Huiting Mao

Do We Understand What the Mercury Speciation Instruments Are Actually Measuring? Results of RAMIX

From August 22 to September 16, 2012, atmospheric mercury (Hg) was measured from a common manifold in the field during the Reno Atmospheric Mercury Intercomparison eXperiment. Data were collected using Tekran systems, laser induced fluorescence, and evolving new methods. The latter included the University of Washington-Detector for Oxidized Mercury, the University of Houston Mercury instrument, and a filter-based system under development by the University of Nevada-Reno. Good transmission of total Hg was found for the manifold. However, despite application of standard protocols and rigorous quality control, systematic differences in operationally defined forms of Hg were measured by the sampling systems. Concentrations of reactive Hg (RM) measured with new methods were at times 2-to-3-fold higher than that measured by Tekran system. The low RM recovery by the latter can be attributed to lack of collection as the system is currently configured. Concentrations measured by all instruments were influenced by their sampling location in-the-manifold and the instrument analytical configuration. On the basis of collective assessment of the data, we hypothesize that reactions forming RM were occurring in the manifold. Results provide a new framework for improved understanding of the atmospheric chemistry of Hg.

Role of meteorological processes in two New England ozone episodes during summer 2001

[i] We examined the impact of dynamical processes on spatial variability in ozone (O 3 ) mixing ratios at closely spaced air monitoring sites in southern New Hampshire (NH) during two O 3 episodes, July 21-25 and August 2, 2001. The Meso-scale Meteorological Model (MM5) and the Community Multiscale Air Quality (CMAQ) photochemical model were applied together with ground-based atmospheric chemistry observations conducted by the Atmospheric Investigation, Regional Modeling, Analysis and Prediction (AIRMAP) program at the University of New Hampshire. Observations and model simulations suggested that during the July episode long-range transport via the nocturnal low-level jet (LLJ) played an important role in producing elevated daytime mixing ratios of O 3 . The marine site Isle of Shoals experienced the highest level of O 3 , possibly a result of having more diverse upwind sources and less ventilation compared to continental sites. Our model results suggest that during daytime the shallow sea breeze circulation contributes to high levels of O 3 at coastal and marine sites while the channeling effect of the Appalachian Mountains influences inland locations. In contrast to the July event, the event on August 2 was characterized by weak and transient synoptic flows, indicating insufficient time for transport of O 3 and its precursors from distant sources to inland sites in NH. Backward trajectories for both events showed that 03-rich air masses from the Boston metropolitan area can contribute to the high levels of O 3 (>120 ppbv) at coastal and marine sites in southern NH. Our results suggest that the International Consortium of Atmospheric Research on Transport and Transformation (ICARTT), an international field campaign based in the northeastern United States in summer 2004, should coordinate mobile platforms to investigate the vertical structure and chemical composition of the LLJ, the sea breeze, and the terrain-forced flows, and estimate the influx of O 3 and its precursors to central New England.

Quantification of ozone formation metrics at Thompson Farm during the New England Air Quality Study (NEAQS) 2002

[1] Several metrics have been estimated to investigate preliminarily ozone (O 3 ) formation dynamics at the University of New Hampshire Atmospheric Observing Station at Thompson Farm, which is associated with the Atmospheric Investigation, Regional Modeling, Analysis, and Prediction program. This paper focuses on the August time frame of the New England Air Quality Study 2002. These metrics include instantaneous and net O 3 production rate (P(O 3 )), instantaneous and average O 3 production efficiency (OPE), and hydrocarbon and carbon monoxide (CO) reactivity. In general, the seacoast region of New Hampshire experiences low P(O 3 ) values compared to other continental locations. Use of a photochemical model yields a range of instantaneous values of 0.2 to 8.5 ppbv h -1 and a range of net values of 0.2 to 8.3 ppbv h -1 . Corresponding calculations for instantaneous OPE range from 0.2 to 2.4, with regression techniques yielding average OPE values of 7.7 and 9.7. These high regression values, the mixing ratios of NO y , and the concentration ratios of O 3 to NO z indicate a NO x -limited atmosphere. Total hydrocarbon and CO reactivity ranges from 0.9 to 20.2 s -1 . In conjunction with back trajectory analysis the metric values calculated for this location indicate that strong peaks in O 3 during this period are most likely a result of mixing of processed, 03-rich air masses rather than direct in situ chemical formation.

Synoptic controls on summertime surface ozone in the northeastern United States

[1] The relationship between synoptic-scale circulation patterns and surface ozone (O 3 ) across the northeastern United States was investigated for summers 2000-2004. Observational data consisted of 1200 UT sea level pressure fields obtained from the National Centers for Environmental Prediction Global Final Analysis and O 3 measurements from 474 Environmental Protection Agency and five AIRMAP monitoring sites. The five most common circulation patterns, or map types (I-V), were identified with a correlation-based synoptic categorization technique, which persisted on 65% of the days during the study period. Map type I, characterized by stagnant warm conditions throughout the northeast, occurred most frequently (21%) with associated episodes of high O 3 . Interannual variability in O 3 varied regionally from a seasonally averaged daily maximum value of 64 ppbv in 2002 to a minimum of 52 ppbv in 2004. By considering both the sea level pressure system intensity and frequency of each map type, 46% of the interannual variability in summertime O 3 was reproduced with intensity being the dominant factor. The remaining interannual variability was possibly due to nonlinear relationships between climate and biogenic emissions and/or recent reductions of power plant emissions of nitrogen oxides (NO x ) over the eastern United States. The storm track of cyclones in the eastern United States was a key determinate of the intensity of circulation patterns.

Characteristics of atmospheric mercury deposition and size-fractionated particulate mercury in urban Nanjing, China

A comprehensive measurement study of mercury wet deposition and size-fractionated particulate mercury (HgP) concurrent with meteorological variables was conducted from June 2011 to February 2012 to evaluate the characteristics of mercury deposition and particulate mercury in urban Nanjing, China. The volume-weighted mean (VWM) concentration of mercury in rainwater was 52.9 ng L −1 with a range of 46.3–63.6 ng L −1. The wet deposition per unit area was averaged 56.5 μg m −2 over 9 months, which was lower than that in most Chinese cities, but much higher than annual deposition in urban North America and Japan. The wet deposition flux exhibited obvious seasonal variation strongly linked with the amount of precipitation. Wet deposition in summer contributed more than 80 % to the total amount. A part of contribution to wet deposition of mercury from anthropogenic sources was evidenced by the association between wet deposition and sulfates, as well as nitrates in rainwater. The ions correlated most significantly with mercury were formate, calcium, and potassium, which suggested that natural sources including vegetation and resuspended soil should be considered as an important factor to affect the wet deposition of mercury in Nanjing. The average Hg P concentration was 1.10± 0.57 ng m−3. A distinct seasonal distribution of HgP concentrations was found to be higher in winter as a result of an increase in the PM 10 concentration. Overall, more than half of the Hg P existed in the particle size range less than 2.1 μm. The highest concentration of Hg P in coarse particles was observed in summer, while Hg P in fine particles dominated in fall and winter. The size distribution of averaged mercury content in particulates was bimodal, with two peaks in the bins of < 0.7 μm and 4.7–5.8 μm. Dry deposition per unit area of Hg P was estimated to be 47.2 μg m −2 using meteorological conditions and a size-resolved particle dry deposition model. This was 16.5 % less than mercury wet deposition. Compared to Hg P in fine particles, HgP in coarse particles contributed more to the total dry deposition due to higher deposition velocities. Negative correlation between precipitation and the Hg P concentration reflected the effect of scavenging of Hg P by precipitation.

Nighttime nitrate radical chemistry at Appledore Island, Maine during the 2004 International Consortium for Atmospheric Research on Transport and Transformation

Received 5 April 2007; revised 22 June 2007; accepted 7 August 2007; published 2 November 2007. [1] Trace gases including nitrogen dioxide (NO2), nitrate radical (NO3), ozone (O3), and a suite of volatile organic compounds (VOCs) were measured within the New England coastal marine boundary layer on Appledore Island (AI), Maine, USA as part of the International Consortium for Atmospheric Research on Transport and Transformation (ICARTT) field campaign. These measurements, together with local meteorological records and published kinetic data were used to investigate nighttime NO3 chemistry at AI during the period of 8–28 July 2004. Among the VOCs, isoprene, monoterpenes and dimethylsulfide (DMS) were the dominant NO3 reactants; on average, DMS accounted for 51 ± 34% of the total reactivity. For three case studies, NO3 mixing ratios were calculated from measured parameters with resultant uncertainties of � 30%. Discrepancies with measured NO3 appeared to result primarily from input parameter variability and exclusion of heterogeneous dinitrogen pentoxide (N2O5) chemistry. We indirectly determined that nighttime NO3 and NOx (=NO + NO2) removal via N2O5 chemistry (gas-phase + heterogeneous) was on average 51–54% and 63–66% of the total respectively. Our analysis suggested that the minimum average NO3 and NOx removal via heterogeneous N2O5 chemistry was � 10% of the total. Reducing gas-phase N2O5 reactivity in accord with Brown et al. (2006a) increased the importance of heterogeneous N2O5 chemistry substantially. It is plausible that the latter pathway was often comparable to gas-phase removal of NO3 and NOx. Overall, 24 h-averaged NOx removal was � 11 ppbv, with nighttime chemical pathways contributing � 50%.

Controls on methanol and acetone in marine and continental atmospheres

We present a regional analysis of CH 3 OH and (CH 3 ) 2 CO in the New England continental and coastal marine atmospheres. Vegetative emissions over land comprise 60-80% of the daily peak-to-peak differences in the diurnal cycles of these oxygenated hydrocarbons. In the morning downward mixing of remnant boundary layer air over land provides an additional source equal to more than half of the vegetative emission strength. The ocean is both a sink and a source of CH 3 OH and (CH 3 ) 2 CO, with dry depositional losses 2-fold greater than their source counterparts of 0.35 and 0.17 ppbv d -1 respectively. Anthropogenic emissions compensate for 59% and 52% of CH 3 OH and (CH 3 ) 2 CO oceanic sink respectively, whereas over land this source is relatively small compared to substantial vegetative sources. Direct measurements of ocean- and land-air fluxes of CH 3 OH and (CH 3 ) 2 CO and boundary layer height are needed to better constrain their regional budgets.

Relationship of surface O3 to large‐scale circulation patterns during two recent winters

[1] We demonstrate a direct connection between large-scale circulation patterns and surface O 3 using atmospheric observations obtained during winters 2002 and 2003. Measurements at two rural sites in the northeastern U.S. revealed that median mixing ratios of O 3 in winter 2003 were increased by up to 80% compared to 2002, and greatly exceeded previous spring annual maximums. To explain this we propose that strong meridional flows in winter 2003 frequently transported O 3 -rich mid-tropospheric air masses from high latitudes to the northeastern U.S. while cooling regional climate 4.4°C below normal. Our measurements also show that an exceptionally elevated spring O 3 maximum occurred in 2003. The impact from this winter enhancement on the levels of O 3 and other species during the following months will be largely driven by actual climatic conditions.

Synoptic influences on springtime tropospheric O 3 and CO over the North American export region observed by TES

The relationship between synoptic circulation patterns over the western North Atlantic Ocean in spring (March, April, and May) and tropospheric O 3 and CO was investigated using retrievals from the Tropospheric Emission Spectrometer (TES) for 2005 and 2006. Seasonal composites of TES retrievals reprocessed to remove the artificial geographic structure added from the a priori revealed a channel of slightly elevated O3 (>55 ppbv) and CO ( >115 ppbv) at the 681 hPa retrieval level between 30 ◦ N and 45 N extending from North America out over the Atlantic Ocean. Ozone and CO in this region were correlated at r=0.22 with a slope value of 0.13 mol mol −1 indicative of the overall impact of photochemical chemical processes in North American continental export. Composites of TES retrievals for the six predominant circulation patterns identified as map types from sea level pressure fields of the NCEP FNL analyses showed large variability in the distribution of tropospheric O3. Map types MAM2 and MAM3 featuring cyclones near the US east coast produced the greatest export to the lower free troposphere with O 3>65 ppbv and a relatively well-defined O3-CO correlation (slope values near 0.20 mol mol −1). The ensembles of HYSPLIT backward trajectories indicated that the high O 3 levels were possibly a result of pollutants lofted to the free troposphere by the warm conveyor belt (WCB) of a cyclone. An important finding was that pollutant export occurred in the main WCB branch to the east of the cyclone and in a secondary branch circling to the back of the cyclone center. Conversely, a map type featuring a large anticyclone dominating the flow over the US east coast (MAM6) restricted export with O 3 levels generally <55 ppbv and CO levels generally <110 ppbv. There was also evidence of stratospheric intrusions particularly to the north of 45 N in the 316 hPa composites predominately Correspondence to: J. Hegarty (jhegarty@ccrc.sr.unh.edu) for MAM1 which featured a large cyclone near Newfoundland. However, the concurrence of these intrusions with pollutant export, specifically in the southwestern North Atlantic Ocean, made it difficult to delineate their respective contributions to the 681 hPa O 3 composites.

Relative contributions of gaseous oxidized mercury and fine and coarse particle‐bound mercury to mercury wet deposition at nine monitoring sites in North America

Relative contributions to mercury wet deposition by gaseous oxidized mercury (%GOM) and fine and coarse particle-bound mercury (%FPBM and %CPBM) were estimated making use of monitored FPBM air concentration and mercury wet deposition at nine North American locations. Scavenging ratios of particulate inorganic ions (K+ and Ca2+, Mg2+ and Na+) were used as a surrogate for those of FPBM and CPBM, respectively. FPBM and CPBM were estimated to contribute 8–36% and 5–27%, respectively, depending on the location, to total wet deposition. The rest of the 39–87% was attributed to the contribution of GOM. The average %GOM, %FPBM and %CPBM among all locations were 65%, 17%, and 18%, respectively. The relative distributions of %GOM, %FPBM, and %CPBM were influenced by Hg(II) gas-particle partitioning, urban site characteristics, and precipitation type. At the regional scale, %GOM dominated over %FPBM and %CPBM. However, the sum of FPBM and CPBM contributed to nearly half of the total Hg wet deposition in urban areas, which was greater than other site categories and is attributed to higher FPBM air concentrations. At four locations, %FPBM exceeded %GOM during winter in contrast to summer, suggesting the efficient snow scavenging of aerosols. The results from this study are useful in improving mercury transport models since most of these models do not estimate CPBM, but frequently use monitored mercury wet deposition data for model evaluation.