unknown source of either HOCl or Cl2 to the marine atmosphere. Photochemical model calculations indicate that photolysis of the observed Cl2* would yield a chlorine atom (Cl•) concentration of order 104–105 cm−3. Oxidation by Cl• would then represent a significant sink for alkanes and dimethylsulfide (DMS) in the marine boundary layer. The cycling of Cl• could provide either a source or a sink for O3, depending on NOX levels.

Global chlorine emissions from biomass burning: Reactive Chlorine Emissions Inventory

Emissions of reactive chlorine-containing compounds from nine discrete classes of biomass burning were estimated on a 1° latitude by 1° longitude grid based on a biomass burning inventory for carbon emissions. Variations on approaches incorporating both emission ratios relative to CO and CO2 and the chlorine content of biomass burning fuels were used to estimate fluxes and associated uncertainties. Estimated, global emissions are 640 Gg Cl yr -1 for CH3Cl; 49 Gg Cl yr -1 for CH2Cl2; 1.8 Gg Cl yr -1 for CHCl3; 13 Gg Cl yr -1 for CH3CCl3; and 6350 Gg Cl yr -1 for the sum of volatile- inorganic and particulate chlorine. Biomass burning appears to be the single largest source of atmospheric CH3Cl and a significant source of CH2Cl2; contributions of CHCl3 and CH3CCl3 are less than 2% of known sources.

Composite global emissions of reactive chlorine from anthropogenic and natural sources: Reactive Chlorine Emissions Inventory

Emission inventories for major reactive tropospheric CI species (particulate CI, HC1, C1NO2, CH3CI, CHCI3, CH3CCI3, C2C14, C2HC13, CH2C12, and CHCIF2) were integrated across source types (terrestrial biogenic and oceanic emissions, sea-salt production and dechlorination, biomass burning, industrial emissions, fossil-fuel combustion, and incinera- tion). Composite emissions were compared with known sinks to assess budget closure; relative contributions of natural and anthropogenic sources were differentiated. Model cal- culations suggest that conventional acid-displacement reactions involving Sov)+O3, S(Iv)+ H202, and H2SO4 and HNO3 scavenging account for minor fractions of sea-salt dechlorina- tion globally. Other important chemical pathways involving sea-salt aerosol apparently pro- duce most volatile chlorine in the troposphere. The combined emissions of CH3CI from known sources account for about half of the modeled sink, suggesting fluxes from known sources were unde:estimated, the OH sink was overestimated, or significant unidentified sources exist. Anthropogenic activities (primarily biomass burning) contribute about half the net CH3CI emitted from known sources. Anthropogenic emissions account for only about 10% of the modeled CHCl3 sink. Although poorly constrained, significant fractions of tropo- spheric CH2C12 (25%), C2HC13 (10%), and C2C14 (5%) are emitted from the surface ocean; the combined contributions of C2C14 and C2HC13 from all natural sources may be substan- tially higher than the estimated oceanic flux.

Tropospheric budget of reactive chlorine

Reactive chlorine in the lower atmosphere (as distinguished from chlorofluorocarbon-derived chlorine in the stratosphere) is important to considerations of precipitation acidity, corrosion, foliar damage, and chemistry of the marine boundary layer. Many of the chlorine-containing gases are difficult to measure, and natural sources appear to dominate anthropogenic sources for some chemical species. As a consequence, no satisfactory budget for reactive chlorine in the lower atmosphere is available. We have reviewed information on sources; source strengths; measurements in gas, aqueous, and aerosol phases; and chemical processes and from those data derive global budgets for nine reactive chlorine species and for reactive chlorine as a whole. The typical background abundance of reactive chlorine in the lower tropospheric is about 1.5 ppbv. The nine species, CH3 Cl, CH3 CCl3, HCl, CHClF2, Cl2* (thought to be HOCl and/or Cl2), CCl2 = CCl2, CH2 Cl2 , COCl2 , and CHCl3, each contribute at least a few percent to that total. The tropospheric reactive chlorine burden of approximately 8.3 Tg Cl is dominated by CH3 Cl (≈45 %) and CH3 CCl3 (≈25 %) and appears to be increasing by several percent per year. By far the most vigorous chlorine cycling appears to occur among seasalt aerosol, HCl, and Cl2*. The principal sources of reactive chlorine are volatilization from seasalt (enhanced by anthropogenically generated reactants), marine algae, volcanoes, and coal combustion (natural sources being thus quite important to the budget). It is anticipated that the concentrations of tropospheric reactive chlorine will continue to increase in the next several decades, particularly near urban areas in the rapidly developing countries.

Aerosol pH in the marine boundary layer: A review and model evaluation

Abstract Impacts of sea-salt-aerosol pH on oxidation processes, sulfur cycling, and surface-ocean fertilization are uncertain; estimates vary from pH 9 and the pH-dependence of some transformations is poorly characterized. We modeled these processes under clean and polluted conditions. At pH 8, S (IV) +O 3 in sea salt is the principal S-oxidation pathway. At pH 5.5, S (IV) oxidation by HOCl dominates. Decreased SO 2 solubility at pH 3 slows S (VI) production. The relative contribution of H 2 SO 4(g) scavenging to S (VI) in sea salt increases with decreasing pH. Significant sea-salt dehalogenation is limited to acidified aerosol. Volatilization rates of BrCl and Br 2 do not vary significantly between pH 5.5 and 3, whereas HCl production via acid displacement increases by a factor of 20. At pH 5.5 and 8, virtually all HNO 3 is scavenged by sea salt. Modeled HNO 3 increases at pH 3 but remains substantially lower than particulate NO - 3 . Discrepancies between measurements and modeled results are assessed based on measurement artifacts, uncertainties in rate and equilibrium constants, organic reactants and surface films, and dynamics.

Organic acidity in precipitation of North America

Abstract Organic anions were measured in 16 precipitation events sampled in central Virginia between 25 April and 1 October 1983. Formic and acetic acids contributed 16% of volume weighted free acidity. The decrease in free acidity in stored aliquots was directly proportional to the disappearance of dissociated HCOO− and CH3COO−. The loss of free acidity between pH measured at field sites and at central laboratories was used to estimate dissociated organic acidity in samples collected by the National Atmospheric Deposition Program (NADP) and the MAP3S Precipitation Chemistry Project. Based on the arithmetic means of the volume weighted averages for each site, we estimate that organic acids contributed 18–35% of free acidity in NADP samples and 16% of free acidity in MAP3S samples. The loss of free acidity in MAP3S samples was three times greater during the growing season than it was during the rest of the year. Because they are rapidly assimilated by microbes, organic acids are unimportant in the long-term acidification of the environment.

Chemical and physical characteristics of nascent aerosols produced by bursting bubbles at a model air‐sea interface

inorganic aerosol constituents were similar to those in ambient air. Ca 2+ was significantly enriched relative to seawater (median factor = 1.2). If in the form of CaCO3, these enrichments would have important implications for pH-dependent processes. Other inorganic constituents were present at ratios indistinguishable from those in seawater. Soluble organic carbon (OC) was highly enriched in all size fractions (median factor for all samples = 387). Number size distributions exhibited two lognormal modes. The number production flux of each mode was linearly correlated with bubble rate. At 80% RH, the larger mode exhibited a volume centroid of 5-mm diameter and included 95% of the inorganic sea-salt mass; water comprised 79% to 90% of volume. At 80% RH, the smaller mode exhibited a number centroid of 0.13-mm diameter; water comprised 87% to 90% of volume. The median mass ratio of organic matter to sea salt in the smallest size fraction (geometric mean diameter = 0.13 mm) was 4:1. These results support the hypothesis that bursting bubbles are an important global source of CN and CCN with climatic implications. Primary marine aerosols also influence radiative transfer via multiphase processing of sulfur and other climate-relevant species.

Natural emissions of chlorine‐containing gases: Reactive Chlorine Emissions Inventory

Although there are many chlorine-containing trace gases in the atmosphere, only those with atmospheric lifetimes of 2 years or fewer appear to have significant natural sources. The most abundant of these gases are methyl chloride, chloroform, dichloromethane, perchloroethylene, and trichloroethylene. Methyl chloride represents about 540 parts per trillion by volume (pptv) Cl, while the others together amount to about 120 pptv Cl. For methyl chloride and chloroform, both oceanic and land-based natural emissions have been identified. For the other gases, there is evidence of oceanic emissions, but the roles of the soils and land are not known and have not been studied. The global annual emission rates from the oceans are estimated to be 460 Gg Cl/yr for CH3Cl, 320 Gg Cl/yr for CHCl3, 160 Gg Cl/yr for CH2Cl2, and about 20 Gg Cl/yr for each of C2HCl3, and C2Cl4. Land-based emissions are estimated to be 100 Gg Cl/yr for CH3Cl and 200 Gg Cl/yr for CHCl3. These results suggest that the oceans account for about 12% of the global annual emissions of methyl chloride, although until now oceans were thought to be the major source. For chloroform, natural emissions from the oceans and lands appear to be the major sources. For further research, the complete database compiled for this work is available from the archive, which includes a monthly emissions inventory on a 1° × 1° latitude-longitude grid for oceanic emissions of methyl chloride.

Seasonal transition from NOx‐ to hydrocarbon‐limited conditions for ozone production over the eastern United States in September

Concentrations of O3, CO, NO, total reactive nitrogen oxides (NOy), H2O2, and HCHO were measured from September 4 to October 1, 1990, at a mountain ridge site in Shenandoah National Park, Virginia. The data show evidence for a transition from NOx-limited to hydrocarbon-limited conditions for O3 production over the course of September. The transition is diagnosed by large decreases of the H2O2/(NOy-NOx) and HCHO/NOy concentration ratios, weakening of the correlation between O3 and NOy- NOx concentrations, and decrease of the slope ΔO3/Δ(NOy-NOx). A high-O3 episode occurring in late September was associated with only 0.34 ppbv H2O2, indicative of hydrocarbon-limited conditions. A seasonal transition in photochemical regime over the eastern United States in September would be expected from theory; the production rate of odd hydrogen radicals decreases by a factor of 2 over the course of the month, due to decreasing UV radiation and humidity, allowing HNO3 production to become the dominant sink for odd hydrogen in the boundary layer and resulting in hydrocarbon-limited conditions for O3 production. Seasonal decline of isoprene emission can greatly accentuate the transition.

Heterogeneous sulfur conversion in sea-salt aerosol particles: the role of aerosol water content and size distribution

Abstract Meteorological and chemical conditions during the July 1988 Bermuda-area sampling appear to have been favorable for conversion of sulfur gases to particulate excess sulfate (XSO4). Observed average XSO4 and SO4 concentrations of 11 and 2.1 nmol m−3, respectively, at 15 m a.s.l. in the marine boundary layer (MBL) upwind of Bermuda, indicate that conversion of SO2 to XSO4, over and above homogeneous conversion, may be necessary to explain the > 5.0 average molar ratio of XSO4 to SO2. Given an observed cloud cover of Aerosol water content, estimated as a function of particle size distribution plus consideration of SO2 mass transfer for the observed particle size distribution, shows that SO2 was rapidly transferred to the sea-salt aerosol particles. Assuming that aqueous-phase SO2 reaction kinetics within the high pH sea-salt aerosol water are controlled by O3 oxidation, and considering mass-transfer limitations, SO2 conversion to XSO4 in the sea-salt aerosol water occurred at rates of approximately 5% h−1 under the low SO2 concentration, Bermuda-area sampling conditions. All of the 2 nmol XSO4 m−3 associated with sea-salt aerosol particles during low-wind-speed, Bermuda-area sampling can be explained by this conversion mechanism. Higher wind speed, greater aerosol water content and higher SO2 concentration conditions over the North Atlantic are estimated to generate more than 4 nmol XSO4 m−3 by heterogeneous conversion of SO2 in sea-salt aerosol particles.