-
Aerosol, rainwater, and sea fog water samples were collected during the cruise conducted over the subarctic western North Pacific Ocean in the summer of 2008, in order to estimate dry, wet, and sea fog deposition fluxes of atmospheric inorganic nitrogen (N). During sea fog events, mean number densities of particles with diameters larger than 0.5 μm decreased by 12–78%, suggesting that particles with diameters larger than 0.5 μm could act preferentially as condensation nuclei (CN) for sea fog droplets. Mean concentrations of nitrate (NO<sub>3</sub><sup>−</sup>), methanesulfonic acid (MSA), and non sea-salt sulfate (nss-SO<sub>4</sub><sup>2−</sup>) in sea fog water were higher than those in rainwater, whereas those of ammonium (NH<sub>4</sub><sup>+</sup>) in both sea fog water and rainwater were similar. These results reveal that sea fog scavenged NO<sub>3</sub><sup>−</sup> and biogenic sulfur species more efficiently than rain. Mean dry, wet, and sea fog deposition fluxes for atmospheric total inorganic N (TIN; i.e. NH<sub>4</sub><sup>+</sup> + NO<sub>3</sub><sup>−</sup>) over the subarctic western North Pacific Ocean were estimated to be 4.9 μmol m<sup>−2</sup> d<sup>−1</sup>, 33 μmol m<sup>−2</sup> d<sup>−1</sup>, and 7.8 μmol m<sup>−2</sup> d<sup>−1</sup>, respectively. While NO<sub>3</sub><sup>−</sup> was the dominant inorganic N species in dry and sea fog deposition, inorganic N supplied to surface waters by wet deposition was predominantly by NH<sub>4</sub><sup>+</sup>. The contribution of dry, wet, and sea fog deposition to total deposition flux for TIN (46 μmol m<sup>−2</sup> d<sup>−1</sup>) were 11%, 72%, and 17%, respectively, suggesting that ignoring sea fog deposition would lead to underestimate of the total influx of atmospheric inorganic N into the subarctic western North Pacific Ocean, especially in summer periods.
-
Recently, secondary organic aerosols (SOAs) generated from anthropogenic volatile organic compounds have been proposed as a possible source of light-absorbing organic compounds, "brown carbon," in the urban atmosphere. However, the atmospheric importance of these SOAs remains unclear due to limited information about their optical properties. In this study, the complex refractive index (RI, <i>m</i> = <i>n</i>-<i>ki</i> values at 405, 532, and 781 nm of the SOAs generated during the photooxidation of toluene (toluene-SOAs) under a variety of initial nitrogen oxide (NO<sub>x</sub> = NO + NO<sub>2</sub>) conditions were examined by photoacoustic spectroscopy (PAS) and cavity ring-down spectroscopy (CRDS). The complex RI-values obtained in the present study and reported in the literature indicate that the <i>k</i>-value, which represents the light absorption of the toluene-SOAs, increased to shorter wavelengths at <532 nm, and the <i>n</i>-value also increased to shorter wavelengths from 781 to 355 nm. The <i>k</i>-values at 405 nm were found to increase from 0.0018 to 0.0072 with increasing initial NO<sub>x</sub> concentration from 109 to 571 ppbv. The nitrate to organics ratio of the SOAs determined using a high-resolution time-of-flight aerosol mass spectrometer (H-ToF-AMS) also increased with increasing initial NO<sub>x</sub> concentration. The RI-values of the SOAs generated during the photooxidation of 1,3,5-trimethylbenzene in the presence of NO<sub>x</sub> (1,3,5-TMB-SOAs) were also determined to investigate the influence of the chemical structure of the precursor on the optical properties of the SOAs, and it was found that the light absorption of the 1,3,5-TMB-SOAs is negligible at all of the wavelengths investigated (405, 532, and 781 nm). These results can be reasonably explained by the hypothesis that nitroaromatic compounds, such as nitrocresols, are the major contributors to the light absorption of the toluene-SOAs. Using the obtained RI-values, mass absorption cross sections of the toluene-SOAs at 405 nm were estimated to be 0.08–0.52 m<sup>2</sup>g<sup>−1</sup> under typical conditions in an urban atmosphere during the daytime. These results indicate that light absorption by the SOAs potentially contributes to the radiation balance at ultraviolet wavelengths below ~400 nm, specifically when the mass concentrations of the anthropogenic SOAs are significant compared with other light-absorbing particles.
-
We present the first systematic analysis for new particle formation (NPF), growth and shrinkage of new particles at four different sites in subtropical central Taiwan. A total of 14 NPF events were identified from 137 days of ambient measurements during a cold and warm season. The measured formation rates of 10 nm particles (<i>J</i><sub>10</sub>) and growth rates were in the range of 4.4–30 cm<sup>−3</sup> s<sup>−1</sup> and 7.4–24 nm h<sup>−1</sup>, respectively. The onset of NPF events coincided with decreases of condensation sink (CS) and increases of SO<sub>2</sub> under enhanced atmospheric mixing and dilution. However, the lower or comparable SO<sub>2</sub> on event days than on non-event days suggests that SO<sub>2</sub> was not a limiting factor for NPF. On non-event days, the particle number concentrations were mostly driven by traffic emissions. We also observed shrinkage of new particles, the reversal of growth, during five out of the identified secondary formation. UFP particles events. In intense cases, the grown particles shrank back to the smallest measurable size of ~10 nm, thereby creating a unique "arch-like" shape in the size distribution contour plot. The particle shrinkage rates ranged from −5.1 to −7.6 nm h<sup>−1</sup>. The corresponding particle volume losses suggest that a notable fraction of the condensable species that contributed to growth was semi-volatile. The particle shrinkage was related to enhanced atmospheric dilution, high ambient temperature and low relative humidity, thus favoring the evaporation of semi-volatile species from the particulate phase to the gas phase. Our observations show that the new particle growth could be a reversible process, in which the evaporating semi-volatile species are important for the growth of new particles to sizes of environmental health concerns.
-
The hydroperoxyl radical (HO<sub>2</sub>) is a major precursor of OH and tropospheric ozone. OH is the main atmospheric oxidant, while tropospheric ozone is an important surface pollutant and greenhouse gas. Standard gas-phase models for atmospheric chemistry tend to overestimate observed HO<sub>2</sub> concentrations, and this has been tentatively attributed to heterogeneous uptake by aerosol particles. It is generally assumed that HO<sub>2</sub> uptake by aerosol involves conversion to H<sub>2</sub>O<sub>2</sub>, but this is of limited efficacy as an HO<sub>2</sub> sink because H<sub>2</sub>O<sub>2</sub> can photolyze to regenerate OH and from there HO<sub>2</sub>. Joint atmospheric observations of HO<sub>2</sub> and H<sub>2</sub>O<sub>2</sub> suggest that HO<sub>2</sub> uptake by aerosols may in fact not produce H<sub>2</sub>O<sub>2</sub>. Here we propose a catalytic mechanism involving coupling of the transition metal ions Cu(I)/Cu(II) and Fe(II)/Fe(III) to rapidly convert HO<sub>2</sub> to H<sub>2</sub>O in aqueous aerosols. The implied HO<sub>2</sub> uptake and conversion to H<sub>2</sub>O significantly affects global model predictions of tropospheric OH, ozone, carbon monoxide (CO) and other species, improving comparisons to observations in the GEOS-Chem model. It represents a previously unrecognized positive radiative forcing of aerosols through the effects on the chemical budgets of major greenhouse gases including methane and hydrofluorocarbons (HFCs).
-
Prior work indicates that the frequency of summertime mid-latitude cyclones tracking across the Great Lakes Storm Track (GLST, bounded by: 70° W, 90° W, 40° N, and 50° N) are strongly anticorrelated with ozone (O<sub>3</sub>) pollution episodes over the Northeastern United States (US). We apply the MAP Climatology of Mid-latitude Storminess (MCMS) algorithm to 6-hourly sea level pressure fields from over 2500 yr of simulations with the GFDL CM3 global coupled chemistry-climate model. These simulations include (1) 875 yr with constant 1860 emissions and forcings (Pre-industrial Control), (2) five ensemble members for 1860–2005 emissions and forcings (Historical), and (3) future (2006–2100) scenarios following the Representative Concentration Pathways (RCP 4.5 and RCP 8.5) and a sensitivity simulation to isolate the role of climate warming from changes in O<sub>3</sub> precursor emissions (RCP 4.5*). The GFDL CM3 Historical simulations capture the mean and variability of summertime cyclones traversing the GLST within the range determined from four reanalysis datasets. Over the 21st century (2006–2100), the frequency of summertime mid-latitude cyclones in the GLST decreases under the RCP 8.5 scenario and in the RCP 4.5 ensemble mean. These trends are significant when assessed relative to the variability in the Pre-industrial Control simulation. In addition, the RCP 4.5* scenario enables us to determine the relationship between summertime GLST cyclones and high-O<sub>3</sub> events (> 95th percentile) in the absence of emission changes. The summertime GLST cyclone frequency explains less than 10% of the variability in high-O<sub>3</sub> events over the Northeastern US in the model, implying that other factors play an equally important role in determining high-O<sub>3</sub> events.
-
Warm rain in real clouds is produced by the collision and coalescence of an initial population of small droplets. The production of rain in warm cumulus clouds is still one of the open problems in cloud physics, and although several mechanisms have been proposed in the past, at present there is no complete explanation for the rapid growth of cloud droplets within the size range of diameters from 10 to 50 μm. By using a collection kernel <i>enhanced by turbulence</i> and a fully stochastic simulation method, the formation of a runaway droplet is modeled through the turbulent collection process. When the runaway droplet forms, the traditional calculation using the kinetic collection equation is no longer valid, since the assumption of a continuous distribution breaks down. There is in essence a phase transition in the system from a continuous distribution to a continuous distribution <i>plus</i> a runaway droplet. This transition can be associated to <i>gelation</i> (also called <i>sol–gel transition</i>) and is proposed here as a mechanism for the formation of large droplets required to trigger warm rain development in cumulus clouds. The fully stochastic turbulent model reveals gelation and the formation of a droplet with mass comparable to the mass of the initial system. The time when the sol–gel transition occurs is estimated with a Monte Carlo method when the parameter ρ (the ratio of the standard deviation for the largest droplet mass over all the realizations to the averaged value) reaches its maximum value. Moreover, we show that the non-turbulent case does not exhibit the sol–gel transition that can account for the impossibility of producing raindrop embryos in such a system. In the context of cloud physics theory, <i>gelation</i> can be interpreted as the formation of the "lucky droplet" that grows at a much faster rate than the rest of the population and becomes the embryo for runaway raindrops.
-
-
A volatility basis set (VBS) approach for the simulation of secondary organic aerosol (SOA) formation is incorporated in the online coupled atmospheric model system COSMO-ART and applied over Europe during the EUCAARI May 2008 campaign. Organic aerosol performance is improved when compared to the default SOA module of COSMO-ART (SORGAM) against high temporal resolution aerosol mass spectrometer ground measurements. The impact of SOA on the overall radiative budget was investigated. The mean direct surface radiative cooling averaged over Europe is −1.2 W m<sup>−2</sup>, representing approximately 20% of the total effect of aerosols on the radiative budget. However, responses are not spatially correlated with the radiative forcing, due to the nonlinear interactions among changes in particle chemical composition, water content, size distribution and cloud cover. These interactions initiated~by the effect of SOA on radiation are found to result even in a positive forcing in specific areas. Further model experiments showed that the availability of nitrogen oxides slightly affects SOA production, but that the aging rate constant used in the VBS approximation and boundary concentrations assumed in the model should be carefully selected. The aging of SOA is found to reduce hourly nitrate levels by up to 30%, while the condensation of inorganic species upon pre-existing, SOA-rich particles results in a monthly average increase of 5% in sulfate and ammonium formation in the accumulation mode.
-
The interrelated geographic and temporal variability seen in more than seven years of tropical and subtropical upper tropospheric (215 hPa) ozone, carbon monoxide and cloud ice water content (IWC) observations by the Aura Microwave Limb Sounder (MLS) are presented. Observed ozone abundances and their variability (geographic and temporal) agree to within 10–15 ppbv with records from sonde observations. MLS complements these (and other) observations with global coverage and simultaneous measurements of related parameters. Previously-reported phenomena such as the ozone "wave one" feature are clearly seen in the MLS observations, as is a double peak in ozone abundance over tropical East Africa, with enhanced abundances in both May to June and September to November. While repeatable seasonal cycles are seen in many regions, they are often accompanied by significant interannual variability. Ozone seasonal cycles in the southern tropics and subtropics tend to be more distinct (i.e., annually repeatable) than in the northern. By contrast, carbon monoxide shows distinct seasonal cycles in many northern subtropical regions, notably from India to the Eastern Pacific. Deep convection (as indicated by large values of IWC) is typically associated with reductions in upper tropospheric ozone. Convection over polluted regions is seen to significantly enhance upper tropospheric carbon monoxide. While some regions show statistically significant correlations among ozone, carbon monoxide and IWC, simple correlations fall well short of accounting for the observed variability. The observed interrelated variations and metrics of annual and interannual variability described here represent a new resource for validation of atmospheric chemistry models.
-
A series of 12 high volume air samples collected from the S2 firn core during the North Greenland Eemian Ice Drilling (NEEM) 2009 campaign have been measured for mixing ratio and stable carbon isotope composition of the chlorofluorocarbon CFC-12 (CCl<sub>2</sub>F<sub>2</sub>). While the mixing ratio measurements compare favorably to other firn air studies, the isotope results show extreme <sup>13</sup>C depletion at the deepest measurable depth (65 m), to values lower than δ<sup>13</sup>C = −80‰ vs. VPDB (the international stable carbon isotope scale), compared to present day surface tropospheric measurements near −40‰. Firn air modeling was used to interpret these measurements. Reconstructed atmospheric time series indicate even larger depletions (to −120‰) near 1950 AD, with subsequent rapid enrichment of the atmospheric reservoir of the compound to the present day value. Mass-balance calculations show that this change is likely to have been caused by a large change in the isotopic composition of anthropogenic CFC-12 emissions, probably due to technological advances in the CFC production process over the last 80 yr, though direct evidence is lacking.
