Long-term trends and transport of surface pollutants to UTLS  during the Asian summer monsoon

2021 ◽  
Author(s):  
William K.M. Lau ◽  
Kyu-Myong Kim
Keyword(s):  
Summer Monsoon ◽  
Asian Summer Monsoon ◽  
Linear Trend ◽  
Monsoon Season ◽  
Radiative Heating ◽  
Aerosol Layer ◽  
Near Surface ◽  
Upper Troposphere ◽  
Asian Summer

<p>Using MERRA2 reanalyses, we have examined the long-term (2000-2019) trends and transport of surface pollutants, CO, BC and OC from surface to the upper troposphere and lower stratosphere (UTLS) during the Asian summer monsoon.    We find a strong linear trend indicating an expansion and strengthening of the Asian Monsoon Anticyclone (AMA), in conjunction with increased concentration of CO, BC and OC in the UTLS, including the Aerosol Tropopause Aerosol Layer (ATAL). </p><p>The UTLS trend in CO can be tracked to increased upward transport primarily from surface sources near 25-35<sup>o</sup>N, in association with the expansion/strengthening of the AMA, and a northward displacement of ascending branch of the monsoon meridional circulation.  In contrast, near 25-35<sup>o</sup>N, BC and OC trends show significant reduction from surface to mid-troposphere, coupled a weak increase at UTLS (above 250 -100 hPa).  The reduction in surface and tropospheric BC and OC likely reflects reduced emission due to the clean air acts in East Asia.  Additionally, heavier rainfall associated with the enhanced ascent and wet scavenging may also contribute to the strong reduction in tropospheric BC and OC.  The increase in UTLS OC/BC appears to stem from increased and extended biomass burning near surface sources located in extratropical latitudes (70-130<sup>o</sup> E, 55-70<sup>o</sup> N).  The OC/BC aerosols are transported upward by vertical mixing over the source regions, and enter the tropical UTLS through horizonal diffusive processes.   Additionally, enhanced penetrative convection in the anomalous ascent regions during the peak monsoon season may also play a role in further enhancing the monsoon ascent, lifting ambient hydrophobic OC/BC and water vapor in the mid-to-upper troposphere to higher elevations, resulting in enhanced ice-cloud fraction, increased latent and radiative heating in the UTLS/ATAL region.</p><p> </p>

scholarly journals Transport of aerosol pollution in the UTLS during Asian summer monsoon as simulated by ECHAM5-HAMMOZ model

2012 ◽  
Vol 12 (11) ◽  
pp. 30081-30117 ◽  
Author(s):  
S. Fadnavis ◽  
K. Semeniuk ◽  
L. Pozzoli ◽  
M. G. Schultz ◽  
S. D. Ghude ◽  
...  
Keyword(s):  
Summer Monsoon ◽  
Asian Summer Monsoon ◽  
Indian Subcontinent ◽  
Aerosol Layer ◽  
Anthropogenic Aerosols ◽  
Upper Troposphere ◽  
Aerosol Pollution ◽  
Asian Summer ◽  
Cloud Ice ◽  
The Impact

Abstract. An eight member ensemble of ECHAM5-HAMMOZ simulations for the year 2003 is analyzed to study the transport of aerosols in the Upper Troposphere and Lower Stratosphere (UTLS) during the Asian Summer Monsoon (ASM). Simulations show persistent maxima in black carbon, organic carbon, sulfate, and mineral dust aerosols within the anticyclone in the UTLS throughout the ASM (period from July to September) when convective activity over the Indian subcontinent is highest. Model simulations indicate boundary layer aerosol pollution as the source of this UTLS aerosol layer and identify ASM convection as the dominant transport process. Evidence of ASM transport of aerosols into the stratosphere is observed in HALogen Occultation Experiment (HALOE) and Stratospheric Aerosol and Gas Experiment (SAGE) II aerosol extinction. The impact of aerosols in the UTLS region is analyzed by evaluating the differences between simulations with (CTRL) and without aerosol (HAM-off) loading. The transport of anthropogenic aerosols in the UTLS increases cloud ice, water vapour and temperature, indicating that aerosols play an important role in enhancement of cloud ice in the Upper-Troposphere (UT). Aerosol induced circulation changes include a weakening of the main branch of the Hadley circulation and increased vertical transport around the southern flank of the Himalayas and reduction in monsoon precipitation over the India region.

scholarly journals Lower-stratospheric aerosol measurements in eastward shedding vortices over Japan from the Asian summer monsoon anticyclone during the summer of 2018

2020 ◽  
Author(s):  
Masatomo Fujiwara ◽  
Tetsu Sakai ◽  
Koichi Shiraishi ◽  
Yoichi Inai ◽  
Sergey Khaykin ◽  
...  
Keyword(s):  
Summer Monsoon ◽  
Forest Fires ◽  
Lower Stratosphere ◽  
Asian Summer Monsoon ◽  
Volcanic Eruptions ◽  
Stratospheric Aerosol ◽  
Aerosol Layer ◽  
Upper Troposphere ◽  
Asian Summer ◽  
The Japanese Archipelago

Abstract. Eastward airmass transport from the Asian summer monsoon (ASM) anticyclone in the upper troposphere and lower stratosphere (UTLS) often involves eastward shedding vortices, which can cover most of the Japanese archipelago. We investigated the aerosol characteristics of these vortices by analysing data from two lidar systems in Japan, at Tsukuba (36.1° N, 140.1° E) and Fukuoka (33.55° N, 130.36° E), during the summer of 2018. We observed several events with enhanced particle signals at Tsukuba at 15.5–18 km altitude (at or above the local tropopause) during August–September 2018, with a backscattering ratio of ~1.10 and particle depolarization of ~5 % (i.e., not spherical, but more spherical than ice crystals). These particle characteristics may be consistent with those of solid aerosol particles, such as ammonium nitrate. Each event had a timescale of a few days. During the same study period, we also observed similar enhanced particle signals in the lower stratosphere at Fukuoka. The upper troposphere is often covered by cirrus clouds at both lidar sites. Backward trajectory calculations for these sites for days with enhanced particle signals in the lower stratosphere and days without indicate that the former airmasses originated within the ASM anticyclone, and the latter more from edge regions. Reanalysis carbon-monoxide and satellite water-vapour data indicate that eastward shedding vortices were involved in the observed aerosol enhancements. Satellite aerosol data confirm that the period and latitudinal region were free from the direct influence of documented volcanic eruptions and high latitude forest fires. Our results indicate that the Asian Tropopause Aerosol Layer (ATAL) over the ASM region extends east towards Japan in association with the eastward shedding vortices, and that lidar systems in Japan can detect at least the lower stratospheric portion of the ATAL during periods when the lower stratosphere is undisturbed by volcanic eruptions and forest fires. The upper tropospheric portion of the ATAL is either depleted by tropospheric processes (convection and wet scavenging) during eastward transport or is obscured by much stronger cirrus cloud signals.

scholarly journals Lower-stratospheric aerosol measurements in eastward-shedding vortices over Japan from the Asian summer monsoon anticyclone during the summer of 2018

2021 ◽  
Vol 21 (4) ◽  
pp. 3073-3090
Author(s):  
Masatomo Fujiwara ◽  
Tetsu Sakai ◽  
Tomohiro Nagai ◽  
Koichi Shiraishi ◽  
Yoichi Inai ◽  
...  
Keyword(s):  
Summer Monsoon ◽  
Forest Fires ◽  
Lower Stratosphere ◽  
Asian Summer Monsoon ◽  
Volcanic Eruptions ◽  
Stratospheric Aerosol ◽  
Aerosol Layer ◽  
Upper Troposphere ◽  
Asian Summer ◽  
The Japanese Archipelago

Abstract. Eastward air-mass transport from the Asian summer monsoon (ASM) anticyclone in the upper troposphere and lower stratosphere (UTLS) often involves eastward-shedding vortices, which can cover most of the Japanese archipelago. We investigated the aerosol characteristics of these vortices by analysing data from two lidar systems in Japan, at Tsukuba (36.1∘ N, 140.1∘ E) and Fukuoka (33.55∘ N, 130.36∘ E), during the summer of 2018. We observed several events with enhanced particle signals at Tsukuba at 15.5–18 km of altitude (at or above the local tropopause) during August–September 2018, with a backscattering ratio of ∼ 1.10 and particle depolarization of ∼ 5 % (i.e. not spherical, but more spherical than ice crystals). These particle characteristics may be consistent with those of solid aerosol particles, such as ammonium nitrate. Each event had a timescale of a few days. During the same study period, we also observed similar enhanced particle signals in the lower stratosphere at Fukuoka. The upper troposphere is often covered by cirrus clouds at both lidar sites. Backward trajectory calculations for these sites for days with enhanced particle signals in the lower stratosphere and days without indicate that the former air masses originated within the ASM anticyclone and the latter more from edge regions. Reanalysis carbon monoxide and satellite water vapour data indicate that eastward-shedding vortices were involved in the observed aerosol enhancements. Satellite aerosol data confirm that the period and latitudinal region were free from the direct influence of documented volcanic eruptions and high-latitude forest fires. Our results indicate that the Asian tropopause aerosol layer (ATAL) over the ASM region extends east towards Japan in association with the eastward-shedding vortices and that lidar systems in Japan can detect at least the lower-stratospheric portion of the ATAL during periods when the lower stratosphere is undisturbed by volcanic eruptions and forest fires. The upper-tropospheric portion of the ATAL is either depleted by tropospheric processes (convection and wet scavenging) during eastward transport or is obscured by much stronger cirrus cloud signals.

scholarly journals Potential impact of carbonaceous aerosols on the Upper Troposphere and Lower Stratosphere (UTLS) during Asian summer monsoon in a global model simulation

2017 ◽  
Author(s):  
Suvarna Fadnavis ◽  
Gayatry Kalita ◽  
K. Ravi Kumar ◽  
Blaz Gasparini ◽  
Jui-Lin Frank Li
Keyword(s):  
Summer Monsoon ◽  
Radiative Forcing ◽  
Asian Summer Monsoon ◽  
Radiative Heating ◽  
The Tibetan Plateau ◽  
Carbonaceous Aerosols ◽  
The South ◽  
Upper Troposphere ◽  
North East ◽  
Asian Summer

Abstract. Recent satellite observations show efficient vertical transport of Asian pollutants from the surface to the upper level anticyclone by deep monsoon convection. In this paper, we examine the transport of carbonaceous aerosols including Black Carbon (BC) and Organic Carbon (OC) into the monsoon anticyclone using of ECHAM6-HAM, a global aerosol climate model. Further, we investigate impacts of enhanced (doubled) carbonaceous aerosols emissions on the UTLS from sensitivity simulations. These model simulations show that boundary layer aerosols are transported into the monsoon anticyclone by the strong monsoon convection from the Bay of Bengal, southern slopes of the Himalayas and the South China Sea. Doubling of emissions of BC and OC aerosols, each, over the South East Asia (10° S–50° N; 65° E–155° E) shows that lofted aerosols produce significant warming in the mid/upper troposphere. These aerosols lead to an increase in temperature by 1 K–3 K in the mid/upper troposphere and in radiative heating rates by 0.005 K/day near the tropopause. They alter aerosol radiative forcing at the surface by −1.4 W/m2; at the Top Of the Atmosphere (TOA) by +1.2 W/m2 and in the atmosphere by 2.7 W/m2 over the Asian summer monsoon region (20° N–40° N, 60° E–120° E). Atmospheric warming increases vertical velocities and thereby cloud ice in the upper troposphere. An anomalous warming over the Tibetan Plateau (TP) facilitate the relative strengthening of the monsoon Hadley circulation and elicit enhancement in precipitation over India and north east China.

scholarly journals Transport pathways of peroxyacetyl nitrate in the upper troposphere and lower stratosphere from different monsoon systems during the summer monsoon season

2015 ◽  
Vol 15 (20) ◽  
pp. 11477-11499 ◽  
Author(s):  
S. Fadnavis ◽  
K. Semeniuk ◽  
M. G. Schultz ◽  
M. Kiefer ◽  
A. Mahajan ◽  
...  
Keyword(s):  
Summer Monsoon ◽  
Lower Stratosphere ◽  
Asian Summer Monsoon ◽  
Monsoon Season ◽  
Peroxyacetyl Nitrate ◽  
Transport Pathways ◽  
Upper Troposphere ◽  
Asian Summer ◽  
Emission Change ◽  
Transport Patterns

Abstract. The Asian summer monsoon involves complex transport patterns with large-scale redistribution of trace gases in the upper troposphere and lower stratosphere (UTLS). We employ the global chemistry–climate model ECHAM5–HAMMOZ in order to evaluate the transport pathways and the contributions of nitrogen oxide species peroxyacetyl nitrate (PAN), NOx and HNO3 from various monsoon regions, to the UTLS over southern Asia and vice versa. Simulated long-term seasonal mean mixing ratios are compared with trace gas retrievals from the Michelson Interferometer for Passive Atmospheric Sounding aboard ENVISAT(MIPAS-E) and aircraft campaigns during the monsoon season (June–September) in order to evaluate the model's ability to reproduce these transport patterns. The model simulations show that there are three regions which contribute substantial pollution to the South Asian UTLS: the Asian summer monsoon (ASM), the North American monsoon (NAM) and the West African monsoon (WAM). However, penetration due to ASM convection reaches deeper into the UTLS compared to NAM and WAM outflow. The circulation in all three monsoon regions distributes PAN into the tropical latitude belt in the upper troposphere (UT). Remote transport also occurs in the extratropical UT where westerly winds drive North American and European pollutants eastward where they can become part of the ASM convection and lifted into the lower stratosphere. In the lower stratosphere the injected pollutants are transported westward by easterly winds. Sensitivity experiments with ECHAM5–HAMMOZ for simultaneous NOx and non-methane volatile organic compounds (NMVOCs) emission change (−10 %) over ASM, NAM and WAM confirm similar transport. Our analysis shows that a 10 % change in Asian emissions transports ~ 5–30 ppt of PAN in the UTLS over Asia, ~ 1–10 ppt of PAN in the UTLS of northern subtropics and mid-latitudes, ~ 7–10 ppt of HNO3 and ~ 1–2 ppb of ozone in UT over Asia. Comparison of emission change over Asia, North America and Africa shows that the highest transport of HNO3 and ozone occurs in the UT over Asia and least over Africa. The intense convective activity in the monsoon regions is associated with lightning and thereby the formation of additional NOx. This also affects the distribution of PAN in the UTLS. Simulations with and without lightning show an increase in the concentrations of PAN (~ 40 %), HNO3 (75 %), NOx (70 %) and ozone (30 %) over the regions of convective transport. Lightning-induced production of these species is higher over equatorial Africa and America compared to the ASM region. This indicates that the contribution of anthropogenic emissions to PAN in the UTLS over the ASM is higher than that of lightning.

scholarly journals Efficient transport of tropospheric aerosol into the stratosphere via the Asian summer monsoon anticyclone

2017 ◽  
Vol 114 (27) ◽  
pp. 6972-6977 ◽  
Author(s):  
Pengfei Yu ◽  
Karen H. Rosenlof ◽  
Shang Liu ◽  
Hagen Telg ◽  
Troy D. Thornberry ◽  
...  
Keyword(s):  
Summer Monsoon ◽  
Lower Stratosphere ◽  
Model Simulation ◽  
Asian Summer Monsoon ◽  
Volcanic Eruptions ◽  
Aerosol Layer ◽  
Upper Troposphere ◽  
Tropospheric Aerosol ◽  
Asian Summer

An enhanced aerosol layer near the tropopause over Asia during the June–September period of the Asian summer monsoon (ASM) was recently identified using satellite observations. Its sources and climate impact are presently not well-characterized. To improve understanding of this phenomenon, we made in situ aerosol measurements during summer 2015 from Kunming, China, then followed with a modeling study to assess the global significance. The in situ measurements revealed a robust enhancement in aerosol concentration that extended up to 2 km above the tropopause. A climate model simulation demonstrates that the abundant anthropogenic aerosol precursor emissions from Asia coupled with rapid vertical transport associated with monsoon convection leads to significant particle formation in the upper troposphere within the ASM anticyclone. These particles subsequently spread throughout the entire Northern Hemispheric (NH) lower stratosphere and contribute significantly (∼15%) to the NH stratospheric column aerosol surface area on an annual basis. This contribution is comparable to that from the sum of small volcanic eruptions in the period between 2000 and 2015. Although the ASM contribution is smaller than that from tropical upwelling (∼35%), we find that this region is about three times as efficient per unit area and time in populating the NH stratosphere with aerosol. With a substantial amount of organic and sulfur emissions in Asia, the ASM anticyclone serves as an efficient smokestack venting aerosols to the upper troposphere and lower stratosphere. As economic growth continues in Asia, the relative importance of Asian emissions to stratospheric aerosol is likely to increase.

scholarly journals Transport pathways of peroxyacetyl nitrate in the upper troposphere and lower stratosphere from different monsoon systems during the summer monsoon season

2014 ◽  
Vol 14 (14) ◽  
pp. 20159-20195 ◽  
Author(s):  
S. Fadnavis ◽  
K. Semeniuk ◽  
M. G. Schultz ◽  
A. Mahajan ◽  
L. Pozzoli ◽  
...  
Keyword(s):  
Summer Monsoon ◽  
North American ◽  
Lower Stratosphere ◽  
Asian Summer Monsoon ◽  
Monsoon Season ◽  
Convective Transport ◽  
Pollution Transport ◽  
Transport Pathways ◽  
Upper Troposphere ◽  
Asian Summer

Abstract. The Asian summer monsoon involves complex transport patterns with large scale redistribution of trace gases in the upper troposphere and lower stratosphere (UTLS). We employ the global chemistry–climate model ECHAM5-HAMMOZ in order to evaluate the transport pathways and the contributions of nitrogen oxide reservoir species PAN, NOx, and HNO3 from various monsoon regions, to the UTLS over Southern Asia and vice versa. The model is evaluated with trace gas retrievals from the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS-E) and aircraft campaigns during the monsoon season (June–September). There are three regions which contribute substantial pollution to the UTLS during the monsoon: the Asian summer monsoon (ASM), the North American Monsoon (NAM) and the West African monsoon (WAM). However, penetration due to ASM convection is deeper into the UTLS as compared to NAM and WAM outflow. The circulation in these monsoon regions distributes PAN into the tropical latitude belt in the upper troposphere. Remote transport also occurs in the extratropical upper troposphere where westerly winds drive North American and European pollutants eastward to partly merge with the ASM plume. Strong ASM convection transports these remote and regional pollutants into the lower stratosphere. In the lower stratosphere the injected pollutants are transported westward by easterly winds. The intense convective activity in the monsoon regions is associated with lightning generation and thereby the emission of NOy species. This will affect the distribution of PAN in the UTLS. The estimates of lightning produced PAN, HNO3, NOx and ozone obtained from control and lightning-off simulations shows high percentage changes over the regions of convective transport especially equatorial Africa and America and comparatively less over the ASM. This indicates higher anthropogenic pollution transport from the ASM region into the UTLS.

scholarly journals Transport pathways of peroxyacetyl nitrate in the upper troposphere and lower stratosphere from different monsoon systems during the summer monsoon season

2015 ◽  
Vol 15 (11) ◽  
pp. 15087-15135 ◽  
Author(s):  
S. Fadnavis ◽  
K. Semeniuk ◽  
M. G. Schultz ◽  
M. Kiefer ◽  
A. Mahajan ◽  
...  
Keyword(s):  
Summer Monsoon ◽  
North American ◽  
Lower Stratosphere ◽  
Asian Summer Monsoon ◽  
Monsoon Season ◽  
Convective Transport ◽  
Transport Pathways ◽  
Upper Troposphere ◽  
Asian Summer ◽  
Transport Patterns

Abstract. The Asian summer monsoon involves complex transport patterns with large scale redistribution of trace gases in the upper troposphere and lower stratosphere (UTLS). We employ the global chemistry-climate model ECHAM5-HAMMOZ in order to evaluate the transport pathways and the contributions of nitrogen oxide species PAN, NOx, and HNO3 from various monsoon regions, to the UTLS over Southern Asia and vice versa. Simulated long term seasonal mean mixing ratios are compared with trace gas retrievals from the Michelson Interferometer for Passive Atmospheric Sounding aboard ENVISAT(MIPAS-E) and aircraft campaigns during the monsoon season (June–September) in order to evaluate the model's ability to reproduce these transport patterns. The model simulations show that there are three regions which contribute substantial pollution to the South Asian UTLS: the Asian summer monsoon (ASM), the North American Monsoon (NAM) and the West African monsoon (WAM). However, penetration due to ASM convection reaches deeper into the UTLS as compared to NAM and WAM outflow. The circulation in all three monsoon regions distributes PAN into the tropical latitude belt in the upper troposphere. Remote transport also occurs in the extratropical upper troposphere where westerly winds drive North American and European pollutants eastward where they can become part of the ASM convection and be lifted into the lower stratosphere. In the lower stratosphere the injected pollutants are transported westward by easterly winds. The intense convective activity in the monsoon regions is associated with lightning and thereby the formation of additional NOx. This also affects the distribution of PAN in the UTLS. According to sensitivity simulations with and without lightning, increase in concentrations of PAN (~ 40%), HNO3 (75%), NOx (70%) and ozone (30%) over the regions of convective transport, especially over equatorial Africa and America and comparatively less over the ASM. This indicates that PAN in the UTLS over the ASM region is primarily of anthropogenic origin.

scholarly journals First detection of ammonia (NH<sub>3</sub>) in the Asian summer monsoon upper troposphere

2016 ◽  
Vol 16 (22) ◽  
pp. 14357-14369 ◽  
Author(s):  
Michael Höpfner ◽  
Rainer Volkamer ◽  
Udo Grabowski ◽  
Michel Grutter ◽  
Johannes Orphal ◽  
...  
Keyword(s):  
Summer Monsoon ◽  
Asian Summer Monsoon ◽  
Michelson Interferometer ◽  
Emission Spectra ◽  
Global Scale ◽  
Aerosol Layer ◽  
Upper Troposphere ◽  
Mixing Ratios ◽  
Monsoon Area ◽  
Asian Summer

Abstract. Ammonia (NH3) has been detected in the upper troposphere by the analysis of averaged MIPAS (Michelson Interferometer for Passive Atmospheric Sounding) infrared limb-emission spectra. We have found enhanced amounts of NH3 within the region of the Asian summer monsoon at 12–15 km altitude. Three-monthly, 10° longitude  ×  10° latitude average profiles reaching maximum mixing ratios of around 30 pptv in this altitude range have been retrieved, with a vertical resolution of 3–8 km and estimated errors of about 5 pptv. These observations show that loss processes during transport from the boundary layer to the upper troposphere within the Asian monsoon do not deplete the air entirely of NH3. Thus, ammonia might contribute to the so-called Asian tropopause aerosol layer by the formation of ammonium aerosol particles. On a global scale, outside the monsoon area and during different seasons, we could not detect enhanced values of NH3 above the actual detection limit of about 3–5 pptv. This upper bound helps to constrain global model simulations.

scholarly journals Characterization of non-methane hydrocarbons in Asian summer monsoon outflow observed by the CARIBIC aircraft

2011 ◽  
Vol 11 (2) ◽  
pp. 503-518 ◽  
Author(s):  
A. K. Baker ◽  
T. J. Schuck ◽  
F. Slemr ◽  
P. van Velthoven ◽  
A. Zahn ◽  
...  
Keyword(s):  
Carbon Monoxide ◽  
Summer Monsoon ◽  
Fossil Fuels ◽  
Asian Summer Monsoon ◽  
Trace Gases ◽  
Northern Region ◽  
Monsoon Circulation ◽  
Upper Troposphere ◽  
Mixing Ratios ◽  
Asian Summer

Abstract. Between April and December 2008 the CARIBIC commercial aircraft conducted monthly measurement flights between Frankfurt, Germany and Chennai, India. These flights covered the period of the Asian summer monsoon (June–September), during which enhancements in a number of atmospheric species were observed in the upper troposphere over southwestern Asia. In addition to in situ measurements of trace gases and aerosols, whole air samples were collected during the flights, and these were subsequently analyzed for a suite of trace gases that included a number of C2–C8 non-methane hydrocarbons. Non-methane hydrocarbons are relatively short-lived compounds and the large enhancements in their mixing ratios in the upper troposphere over southwestern Asia during the monsoon, sometimes more than double their spring and fall means, provides qualitative evidence for the influence of convectively uplifted boundary layer air. The particularly large enhancements of the combustion tracers benzene and ethyne, along with the similarity of their ratios with carbon monoxide and emission ratios from the burning of household biofuels, indicate a strong influence of biofuel burning to NMHC emissions in this region. Conversely, the ratios of ethane and propane to carbon monoxide, along with the ratio between i-butane and n-butane, indicate a significant source of these compounds from the use of fossil fuels, and comparison to previous campaigns suggests that this source could be increasing. Photochemical aging patterns of NMHCs showed that the CARIBIC samples were collected in two distinctly different regions of the monsoon circulation: a southern region where air masses had been recently influenced by low level contact and a northern region, where air parcels had spent substantial time in transit in the upper troposphere before being probed. Estimates of age using ratios of individual NMHCs have ranges of 3–6 days in the south and 9–12 days in the north.

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