Abstract. We report measurements of ambient amines and ammonia with a fast response chemical ionization mass spectrometer (CIMS) in a southeastern US forest and a moderately polluted midwestern site during the summer. At the forest site, mostly C3-amines (from pptv to tens of pptv) and ammonia (up to 2 ppbv) were detected, and they both showed temperature dependencies. Aerosol-phase amines measured thermal-desorption chemical ionization mass spectrometer (TDCIMS) showed a higher mass fraction in the evening with cooler temperatures and lower in the afternoon with warmer temperatures, a trend opposite to the gas-phase amines. Concentrations of aerosol-phase primary amines measured with Fourier transform infrared spectroscopy (FTIR) from micron and submicron particles were 2 orders of magnitude higher than the gas-phase amines. These results indicate that gas to particle conversion is one of the major processes that control the ambient amine concentrations at this forest site. Temperature dependencies of C3-amines and ammonia also imply reversible processes of evaporation of these nitrogen-containing compounds from soil surfaces in daytime and deposition to soil surfaces at nighttime. During the transported biomass burning plume events, various amines (C1–C6) appeared at the pptv level, indicating that biomass burning is a substantial source of amines in the southeastern US. At the moderately polluted Kent site, there were higher concentrations of C1- to C6-amines (pptv to tens of pptv) and ammonia (up to 6 ppbv). C1- to C3-amines and ammonia were well correlated with the ambient temperature. C4- to C6-amines showed frequent spikes during the nighttime, suggesting that they were emitted from local sources. These abundant amines and ammonia may in part explain the frequent new particle formation events reported from Kent. Higher amine concentrations measured at the polluted site than at the rural forested site highlight the importance of constraining anthropogenic emission sources of amines.

Abstract. Production of new particles over forests is an important source of cloud condensation nuclei that can affect climate. While such particle formation events have been widely observed, their formation mechanisms over forests are poorly understood. Our observations made in a mixed deciduous forest with large isoprene emissions during the summer displayed a surprisingly rare occurrence of new particle formation (NPF). Typically, NPF events occur around noon but no NPF events were observed during the 5 weeks of measurements. The exceptions were two evening ultrafine particle events. During the day, sulfuric acid concentrations were in the 10 6 cm −3 range with very low preexisting aerosol particles, a favorable condition for NPF to occur even during the summer. The ratio of emitted isoprene carbon to monoterpene carbon at this site was similar to that in Amazon rainforests (ratio >10), where NPF events are also very rare, compared with a ratio <0.5 in Finland boreal forests, where NPF events are frequent. Our results suggest that large isoprene emissions can suppress NPF formation in forests although the underlying mechanism for the suppression is unclear. The two evening ultrafine particle events were associated with the transported anthropogenic sulfur plumes and ultrafine particles were likely formed via ion-induced nucleation. Changes in landcover and environmental conditions could modify the isoprene suppression of NPF in some forest regions resulting in a radiative forcing that could have influence on the climate.

In Part I of this two-part paper, a formulation was developed to treat fragmentation in ice–ice collisions. In the present Part II, the formulation is implemented in two microphysically advanced cloud models simulating a convective line observed over the U.S. high plains. One model is 2D with a spectral bin microphysics scheme. The other has a hybrid bin–two-moment bulk microphysics scheme in 3D. The case consists of cumulonimbus cells with cold cloud bases (near 0°C) in a dry troposphere. Only with breakup included in the simulation are aircraft observations of particles with maximum dimensions &gt;0.2 mm in the storm adequately predicted by both models. In fact, breakup in ice–ice collisions is by far the most prolific process of ice initiation in the simulated clouds (95%–98% of all nonhomogeneous ice), apart from homogeneous freezing of droplets. Inclusion of breakup in the cloud-resolving model (CRM) simulations increased, by between about one and two orders of magnitude, the average concentration of ice between about 0° and −30°C. Most of the breakup is due to collisions of snow with graupel/hail. It is broadly consistent with the theoretical result in Part I about an explosive tendency for ice multiplication. Breakup in collisions of snow (crystals &gt;~1 mm and aggregates) with denser graupel/hail was the main pathway for collisional breakup and initiated about 60%–90% of all ice particles not from homogeneous freezing, in the simulations by both models. Breakup is predicted to reduce accumulated surface precipitation in the simulated storm by about 20%–40%.

In this study, we present long-term near-surface measurements of sulfur dioxide (SO 2 ), oxides of nitrogen (NO x ), carbon monoxide (CO), and ozone (O 3 ) carried out at an urban location, Kanpur (26.46°N, 80.33°E, 125 m amsl), in Northern India from June 2009 to May 2013. The mean concentrations of SO 2 , NO x , CO, and O 3 over the entire study period were 3.0, 5.7, 721, and 27.9 ppb, respectively. SO 2 , NO x and CO concentrations were highest during the winter season, whereas O 3 concentration peaked during summer. The former could be attributed mainly to the near-surface anthropogenic sources (e.g. automobiles, residential cooking, brick kilns, coal-burning power plants, agricultural land-clearing, and biomass burning) and low mixing height in winter, whereas the latter was clearly due to enhanced chemical production of O 3 during the pre-monsoon (i.e. summer) season. The lowest concentration of all trace gases were observed during the monsoon season, due to efficient wet scavenging by precipitation. The averaged diurnal patterns also showed similar seasonal variation. NO x and CO showed peaks during morning and evening traffic hours and a valley in the afternoon irrespective of the seasons, clearly linked to the boundary layer height evolution. Contrarily, O 3 depicted a reverse pattern with highest concentrations during afternoon hours and lowest in the morning hours. The mean rate of change of O 3 concentrations (dO 3 /dt) during the morning hours (08:00 to 11:00 h) and evening hours (17:00 to 19:00 h) at Kanpur were 3.3 ppb h −1 and −2.6 ppb h −1 , respectively. O 3 followed a positive linear relationship with temperature, except in post-monsoon season while the strong negative with the relative humidity in all seasons. The ventilation coefficient was found to be highest in the pre-monsoon season (15,622 m 2 s −1 ) and lowest during winter (2564 m 2 s −1 ), indicative of excellent pollution dispersion efficiency during the pre-monsoon season. However, the low ventilation coefficient during winter and post-monsoon seasons indicated that the highpollution potential occurs at this site.

[1] This paper attempts to analyze the chemical compositions of the near surface aerosols at a typical location in the Ganga basin with an emphasis on delineating the source of aerosols in foggy/hazy conditions. Collocated measurements of a number of atmospheric and aerosol parameters along with simultaneous sampling of near surface aerosols of size less than 10 mm (PM 10 ) were made as part of an intense field campaign launched under the Indian Space Research Organization Geosphere Biosphere Program (ISRO-GBP) in December 2004. PM 10 and black carbon (BC) mass concentration was found to be significantly higher during the foggy/hazy period. Much of the PM 10 mass ($81%) was due to fine/accumulation mode particles (0.1-0.95 mm). associated with very low values (<5 ppmv) of SO 2 despite considerable plausible emissions due to fossil fuel and biomass burning in the region suggests that loading of fine mode aerosols in the region could have been enhanced through reactions of gaseous pollutants on the solid surfaces. These results along with the findings presented in the companion paper indicate that prolonged foggy/hazy conditions in the region may be due to the increased anthropogenic emissions.

This paper attempts to characterize the physical and optical properties of the aerosols along with relevant meteorological parameters at a typical location in the Ganga basin. The emphasis is on delineating the prolonged foggy/hazy conditions, a phenomenon believed to be of relatively recent origin, faced by millions of people during the winter months of December and January. Collocated measurements of a number of aerosol and atmospheric parameters were made using ground‐based instruments as part of an intense field campaign launched under the Indian Space Research Organization Geosphere Biosphere Programme in December 2004. The meteorological conditions suggest limited mixing due to shallow boundary layer thickness and essentially calm wind conditions. Monthly mean aerosol optical depth was high (0.77 ± 0.3 at 0.5 μm wavelength) and showed high spectral variation (first‐order Ångström exponent for all wavelengths, α = 1.24 ± 0.24). The second‐order Ångström exponent α′ derived for 0.34, 0.5, and 1.02 μm wavelengths showed much higher curvature in the aerosol optical depth spectrum on the hazy/foggy days (0.93 ± 0.36) as compared to that during the clear days (0.59 ± 0.3). Single‐scattering albedo (0.87–0.97) showed strong spectral variation. Aerosol mass concentration was high with monthly average 125.9 ± 47.1 μg m−3. Fine mode particles (<1 μm) contributed ∼75% to the total mass of aerosols. Similarly, aerosol number concentration was found to vary in the range 1.5–2 × 103 cm−3, with fine mode particles contributing to ∼99.6%. The hazy/foggy conditions typically prevailed when higher daytime relative humidity, lower maximum temperature, and higher fine/accumulation mode particles were observed. The companion paper suggests that the rise in aerosol mass/number concentration could be attributed to the aqueous‐phase heterogeneous reactions mediated by anthropogenic pollutants and the associated reduction in boundary layer thickness and suppressed mixing.

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Abstract. 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 (J 10 ) and growth rates were in the range of 4.4-30 cm −3 s −1 and 7.4-24 nm h −1 , respectively. The onset of NPF events coincided with decreases of condensation sink (CS) and increases of SO 2 under enhanced atmospheric mixing and dilution. However, the lower or comparable SO 2 on event days than on non-event days suggests that SO 2 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 −1 . 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.