Improved continuous measurement system for atmospheric total peroxy and total organic nitrate under the high NOx condition

We improved the thermal dissociation cavity attenuated phase shift spectroscopy (TD-CAPS) instrument to measure atmospheric total peroxy nitrates (PNs) and organic nitrates (ONs) continuously under the condition of high NOx. In TD-CAPS, PNs and ONs are dissociated in heated quartz tubes to form NO2, and the NO2 concentration is measured by cavity attenuated phase shift spectroscopy (CAPS). The original TD-CAPS system overestimates PN and ON concentrations in the presence of high NO concentrations. Our laboratory experiments and numerical simulations showed that the main cause of the overestimation was NO oxidation to NO2 by peroxy radicals generated in the heated quartz tubes. In the improved system, NO was converted to NO2 by adding excess O3 after the quartz tubes so that CAPS detected NOx (NO and NO2) instead of NO2. The uncertainty of the improved system was less than 20% with ∼15 parts per billion by volume (ppbv) NO and ∼80 ppbv NO2. The estimated detection limit (3σ) was 0.018 ppbv with an integration time of 2 min in the presence of 64 ppbv NO2. The improved system was tested for measurement of PNs and ONs in an urban area, and the results indicated that interference from NO was successfully suppressed.

A WRF-CMAQ modeling of atmospheric peroxyacetyl nitrate and source apportionment in Central China

Atmospheric peroxyacetyl nitrate (PAN), as an essential constituent in the photochemical smog, is formed from photochemical reactions between volatile organic compounds (VOCs) and NOx. However, limited regional studies on distribution, formation and sources of PAN restrict the further understanding of the atmospheric behavior and environmental significance of PAN. In this study, the variation characteristics of PAN and the influencing factors to PAN concentrations were investigated using the WRF-CMAQ model simulation in the central China during July 2019. The results showed that the monthly mean concentration of PAN in the near-surface layer was 0.4 ppbv and increased with the height rising, accompanied by strong intra-day variation. The process analysis suggested that the removal was mainly controlled by dry deposition (57 %), followed by the gas-phase chemistry (43 %) which was mainly attributed to the thermal decomposition. Based on the sensitivity simulation, PAN concentrations decreased effectively in most of the simulated regions when precursors of VOCs and NOx emissions were reduced, and PAN concentrations were more sensitive to VOCs emissions than NOx emissions. The reduction of NOx and VOCs could lead to enhanced atmospheric oxidation in east-central region, which in turn hindered the decrease of PAN concentrations. During the simulation period, we found that emissions from industry and transportation sectors had the greatest impact on PAN concentrations in the central China, with contributions of 39 %-49 % and 33 %-41 %, respectively.

A review of measurements of air-surface exchange of reactive nitrogen in natural ecosystems across North America

This review summarizes the state of the science of measurements of dry deposition of reactive nitrogen (Nr) compounds in North America, beginning with current understanding of the importance of dry deposition at the U.S. continental scale followed by a review of micrometeorological flux measurement methods. Measurements of Nr air-surface exchange in natural ecosystems of North America are then summarized, focusing on the U.S. and Canada. Drawing on this synthesis, research needed to address the incompleteness of dry deposition budgets, more fully characterize temporal and geographical variability of fluxes, and better understand air-surface exchange processes is identified. Our assessment points to several data and knowledge gaps that must be addressed to advance dry deposition budgets and air-surface exchange modeling for North American ecosystems. For example, recent studies of particulate (NO3-) and gaseous (NOx, HONO, peroxy nitrates) oxidized N fluxes challenge the fundamental framework of unidirectional flux from the atmosphere to the surface employed in most deposition models. Measurements in forest ecosystems document the importance of in-canopy chemical processes in regulating the net flux between the atmosphere and biosphere, which can result in net loss from the canopy. These results emphasize the need for studies to quantify within- and near-canopy sources and sinks of the full suite of components of the Nr chemical system under study (e.g., NOy or HNO3-NH3-NH4NO3). With respect to specific ecosystems and geographical locations, additional flux measurements are needed particularly in agricultural regions (NH3), coastal zones (NO3- and organic N), and arid ecosystems and along urban to rural gradients (NO2). Measurements that investigate non-stomatal exchange processes (e.g., deposition to wet surfaces) and the biogeochemical drivers of bidirectional exchange (e.g., NH3) are considered high priority. Establishment of long-term sites for process level measurements of reactive chemical fluxes should be viewed as a high priority long-term endeavor of the atmospheric chemistry and ecological communities.

Wintertime characteristic of peroxyacetyl nitrate in the Chengyu district of southwestern China

Atmospheric concentrations of peroxyacetyl nitrate (PAN) were measured in Ziyang in December 2012 to provide basic knowledge of PAN in the Chengyu district and offer recommendations for air pollution management. The PAN pollution was relatively severe in Ziyang in winter, with the maximum and average PAN concentrations of 1.61 and 0.55 ppbv, respectively, and a typical single-peak diurnal trend in PAN and theoretical PAN lost by thermal decomposition (TPAN) were observed. PAN and O₃ concentrations were correlated (R² = 0.52) and the ratios of daily maximum PAN to O₃ ([PAN]/[O₃] ratio) ranged from 0.013 to 0.108, with an average of 0.038. Both acetone and methyl ethyl ketone (MEK) were essential for producing the acetylperoxy radicals (PA) and subsequently PAN in Ziyang in winter, and PAN concentrations at the sampling site exhibited more sensitivity to volatile organic compound (VOC) concentrations than nitrogen oxide (NO_x) levels. Therefore, management should focus on reducing VOCs emissions, in particular those that produce acetone and MEK through photolysis and oxidizing reactions. In addition, the influence of relative humidity (RH) on the heterogeneous reactions between PAN and PM_2.5 in the atmospheric environment may have led to the strong correlation between observed PM_2.5 and PAN in Ziyang in winter. Furthermore, a typical air pollution event was observed on 17-18 December 2012, which Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) and PSCF simulations suggest that it was caused by the local formation and the regional transport of polluted air masses from Hanzhong, Nanchong, and Chengdu.

Comparison of micrometeorological and two-film estimates of air-water gas exchange for alpha-hexachlorocyclohexane in the Canadian archipelago

The air-sea gas exchange of alpha-hexachlorocyclohexane (α-HCH) in the Canadian Arctic was estimated using a micrometeorological approach and the commonly used Whitman two-film model. Concurrent shipboard measurements of α-HCH in air at two heights (1 and 15 m) and in surface seawater were conducted during the Circumpolar Flaw Lead study in 2008. Sampling was carried out during eight events in the early summer time when open water was encountered. The micrometeorological technique employed the vertical gradient in air concentration and the wind speed to estimate the flux; results were corrected for atmospheric stability using the Monin-Obukhov stability parameter. The Whitman two-film model used the concentrations of α-HCH in surface seawater, in bulk air at 1 and 15 m above the surface, and the Henry's law constant adjusted for temperature and salinity to derive the flux. Both approaches showed that the overall net flux of α-HCH was from water to air. Mean fluxes calculated using the micrometeorological technique ranged from -3.5 to 18 ng m(-2) day(-1) (mean 7.4), compared to 3.5 to 14 ng m(-2) day(-1) (mean 7.5) using the Whitman two-film model. Flux estimates for individual events agreed in direction and within a factor of two in magnitude for six of eight events. For two events, fluxes estimated by micrometeorology were zero or negative, while fluxes estimated with the two-film model were positive, and the reasons for these discrepancies are unclear. Improvements are needed to shorten air sampling times to ensure that stationarity of meteorological conditions is not compromised over the measurement periods. The micrometeorological technique could be particularly useful to estimate fluxes of organic chemicals over water in situations where no water samples are available.

Summertime elemental mercury exchange of temperate grasslands on an ecosystem-scale

In order to estimate the air-surface mercury exchange of grasslands in temperate climate regions, fluxes of gaseous elemental mercury (GEM) were measured at two sites in Switzerland and one in Austria during summer 2006. Two classic micrometeorological methods (aerodynamic and modified Bowen ratio) have been applied to estimate net GEM exchange rates and to determine the response of the GEM flux to changes in environmental conditions (e.g. heavy rain, summer ozone) on an ecosystem-scale. Both methods proved to be appropriate to estimate fluxes on time scales of a few hours and longer. Average dry deposition rates up to 4.3 ng m^-2 h^-1 and mean deposition velocities up to 0.10 cm s^-1 were measured, which indicates that during the active vegetation period temperate grasslands are a small net sink for atmospheric mercury. With increasing ozone concentrations depletion of GEM was observed, but could not be quantified from the flux signal. Night-time deposition fluxes of GEM were measured and seem to be the result of mercury co-deposition with condensing water. Effects of grass cuts could also be observed, but were of minor magnitude.