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Abstract: The Multilayer Model (MLM) has been used for many years to infer dry deposition fluxes from measured trace species concentrations and standard meteorological measurements for national networks in the U.S., including the U.S. Environmental Protection Agency's Clean Air Status and Trends Network (CASTNet). MLM utilizes a resistance analogy to calculate deposition velocities appropriate for whole vegetative canopies, while employing a multilayer integration to account for vertically varying meteorology, canopy morphology and radiative transfer within the canopy. However, the MLM formulation, as it was originally presented and as it has been subsequently employed, contains a non-physical representation related to the leaf-level quasi-laminar boundary layer resistance that affects the calculation of the total canopy resistance. In this note, the non-physical representation of the canopy resistance as originally formulated in MLM is discussed and a revised, physically consistent, formulation is suggested as a replacement. The revised canopy resistance formulation reduces estimates of HNO3 deposition velocities by as much as 38% during mid-day as compared to values generated by the original formulation. Inferred deposition velocities for SO2 and O3 are not significantly altered by the change in formulation (<3%). Inferred deposition loadings of oxidized and total nitrogen from CASTNet data may be reduced by 10–20% and 5–10%, respectively, for the Eastern U. S. when employing the revised formulation of MLM as compared to the original formulation. [Copyright &y& Elsevier]

Abstract: Continuous ammonia (NH3) measurements with a temporal resolution of 5min were implemented at selected SEARCH sites in the southeastern U. S. during 2007. The SEARCH continuous NH3 instrument uses a citric acid denuder difference technique employing a dual-channel nitric oxide-ozone chemiluminescence analyzer. Data from two SEARCH sites are presented, Jefferson Street, Atlanta (JST) (urban), and Yorkville, Georgia (YRK) (rural), for the period July–December, 2007. Highest NH x (total ammonia=gaseous NH3 +PM2.5 NH4 +) values were observed in August and September at both JST and YRK. Highest NH3 values occurred in August and September at JST, but in August through October at YRK. Lowest NH3 and NH x values occurred in December at both sites. YRK is significantly impacted by nearby poultry sources, routinely experiencing hourly average NH3 mixing ratios above 20ppbv. Wind sector analysis clearly implicates the nearby poultry operations as the source of the high NH3 values. Weekday versus weekend differences in composite hourly mean diurnal profiles of NH3 at JST indicate that mobile sources have a measurable but relatively small impact on NH3 observed at that site, and little or no impact on NH3 observed at YRK. A distinctive composite mean hourly diurnal variation was observed at both JST and YRK, exhibiting maxima in the morning and evening with a broad minimum during midday. Analysis of observed NH3 diurnal variations from the literature suggests a hypothesized mechanism for the observed behavior based on interaction of local emissions and dry deposition with the formation and collapse of the dynamically mixed atmospheric boundary layer during the day and shallow nocturnal layer at night. Simple mixed layer concentration box model simulations confirm the plausibility of the suggested mechanism. [Copyright &y& Elsevier]

Twenty-four-hour integrated gaseous and fine particulate were measured during 2004 at eight sites in the southeastern U.S. Mean concentrations for 2004 ranged from 2. 44ppbv at an urban-industrial site in North Birmingham, AL, to 0. 23ppbv at a rural-forested site near Centreville, AL. Mixing ratios were found to be higher at urban sites than at nearby rural (or suburban sites) except for sites which were directly influenced by local sources. Only weak correlations with temperature were observed for at the sites; slightly greater correlations were observed for total ammonia vs. temperature. A weak seasonal variation was observed for mixing ratios at all sites, with all but one site exhibiting biannual maxima in spring and late summer/fall. Mean concentrations ranged from in Atlanta, GA, to at Oak Grove, MS, and were more uniform across the network than mixing ratios, with only slightly larger values at urban sites as compared to nearby rural (or suburban) sites. All sites exhibited highest between July and September and lowest in November and December. The gaseous fraction was observed to decrease with increasing values of at all sites. At two rural-forested sites and two sites near the Gulf of Mexico, the gaseous fraction approached a relatively constant value of 5–10% as increased beyond 5–, suggesting that availability at these locations limits aerosol neutralization. [Copyright &y& Elsevier]

Abstract: A variety of linear regression techniques and simple slope estimators are evaluated for use in the elemental carbon (EC) tracer method of secondary organic carbon (OC) estimation. Linear regression techniques based on ordinary least squares are not suitable for situations where measurement uncertainties exist in both regressed variables. In the past, regression based on the method of Deming [1943. Statistical Adjustment of Data. Wiley, London] has been the preferred choice for EC tracer method parameter estimation. In agreement with Chu [2005. Stable estimate of primary OC/EC ratios in the EC tracer method. Atmospheric Environment 39, 1383–1392], we find that in the limited case where primary non-combustion OC (OCnon-comb) is assumed to be zero, the ratio of averages (ROA) approach provides a stable and reliable estimate of the primary OC–EC ratio, (OC/EC)pri. In contrast with Chu [2005. Stable estimate of primary OC/EC ratios in the EC tracer method. Atmospheric Environment 39, 1383–1392], however, we find that the optimal use of Deming regression (and the more general York et al. [2004. Unified equations for the slope, intercept, and standard errors of the best straight line. American Journal of Physics 72, 367–375] regression) provides excellent results as well. For the more typical case where OCnon-comb is allowed to obtain a non-zero value, we find that regression based on the method of York is the preferred choice for EC tracer method parameter estimation. In the York regression technique, detailed information on uncertainties in the measurement of OC and EC is used to improve the linear best fit to the given data. If only limited information is available on the relative uncertainties of OC and EC, then Deming regression should be used. On the other hand, use of ROA in the estimation of secondary OC, and thus the assumption of a zero OCnon-comb value, generally leads to an overestimation of the contribution of secondary OC to total measured OC. [Copyright &y& Elsevier]