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Seven techniques for the field measurement of trace atmospheric SO2 were compared simultaneously over 1 month in 1994 using samples produced in situ by dynamic dilution. Samples included SO2 in dry air, in humid air, and in air with potentially interfering gases added. In addition, 2 days of comparison using diluted ambient air were conducted. Six of the seven techniques compared well, with good linear response and no serious interferences but with a range of calibration differences of about 50%.

Atmospheric transport and in situ oxidation are important factors influencing mercury concentrations at the surface and wet and dry deposition rates. Contributions of both natural and anthropogenic processes can significantly impact burdens of mercury on local, regional and global scales. To address these key issues in atmospheric mercury research, airborne measurements of mercury speciation and ancillary parameters were conducted over a region near Tullahoma, Tennessee, USA, from August 2012 to June 2013. Here, for the first time, we present vertical profiles of Hg speciation from aircraft for an annual cycle over the same location. These airborne measurements included gaseous elemental mercury (GEM), gaseous oxidized mercury (GOM) and particulate bound mercury (PBM), as well as ozone (O3), sulfur dioxide (SO2), condensation nuclei (CN) and meteorological parameters. The flights, each lasting ~3 h, were conducted typically one week out of each month to characterize seasonality in mercury concentrations. Data obtained from 0 to 6 km altitudes show that GEM exhibited a relatively constant vertical profile for all seasons with an average concentration of 1.38 ± 0.17 ng⋅m-3. A pronounced seasonality of GOM was observed, with the highest GOM concentrations up to 120 pg⋅m-3 in the summer flights and lowest (0-20 pg⋅m-3) in the winter flights. Vertical profiles of GOM show the maximum levels at altitudes between 2 and 4 km. Limited PBM measurements exhibit similar levels to GOM at all altitudes. HYSPLIT back trajectories showed that the trajectories for elevated GOM (>70 pg⋅m-3) or PBM concentrations (>30 pg⋅m-3) were largely associated with air masses coming from west/northwest, while events with low GOM (<20 pg⋅m-3) or PBM concentrations (<5 pg⋅m-3) were generally associated with winds from a wider range of wind directions. This is the first set of speciated mercury vertical profiles collected in a single location over the course of a year. Even though there are current concerns that the KCl denuders used in this study may under-collect GOM, especially in the presence of elevated ozone, the collected data in this region shows the strong seasonality of oxidized mercury concentrations throughout the low to middle free troposphere. [ABSTRACT FROM AUTHOR]

The article presents results from two intensive studies during the summer of 2010 and spring of 2011 on measurements of mercury species including gaseous elemental mercury (GEM), gaseous oxidized mercury (GOM), and particulate-bound mercury (PBM) at an Atmospheric Mercury Network (AMNet) site at the Grand Bay National Estuarine Research Reserve (NERR) in Moss Point, Mississippi, in the northern Gulf of Mexico region.

Abstract: Concentrations of CO, SO2, NO, NO2, and NOY were measured atop the University of Houston''s Moody Tower supersite during the 2006 TexAQS-II Radical and Aerosol Measurement Project (TRAMP). The lowest concentrations of all primary and secondary species were observed in clean marine air in southerly flow. SO2 concentrations were usually low, but increased dramatically in sporadic midday plumes advected from sources in the Houston Ship Channel (HSC), located NE of the site. Concentrations of CO and NOx displayed large diurnal variations in keeping with their co-emission by mobile sources in the Houston Metropolitan Area (HMA). CO/NOx emission ratios of 5.81 ± 0.94 were observed in the morning rush hour. Nighttime concentrations of NOx (NOx = NO + NO2) and NOY (NOY = NO + NO2 + NO3 + HNO3 + HONO + 2∗N2O5 + HO2NO2 + PANs + RONO2 + p-NO3 − + …) were highest in winds from the NNW-NE due to emission from mobile sources. Median ratios of NOx/NOY were approximately 0.9 overnight, reflecting the persistence and/or generation of NOZ (NOZ = NOY − NOx) species in the nighttime Houston boundary layer, and approached unity in the morning rush hour. Daytime concentrations of NOx and NOY were highest in winds from the HSC. NOx/NOY ratios reached their minimum values (median ca 0.63) from 1300 to 1500 CST, near local solar noon, and air masses often retained enough NOx to sustain additional O3 formation farther downwind. HNO3 and PANs comprised the dominant NOZ species in the HMA, and on a median basis represented 17–20% and 12–15% of NOY, respectively, at midday. Concentrations of HNO3, PANs, and NOZ, and fractional contributions of these species to NOY, were at a maximum in NE flow, reflecting the source strength and reactivity of precursor emissions in the HSC. As a result, daytime O3 concentrations were highest in air masses with HSC influence. Overall, our findings confirm the impact of the HSC as a dominant source region within the HMA. A comparison of total NOY measurements with the sum of measured NOY species (NOYi = NOx + HNO3 + PANs + HONO + p-NO3 −) yielded excellent overall agreement during both day ([NOY](ppb) = ([NOYi](ppb)∗1.03 ± 0.16) − 0.42; r 2 = 0.9933) and night ([NOY](ppb) = ([NOYi](ppb)∗1.01 ± 0.16) + 0.18; r 2 = 0.9975). A similar comparison between NOY–NOx concentrations and the sum of NOZi (NOZi = HNO3 + PANs + HONO + p-NO3 −) yielded good overall agreement during the day ([NOZ](ppb) = ([NOZi](ppb)∗1.01 ± 0.30) + 0.044 ppb; r 2 = 0.8527) and at night ([NOZ](ppb) = ([NOZi](ppb)∗1.12 ± 0.69) + 0.16 ppb; r 2 = 0.6899). Median ratios of NOZ/NOZi were near unity during daylight hours but increased to approximately 1.2 overnight, a difference of 0.15–0.50 ppb. Differences between NOZ and NOZi rarely exceeded combined measurement uncertainties, and variations in NOZ/NOZi ratios may have resulted solely from errors in conversion efficiencies of NOY species and changes in NOY composition. However, nighttime NOZ/NOZi ratios and the magnitude of NOZ − NOZi differences were generally consistent with recent observations of ClNO2 in the nocturnal Houston boundary layer. [Copyright &y& Elsevier]

Nitrogen dioxide (NO2) plays a central role in atmospheric chemistry, air pollution, and biogeochemical cycles. Many analytical techniques have been developed to detect NO2, but only chemiluminescence-based instruments are commonly, commercially available. There remains a need for a fast, light, and simple method to directly measure NO2. In this work we describe the modification and characterization of a small, commercially available cavity ring-down spectroscopy (CRDS) NO2 detector suitable for surface and aircraft monitoring. A metal oxide scrubber was added to remove NO2, and provide a chemical zero, improving the detection limit (3σ of the background noise) from several parts per billion by volume (ppbv) to 0.06 ppbv, integrated over 60 s. Known interferences by water and particles were removed using Nafion tubing and a 1 μm Teflon® filter, respectively. A 95% response time of 18±1 s was observed for a step change in concentration. The CRDS detector was run in parallel to an ozone chemiluminescence device with photolytic conversion of NO2 to NO. The two instruments measured ambient air in suburban Maryland. A least-squares fit to the comparison data resulted a slope of 0.960±0.002 and R of 0.995, showing agreement within experimental uncertainty. [ABSTRACT FROM AUTHOR]