Photochemical production of nitrous acid on glass sample manifold surface

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/94760/1/grl15858.pd

Inland Sea Spray Aerosol Transport and Incomplete Chloride Depletion: Varying Degrees of Reactive Processing Observed during SOAS

Multiphase reactions involving sea spray aerosol (SSA) impact trace gas budgets in coastal regions by acting as a reservoir for oxidized nitrogen and sulfur species, as well as being a source of halogen gases (HCl, ClNO₂, etc.). Whereas most studies of multiphase reactions on SSA have focused on marine environments, far less is known about SSA transported inland. Herein, single-particle measurements of SSA are reported at a site >320 km from the Gulf of Mexico, with transport times of 7–68 h. Samples were collected during the Southern Oxidant and Aerosol Study (SOAS) in June–July 2013 near Centreville, Alabama. SSA was observed in 93% of 42 time periods analyzed. During two marine air mass periods, SSA represented significant number fractions of particles in the accumulation (0.2–1.0 μm, 11%) and coarse (1.0–10.0 μm, 35%) modes. Chloride content of SSA particles ranged from full to partial depletion, with 24% of SSA particles containing chloride (mole fraction of Cl/Na ≥ 0.1, 90% chloride depletion). Both the frequent observation of SSA at an inland site and the range of chloride depletion observed suggest that SSA may represent an underappreciated inland sink for NO_<i>x</i>/SO₂ oxidation products and a source of halogen gases

Bidirectional Ecosystem-Atmosphere Fluxes of Volatile Organic Compounds Across the Mass Spectrum : How Many Matter?

Terrestrial ecosystems are simultaneously the largest source and a major sink of volatile organic compounds (VOCs) to the global atmosphere, and these two-way fluxes are an important source of uncertainty in current models. Here, we apply high-resolution mass spectrometry (proton transfer reaction-quadrupole interface time-of-flight; PTR-QiTOF) to measure ecosystem-atmosphere VOC fluxes across the entire detected mass range (m/z 0-335) over a mixed temperate forest and use the results to test how well a state-of-science chemical transport model (GEOS-Chem CTM) is able to represent the observed reactive carbon exchange. We show that ambient humidity fluctuations can give rise to spurious VOC fluxes with PTR-based techniques and present a method to screen for such effects. After doing so, 377 of the 636 detected ions exhibited detectable gross fluxes during the study, implying a large number of species with active ecosystem-atmosphere exchange. We introduce the reactivity flux as a measure of how Earth-atmosphere fluxes influence ambient OH reactivity and show that the upward total VOC (-VOC) carbon and reactivity fluxes are carried by a far smaller number of species than the downward fluxes. The model underpredicts the -VOC carbon and reactivity fluxes by 40-60% on average. However, the observed net fluxes are dominated (90% on a carbon basis, 95% on a reactivity basis) by known VOCs explicitly included in the CTM. As a result, the largest CTM uncertainties in simulating VOC carbon and reactivity exchange for this environment are associated with known rather than unrepresented species. This conclusion pertains to the set of species detectable by PTR-TOF techniques, which likely represents the majority in terms of carbon mass and OH reactivity, but not necessarily in terms of aerosol formation potential. In the case of oxygenated VOCs, the model severely underpredicts the gross fluxes and the net exchange. Here, unrepresented VOCs play a larger role, accounting for ∼30% of the carbon flux and ∼50% of the reactivity flux. The resulting CTM biases, however, are still smaller than those that arise from uncertainties for known and represented compounds

Bidirectional Ecosystem-Atmosphere Fluxes of Volatile Organic Compounds Across the Mass Spectrum : How Many Matter?

Terrestrial ecosystems are simultaneously the largest source and a major sink of volatile organic compounds (VOCs) to the global atmosphere, and these two-way fluxes are an important source of uncertainty in current models. Here, we apply high-resolution mass spectrometry (proton transfer reaction-quadrupole interface time-of-flight; PTR-QiTOF) to measure ecosystem-atmosphere VOC fluxes across the entire detected mass range (m/z 0-335) over a mixed temperate forest and use the results to test how well a state-of-science chemical transport model (GEOS-Chem CTM) is able to represent the observed reactive carbon exchange. We show that ambient humidity fluctuations can give rise to spurious VOC fluxes with PTR-based techniques and present a method to screen for such effects. After doing so, 377 of the 636 detected ions exhibited detectable gross fluxes during the study, implying a large number of species with active ecosystem-atmosphere exchange. We introduce the reactivity flux as a measure of how Earth-atmosphere fluxes influence ambient OH reactivity and show that the upward total VOC (-VOC) carbon and reactivity fluxes are carried by a far smaller number of species than the downward fluxes. The model underpredicts the -VOC carbon and reactivity fluxes by 40-60% on average. However, the observed net fluxes are dominated (90% on a carbon basis, 95% on a reactivity basis) by known VOCs explicitly included in the CTM. As a result, the largest CTM uncertainties in simulating VOC carbon and reactivity exchange for this environment are associated with known rather than unrepresented species. This conclusion pertains to the set of species detectable by PTR-TOF techniques, which likely represents the majority in terms of carbon mass and OH reactivity, but not necessarily in terms of aerosol formation potential. In the case of oxygenated VOCs, the model severely underpredicts the gross fluxes and the net exchange. Here, unrepresented VOCs play a larger role, accounting for ∼30% of the carbon flux and ∼50% of the reactivity flux. The resulting CTM biases, however, are still smaller than those that arise from uncertainties for known and represented compounds

Unexpected Contributions of Sea Spray and Lake Spray Aerosol to Inland Particulate Matter

Sea spray aerosol (SSA) and lake spray aerosol (LSA) from wave breaking contribute to particulate matter (PM) in coastal regions near oceans and freshwater lakes, respectively. However, SSA and LSA contributions to atmospheric aerosol populations in inland regions are poorly understood because of difficulties differentiating them from other inland sources when using bulk particle measurements. Herein, we show that SSA and LSA episodically contribute to atmospheric aerosol populations at a rural site in northern Michigan >700 and >25 km from the nearest seawater and Great Lakes sources, respectively. During July 2014, individual SSA and LSA particles were identified by single-particle mass spectrometry and electron microscopy and then combined with air mass trajectory analysis for source apportionment. SSA comprised up to 33 and 20% of PM mass (0.5–2.0 μm) during two multiday transport events from Hudson Bay and a 3% average background outside these periods. LSA transported from Lake Michigan reached a maximum of 7% of PM mass (0.5–2 μm) during a daylong high-wind event and contributed a 3% average background during the remainder of the study. The observation of SSA and LSA transported inland motivates further studies of the impacts of wave breaking particles on cloud formation and air quality at inland locations far from marine and freshwater sources

Bidirectional Ecosystem–Atmosphere Fluxes of Volatile Organic Compounds Across the Mass Spectrum: How Many Matter?

Terrestrial ecosystems are simultaneously the largest source and a major sink of volatile organic compounds (VOCs) to the global atmosphere, and these two-way fluxes are an important source of uncertainty in current models. Here, we apply high-resolution mass spectrometry (proton transfer reaction-quadrupole interface time-of-flight; PTR-QiTOF) to measure ecosystem–atmosphere VOC fluxes across the entire detected mass range (<i>m</i>/<i>z</i> 0–335) over a mixed temperate forest and use the results to test how well a state-of-science chemical transport model (GEOS-Chem CTM) is able to represent the observed reactive carbon exchange. We show that ambient humidity fluctuations can give rise to spurious VOC fluxes with PTR-based techniques and present a method to screen for such effects. After doing so, 377 of the 636 detected ions exhibited detectable gross fluxes during the study, implying a large number of species with active ecosystem–atmosphere exchange. We introduce the reactivity flux as a measure of how Earth–atmosphere fluxes influence ambient OH reactivity and show that the upward total VOC (∑VOC) carbon and reactivity fluxes are carried by a far smaller number of species than the downward fluxes. The model underpredicts the ∑VOC carbon and reactivity fluxes by 40–60% on average. However, the observed net fluxes are dominated (90% on a carbon basis, 95% on a reactivity basis) by known VOCs explicitly included in the CTM. As a result, the largest CTM uncertainties in simulating VOC carbon and reactivity exchange for this environment are associated with known rather than unrepresented species. This conclusion pertains to the set of species detectable by PTR-TOF techniques, which likely represents the majority in terms of carbon mass and OH reactivity, but not necessarily in terms of aerosol formation potential. In the case of oxygenated VOCs, the model severely underpredicts the gross fluxes and the net exchange. Here, unrepresented VOCs play a larger role, accounting for ∼30% of the carbon flux and ∼50% of the reactivity flux. The resulting CTM biases, however, are still smaller than those that arise from uncertainties for known and represented compounds

Studies of peroxyacetyl nitrate (PAN) and its interaction with the snowpack at Summit, Greenland

Peroxyacetyl nitrate (PAN) was measured in ambient and snowpack interstitial air at Summit, Greenland, in June and July of 1998 and 1999 and at a rural/forest site in the Keewenaw Peninsula of Michigan in January of 1999. At Summit, we found that PAN typically represented between 30 and 60% of NOy. In the summer of 1999, a significant diel variation in both PAN/NOy and NOx/NOy was observed, but this was much less pronounced in 1998. Experiments during SNOW99 near Houghton, Michigan, indicated that PAN undergoes weak uptake onto snow grain surfaces. At Summit, we found that PAN concentrations in the snowpack interstitial air were significantly elevated (by as much as 2-5 times) relative to ambient levels, implying a flux of PAN out of the snowpack during the study period. We also observed evidence that PAN can be photochemically produced in snow that is exposed to polluted air. These observations indicate that interactions with the snowpack can have a significant impact on PAN concentrations in the boundary layer and point to potential difficulties associated with investigation of long-term changes in PAN uptake into ice cores because of the impact of postdepositional processes