Nocturnal Atmospheric Oxidative Processes in the Indo-Gangetic Plain and Their Variation During the COVID-19 Lockdowns.

This study investigates selected secondary atmospheric responses to the widely reported emission change attributed to COVID-19 lockdowns in the highly polluted Indo-Gangetic Plain (IGP) using ground-based measurements of trace gases and particulate matter. We used a chemical box-model to show that production of nighttime oxidant, NO3, was affected mainly by emission decrease (average nighttime production rates 1.2, 0.8 and 1.5 ppbv hr-1 before, during and relaxation of lockdown restrictions, respectively), while NO3 sinks were sensitive to both emission reduction and seasonal variations. We have also shown that the maximum potential mixing ratio of nitryl chloride, a photolytic chlorine radical source which has not been previously considered in the IGP, is as high as 5.5 ppbv at this inland site, resulting from strong nitrate radical production and a potentially large particulate chloride mass. This analysis suggests that air quality measurement campaigns and modeling explicitly consider heterogeneous nitrogen oxide and halogen chemistry.

Environmental conditions regulate the impact of plants on cloud formation.

The terrestrial vegetation emits large amounts of volatile organic compounds (VOC) into the atmosphere, which on oxidation produce secondary organic aerosol (SOA). By acting as cloud condensation nuclei (CCN), SOA influences cloud formation and climate. In a warming climate, changes in environmental factors can cause stresses to plants, inducing changes of the emitted VOC. These can modify particle size and composition. Here we report how induced emissions eventually affect CCN activity of SOA, a key parameter in cloud formation. For boreal forest tree species, insect infestation by aphids causes additional VOC emissions which modifies SOA composition thus hygroscopicity and CCN activity. Moderate heat increases the total amount of constitutive VOC, which has a minor effect on hygroscopicity, but affects CCN activity by increasing the particles' size. The coupling of plant stresses, VOC composition and CCN activity points to an important impact of induced plant emissions on cloud formation and climate.

Complex refractive indices in the near-ultraviolet spectral region of biogenic secondary organic aerosol aged with ammonia.

Atmospheric absorption by brown carbon aerosol may play an important role in global radiative forcing. Brown carbon arises from both primary and secondary sources, but the mechanisms and reactions of the latter are highly uncertain. One proposed mechanism is the reaction of ammonia or amino acids with carbonyl products in secondary organic aerosol (SOA). We generated SOA in situ by reacting biogenic alkenes (α-pinene, limonene, and α-humulene) with excess ozone, humidifying the resulting aerosol, and reacting the humidified aerosol with gaseous ammonia. We determined the complex refractive indices (RI) in the 360-420 nm range for these aerosols using broadband cavity enhanced spectroscopy (BBCES). The average real part (n) of the measured spectral range of the NH3-aged α-pinene SOA increased from n = 1.50 (±0.01) for the unreacted SOA to n = 1.57 (±0.01) after 1.5 h of exposure to 1.9 ppm NH3, whereas the imaginary component (k) remained below k < 0.001((+0.002)(-0.001)). For the limonene and α-humulene SOA the real part did not change significantly, and we observed a small change in the imaginary component of the RI. The imaginary component increased from k = 0.000 to an average k = 0.029 (±0.021) for α-humulene SOA, and from k < 0.001((+0.002)(-0.001)) to an average k = 0.032 (±0.019) for limonene SOA after 1.5 h of exposure to 1.3 and 1.9 ppm of NH3, respectively. Collected filter samples of the aged and unreacted α-pinene SOA and limonene SOA were analyzed off-line by nanospray desorption electrospray ionization high resolution mass spectrometry (nano-DESI/HR-MS), and in situ using a Time-of-Flight Aerosol Mass Spectrometer (ToF-AMS), confirming that the SOA reacted and that various nitrogen-containing reaction products formed. If we assume that NH3 aging reactions scale linearly with time and concentration, which will not necessarily be the case in the atmosphere, then a 1.5 h reaction with 1 ppm NH3 in the laboratory is equivalent to 24 h reaction with 63 ppbv NH3, indicating that the observed aerosol absorption will be limited to atmospheric regions with high NH3 concentrations.

Hydration-influenced sorption of organic compounds by model and atmospheric humic-like substances (HULIS).

Atmospheric humic-like substances (HULIS) constitute a major fraction of the water soluble organic carbon of aerosol particles. We investigated sorption and desorption of water and two model organic contaminants (toluene and benzyl alcohol) on HULIS and a standard humic substance (Suwannee River fulvic acid; SRFA) under varying relative humidity using a quartz crystal microbalance. Simultaneous sorption of water and benzyl alcohol (capable of specific interactions like hydrogen bonding or charge transfer) on HULIS and SRFA shows significant, humidity-dependent, cooperative sorption at intermediate water activity, as well as a dependence of sorption distribution coefficient on the wetting-drying pathway. In contrast, sorption of toluene (capable of only nonspecific interactions) was humidity-independent. Atmospheric HULIS is thus found to have several sorption features in common with terrestrial and aquatic humic substances and soil organic matter. These features are consistent with the link solvation model (LSM), whereby water assists in cooperative sorption of specifically interacting compounds by the organic matter sorbent, and subsequent changes in sorbent structure result in sorption hysteresis. Sorption of compounds capable of only nonspecific interactions is unaffected by hydration status. Such sorption features can lead to considerable uncertainty in predicting and modeling transport of organic pollutants in the atmosphere.

Complex refractive indices of aerosols retrieved by continuous wave-cavity ring down aerosol spectrometer.

The major uncertainties associated with the direct impact of aerosols on climate call for fast and accurate characterization of their optical properties. Cavity ring down (CRD) spectroscopy provides highly sensitive measurement of aerosols' extinction coefficients from which the complex refractive index (RI) of the aerosol may be retrieved accurately for spherical particles of known size and number density, thus it is possible to calculate the single scattering albedo and other atmospherically relevant optical parameters. We present a CRD system employing continuous wave (CW) single mode laser. The single mode laser and the high repetition rate obtained significantly improve the sensitivity and reliability of the system, compared to a pulsed laser CRD setup. The detection limit of the CW-CRD system is between 6.67 x 10(-10) cm(-1) for an empty cavity and 3.63 x 10(-9) cm(-1) for 1000 particles per cm(3) inside the cavity, at a 400 Hz sampling and averaging of 2000 shots for one sample measurement taken in 5 s. For typical pulsed-CRD, the detection limit for an empty cavity is less than 3.8 x 10(-9) cm(-1) for 1000 shots averaged over 100 s at 10 Hz. The system was tested for stability, accuracy, and RI retrievals for scattering and absorbing laboratory-generated aerosols. Specifically, the retrieved extinction remains very stable for long measurement times (1 h) with an order of magnitude change in aerosol number concentration. In addition, the optical cross section (sigma(ext)) of a 400 nm polystyrene latex sphere (PSL) was determined within 2% error compared to the calculated value based on Mie theory. The complex RI of PSL, nigrosin, and ammonium sulfate (AS) aerosols were determined by measuring the extinction efficiency (Q(ext)) as a function of the size parameter ((piD)/lambda) and found to be in very good agreement with literature values. A mismatch in the retrieved RI of Suwannee River fulvic acid (SRFA) compared to a previous study was observed and is attributed to variation in the sample composition. The small system presented delivers high ability for fast measurements and accurate analysis, making it a good candidate for field aerosol optical properties studies.

CCN activity and hygroscopic growth of organic aerosols following reactive uptake of ammonia.

Recent field observations suggest that ammonium salts of organic acids may be very important in accounting for aerosols' properties in many environments. In this study we present laboratory experiments and calculations on the influence of ammonia reaction with organic aerosol components and its effect upon their (1) subsaturation hygroscopic growth (HG) and (2) supersaturation cloud condensation nuclei (CCN) activity. By using adipic acid (slightly soluble), citric acid (soluble), and di(ethylene glycol) monovinyl ether (DEGMVE, nonacidic compound) aerosols we show the feasibility and importance of atmospherically relevant acid-base neutralization by ammonia for different organic species. It is suggested that the formation of ammonium salts due to reaction of ammonia with slightly soluble organic acids (such as adipic acid) can affect the CCN activity and hygroscopic growth of aerosols with a significant organic component. It is further confined that the reaction involves carboxylic groups, it requires presence of water in the aerosol, and that the effects are stronger for less soluble organic acids.

The complex refractive index of atmospheric and model humic-like substances (HULIS) retrieved by a cavity ring down aerosol spectrometer (CRD-AS).

Atmospheric aerosols absorb and reflect solar radiation causing surface cooling and heating of the atmosphere. The interaction between aerosols and radiation depends on their complex index of refraction, which is related to the particles' chemical composition. The contribution of light absorbing organic compounds, such as HUmic-LIke Substances (HULIS) to aerosol scattering and absorption is among the largest uncertainties in assessing the direct effect of aerosols on climate. Using a Cavity Ring Down Aerosol Spectrometer (CRD-AS), the complex index of refraction of aerosols containing HULIS extracted from pollution, smoke, and rural continental aerosols, and molecular weight-fractionated fulvic acid was measured at 390 nm and 532 nm. The imaginary part of the refractive index (absorption) substantially increases towards the UV range with increasing molecular weight and aromaticity. At both wavelengths, HULIS extracted from pollution and smoke particles absorb more than HULIS from the rural aerosol. Sensitivity calculations for a pollution-type aerosol containing ammonium sulfate, organic carbon (HULIS), and soot suggests that accounting for absorption by HULIS leads in most cases to a significant decrease in the single scattering albedo and to a significant increase in aerosol radiative forcing efficiency, towards more atmospheric absorption and heating. This indicates that HULIS in biomass smoke and pollution aerosols, in addition to black carbon, can contribute significantly to light absorption in the ultraviolet and visible spectral regions.

New analytical method for the determination of levoglucosan, polyhydroxy compounds, and 2-methylerythritol and its application to smoke and rainwater samples.

Biomass burning is an important source of smoke aerosol particles, which contain water-soluble inorganic and organic species, and thus have a great potential of affecting cloud formation, precipitation, and climate on global and regional scales. In this study, we have developed a new chromatographic method for the determination of levoglucosan (a specific tracer for biomass burning particles), related polyhydroxy compounds, and 2-methylerythritol (recently identified as isoprene oxidation product in fine aerosols in the Amazon) in smoke and in rainwater samples. The new method is based on water extraction and utilizes ion-exclusion high-performance liquid chromatography (IEC-HPLC) separation and spectroscopic detection at 194 nm. The new method allows the analysis of wet samples, such as rainwater samples. In addition, aliquots of the same extracts can be used for further analyses, such as ion chromatography. The overall method uncertainty for sample analysis is 15%. The method was applied to the analysis of high-volume and size-segregated smoke samples and to rainwater samples, all collected during and following the deforestation fires season in Rondonia, Brazil. From the analysis of size-segregated samples, it is evident that levoglucosan is a primary vegetation combustion product, emitted mostly in the 0.175-1 microm size bins. Levoglucosan concentrations decrease below the detection limit atthe end of the deforestation fires period, implying that it is not present in significant amounts in background Amazon forest aerosols. The ratio of daytime levoglucosan concentration to particulate matter (PM) concentration was about half the nighttime ratio. This observation is rationalized by the prevalence of flaming combustion during day as opposed to smoldering combustion during night. This work broadens the speciation possibilities

Negative mass instability for interacting particles in a 1D box: theory and application.

We demonstrate that the synchronization effect observed [Pedersen et al., Phys. Rev. Lett. 87, 055001 (2001)]], when a bunch of ions oscillates between two mirrors in an electrostatic ion beam trap, can be explained as a negative mass instability. We derive simple necessary conditions for the existence of a regime in which this dispersionless behavior occurs and demonstrate that in this regime, the ion trap can be used as a high resolution mass spectrometer.

Ion motion synchronization in an ion-trap resonator.

Using a new type of ion trap, we demonstrate that the length of a packet of charged particles oscillating between two electrostatic mirrors will remain constant under special conditions. The effect can be understood in terms of phase synchronization, where, in a rather counterintuitive way, the repulsive Coulomb interaction between the ions actually holds the packet together. Application of this effect for mass spectrometry is discussed.

Analysis of semivolatile organic compounds in atmospheric aerosols by direct sample introduction thermal desorption GC/MS.

A technique for identifying trace amounts of semivolatile organic compounds in atmospheric aerosols and in the NIST Urban Dust standard (SRM1649a) is presented. The technique is based on direct sample introduction (DSI) of small samples followed by thermal desorption in a conventional GC injector. The method enables injection of both solid and liquid samples. Validation of the method, including quantitative determination of EPA-targeted polycyclic aromatic hydrocarbons (PAHs), as well as the reproducibility and recovery efficiency tests are presented. The advantages of using aluminum foil as sampling filter are also discussed. Determination of different classes of compounds such as quinolines, methylquinoline isomers, PAHs, and n-monocarboxylic acids in the ambient size-segregated aerosol sample is also performed. The method was directly applied to the determination of C6-C16 n-monocarboxylic acids, eliminating the need for a complex sample preparation procedure. The small quantities needed for the analysis as well as the lack of complicated sample preparation steps enable fast characterization of semivolatile organic species present in time-resolved or size-segregated aerosol samples. Thus, this method can potentially be employed for air quality monitoring and field measurements as well as for fast screening of the organic content of ambient particles.

Desert dust suppressing precipitation: a possible desertification feedback loop.

The effect of desert dust on cloud properties and precipitation has so far been studied solely by using theoretical models, which predict that rainfall would be enhanced. Here we present observations showing the contrary; the effect of dust on cloud properties is to inhibit precipitation. Using satellite and aircraft observations we show that clouds forming within desert dust contain small droplets and produce little precipitation by drop coalescence. Measurement of the size distribution and the chemical analysis of individual Saharan dust particles collected in such a dust storm suggest a possible mechanism for the diminished rainfall. The detrimental impact of dust on rainfall is smaller than that caused by smoke from biomass burning or anthropogenic air pollution, but the large abundance of desert dust in the atmosphere renders it important. The reduction of precipitation from clouds affected by desert dust can cause drier soil, which in turn raises more dust, thus providing a possible feedback loop to further decrease precipitation. Furthermore, anthropogenic changes of land use exposing the topsoil can initiate such a desertification feedback process.

Fourier transform time-of-flight mass spectrometry in an electrostatic ion beam trap

We report on the application of an electrostatic ion beam trap as a mass spectrometer. The instrument is analogous to an optical resonator; ions are trapped between focusing mirrors. The storage time is limited by the residual gas pressure and reaches up to several seconds, resulting in long ion flight paths. The oscillation of ion bunches between the mirrors is monitored by nondestructive image charge detection in a field-free region and mass spectra are obtained via Fourier transform. The principle of operation is demonstrated by measuring the mass spectrum of trapped Ar+ and Xe+ particles, produced by a standard electron impact ion source. Also, mass spectra of heavier PEGnNa+ and bradykinin ions from a pulsed MALDI ion source were obtained. The long ion flight path, combined with mass-independent charge detection, makes this system particularly interesting for the investigation of large molecules.

A comparative study of a liquid and a solid matrix in matrix-assisted laser desorption/ionization time-of-flight mass spectrometry and collision cross section measurements.

We present experimental matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) results comparing a liquid (glycerol/K(4)[Fe(CN)(6)]) and a solid matrix (2,5-dihydroxybenzoic acid, DHB) with respect to analyte signal stability and initial ion velocity. For applications requiring stable production of analyte ions over a long period of time, the liquid matrix is superior to the solid matrix. The stable analyte ion signal obtained from a liquid matrix allowed the measurement of collision cross sections of small poly(ethylene glycol) (PEG(n)) adduct ions in the flight tube with good resolution. The initial velocity of these adduct ions was measured. It was found that analyte molecules from the liquid matrix have initial ion velocities significantly smaller than those from the solid matrix. MALDI-TOF measurements for large molecules using a liquid matrix are therefore likely to result in smaller systematic errors in mass calibrations due to initial ion velocity.

Mass and collision cross-section determination using a low-vacuum mass spectrometer.

A simple low-vacuum mass spectrometer (LVMS) operating in the milliTorr pressure range was developed. The instrument resolves masses by time-of-flight measurements and employs a high-gain, fast-response detector that can operate at these pressures. This instrument allows simultaneous determination of mass and collision cross sections of the ions with the bath gas. Here we demonstrate the LVMS's abilities to determine total collision cross sections for the collisions of organic ions with three background gases, He, N(2), and SF(6). As a demonstration of the system capabilities, the unimolecular interconversion of photochemically produced C(7)H(7)(+) to the tropylium ion structure is investigated.