Anthropogenic Non-methane Hydrocarbons Research Articles

Internal consistency tests for evaluation of measurements of anthropogenic hydrocarbons in the troposphere

Measurements of tropospheric nonmethane hydrocarbons (NMHCs) made in continental North America should exhibit a common pattern determined by photochemical removal and dilution acting upon the typical North American urban emissions. We analyze 11 data sets collected in the United States in the context of this hypothesis, in most cases by analyzing the geometric mean and standard deviations of ratios of selected NMHCs. In the analysis we attribute deviations from the common pattern to plausible systematic and random experimental errors. In some cases the errors have been independently verified and the specific causes identified. Thus this common pattern provides a check for internal consistency in NMHC data sets. Specific tests are presented which should provide useful diagnostics for all data sets of anthropogenic NMHC measurements collected in the United States. Similar tests, based upon the perhaps different emission patterns of other regions, presumably could be developed. The specific tests include (1) a lower limit for ethane concentrations, (2) specific NMHCs that should be detected if any are, (3) the relatively constant mean ratios of the longer‐lived NMHCs with similar atmospheric lifetimes, (4) the constant relative patterns of families of NMHCs, and (5) limits on the ambient variability of the NMHC ratios. Many experimental problems are identified in the literature and the Southern Oxidant Study data sets. The most important conclusion of this paper is that a rigorous field intercomparison of simultaneous measurements of ambient NMHCs by different techniques and researchers is of crucial importance to the field of atmospheric chemistry. The tests presented here are suggestive of errors but are not definitive; only a field intercomparison can resolve the uncertainties.

Convective transport over the central United States and its role in regional CO and ozone budgets

We have constructed a regional budget for boundary layer carbon monoxide over the central United States (32.5°–50°N, 90°–105°W), emphasizing a detailed evaluation of deep convective vertical fluxes appropriate for the month of June. Deep convective venting of the boundary layer (upward) dominates other components of the CO budget, e.g., downward convective transport, loss of CO by oxidation, anthropogenic emissions, and CO produced from oxidation of methane, isoprene, and anthropogenic nonmethane hydrocarbons (NMHCs). Calculations of deep convective venting are based on the method of Pickering et al. [1992a] which uses a satellite‐derived deep convective cloud climatology along with transport statistics from convective cloud model simulations of observed prototype squall line events. This study uses analyses of convective episodes in 1985 and 1989 and CO measurements taken during several midwestern field campaigns. Deep convective venting of the boundary layer over this moderately polluted region provides a net (upward minus downward) flux of 18.1×108kg CO month−1 to the free troposphere during early summer, assuming the June statistics are typical. Shallow cumulus and synoptic‐scale weather systems together make a comparable contribution (total net flux 16.2×108 kg CO month−1). Boundary layer venting of CO with other O3 precursors leads to efficient free tropospheric O3 formation. We estimate that deep convective transport of CO and other precursors over the central United States in early summer leads to a gross production of 0.66–1.1 Gmol O3 d−1 in good agreement with estimates of O3 production from boundary layer venting in a continental‐scale model [Jacob et al., 1993a, b]. In this respect the central U.S. region acts as a “chimney” for the country, and presumably this O3 contributes to high background levels of O3 in the eastern United States and O3 export to the North Atlantic.

A study of the dependence of rural ozone on ozone precursors in the eastern United States

A comprehensive three‐dimensional mesoscale model, with detailed treatment of meteorology and photochemistry, is applied to the eastern United States and southeastern Canada to address the issue of ozone production over rural regions. Simulations with various anthropogenic emission reductions over a 4‐day high‐pressure period, for which the eastern U.S. experienced elevated O3 levels, are compared to the results of a reference case that incorporates the most recent National Acid Precipitation Assessment Program (1985) emissions inventory. Our focus is on the relative effects of anthropogenic NOx versus nonmethane hydrocarbons (NMHC) emission reductions on O3 concentrations within this photochemical regime. The model results are consistent with earlier studies that predict a reasonably high O3 sensitivity to NMHC emission reductions, and O3 increases for NOx emission reductions in regions of strong urban influence. However, the total area that exhibits these distinctly urban characteristics comprises less than 10% of the continental area within the model. O3 production for the majority of the eastern United States is found to be limited by NOx availability rather than NMHC. Thus the sensitivity to NOx emission reductions averaged over the continental regions of the model domain is about a factor of 3 higher than that of NMHC. Because of large uncertainties in the natural and anthropogenic NMHC emissions, additional sensitivity runs are performed with increased anthropogenic NMHC emissions and natural NMHC emissions excluded. These alterations do not effect our basic conclusions with respect to rural O3 formation but do impact urban locations. When anthropogenic NMHC emissions are increased by a factor of 4, control Of NOx emissions has nearly the same effect in both urban and rural areas. The nonlinear nature of regionally averaged O3 with respect to anthropogenic and natural precursors is also illustrated from the results of the sensitivity studies.

On the nonlinearity of the tropospheric ozone production

The relationship of photochemical ozone production versus photochemical loss of an ozone precursor, that is, either NOx or nonmethane hydrocarbons (NMHCs), is studied by using a box model with particular emphasis on the nonlinearity problem of the relationship with respect to the concentration of the precursor. Model calculations indicate that the composition of NMHCs, the ratio of NMHCs to NOx, and the background concentrations of natural hydrocarbons, CO, and CH4 all play important roles in determining the nonlinearity of O3 production with respect to the loss of NOx. In addition, influences on the nonlinearity due to radical loss via reactions of HO2 with RO2, exchanges between PAN and NO2, and inclusion of nighttime NOx loss processes are also investigated. Mechanisms that contribute to the nonlinearity are discussed. The nonlinear property of O3 production versus loss of hydrocarbons and CO is different from that of NOx. When the sum of CO and all hydrocarbons, including CH4, natural NMHCs, and anthropogenic NMHCs, is used as the reference O3 precursor, the nonlinearity is much less pronounced for ambient conditions usually found in rural air.

Models and observations of the impact of natural hydrocarbons on rural ozone

Large quantities of non-methane hydrocarbons (NMHCs) are emitted into the atmosphere from vegetation1,2. A recent inventory by Lamb et al.2 indicates that the emission of natural hydrocarbons is significant compared to that of anthropogenic NMHCs in most regions of the United States. Because of their chemical activity, the natural NMHCs can play important parts in the formation of trace gases, such as ozone (O3), peroxyacetyl nitrate (PAN) and oxygenated secondary hydrocarbons3–5, which contribute to regional-scale air pollution6 and may be harmful to crops and forest7–9. The impact of natural hydrocarbons on the formation of O3 in the atmosphere has been discussed previously4–13. In general, it has been concluded that their impact is small14–16. But lack of data on the ambient concentrations of key photochemical species and an incomplete analysis of the photochemistry has prevented a definitive evaluation of the impact of natural NMHCs on rural O3. Here we report on concentrations of key trace gases measured concurrently at a rural site in the eastern USA during the summer of 1986 and a modelling study conducted to analyse these measurements. This study demonstrates that natural NMHCs can have a significant impact on ozone formation in rural air.

The photochemistry of anthropogenic nonmethane hydrocarbons in the troposphere

The photochemistry of anthropogenically emitted nonmethane hydrocarbons (NMHC's) in the troposphere was studied by (1) developing a chemical mechanism which includes the reactions of anthropogenically emitted NMHC's and their associated intermediates (alkoxy and alkylperoxy radicals, aldehydes, peroxyacetylnitrate (PAN)) with both hydrocarbon and inorganic species and (2) performing model calculations with the mechanism using a one‐dimensional photochemical model of the troposphere. Calculations represent the region between 30°N and 60°N latitude. The inclusion of the chemistry of NMHC species with short lifetimes against chemical destruction provided detailed information about both the profiles of short‐lived NMHC species and the overall NMHC chemistry. Surface concentrations of emitted hydrocarbons, NOx, and HNO3 were well simulated. Only slight changes in the profiles of OH, HO2, NOx, and HNO3 were obtained when the NMHC chemistry was included. The inorganic nitrogen budget is not significantly altered by the inclusion of NMHC chemistry. The sensitivity of the PAN profile to changes in the magnitudes of NMHC emissions and to the introduction of a heterogeneous loss term to simulate advection was also assessed. Advection may be an important removal process for PAN in the mid‐latitudes.